ISSN: 1683-1489
Mekong River Commission
Fish migration triggers in the
Lower Mekong Basin and other
tropical freshwater systems
MRC Technical Paper
No. 14
December 2006
Meeting the Needs, Keeping the Balance
ISSN: 1683 1489
Mekong River Commission
Fish migration triggers in the Lower
Mekong Basin and other tropical
freshwater systems
MRC Technical Paper
No. 14
December 2006
Published in Vientiane, Lao PDR in December 2006 by the Mekong River Commission
Suggested citation:
BARAN Eric (2006) Fish migration triggers in the Lower Mekong Basin and other tropical
freshwater systems. MRC Technical Paper No. 14, Mekong River Commission, Vientiane.
56 pp.
The opinions and interpretation expressed within are those of the author and do not
necessarily reflect the views of the Mekong River Commission.
Editor: Dr Tim Burnhill
© Mekong River Commission
184 Fa Ngoum Road, Unit 18, Ban Sithane Neua, Sikhottabong District,
Vientiane 01000, Lao PDR
Telephone: (856-21) 263 263 Facsimile: (856-21) 263 264
Email: mrcs@mrcmekong.org
Website: www.mrcmekong.org
Table of Contents
Summary
vii
1.
Introduction
1
Scope of the study
1
Methodology
1
2.
Definitions and Context
3
Migrations, triggers and cues
3
Types of migration
3
Migration in the Mekong River
5
3.
Review of Migration Triggers
7
Discharge, water level and current
7
Mekong River
8
Other tropical rivers
9
Precipitation
10
Mekong River
10
Other tropical rivers
10
Lunar cycle
11
Mekong River
11
Other tropical rivers
12
Other migration triggers
13
Apparition of insects
13
Water colour and turbidity
13
Temperature and photoperiod
13
Issues raised by the literature review
14
Importance of fish density as a complementary biological trigger
14
Importance of spawning triggers in addition to migration triggers
14
Multiplicity of triggers
15
4.
Quantitative Analysis of Mekong Fish Migration Triggers
17
Methodology
17
Migration and triggers of the 768 Mekong fish species
19
Status of knowledge on migration of fish species
19
Status of knowledge on migration triggers of fish species
19
Classification of factors triggering fish migration
21
Fish families most sensitive to triggers
22
Correlation between migration cues
22
Correlation between triggers of fish migrations
22
iii
Correlation between environmental variables
23
Quantitative relationships between discharge and migrations
24
Overview of migration patterns at Khone Falls
24
Representativeness of the Khone Falls studies
25
Catches, species richness and discharge: global patterns
27
Catches and discharge: detailed patterns
29
Response of 47 fish taxa to discharge
30
5.
Conclusions
33
Migration and triggers
33
Discharge, catch and diversity
34
Consequences of flow modifications on fish
35
Dry season flow modifications
35
Rainy season flow modifications
37
Methodological points
37
6.
References
39
Annex 1. Life history, migration and triggers of Mekong fish species
49
Annex 2. Response of fish taxa to discharge at Khone Falls
53
iv
Acknowledgments
The author would like to acknowledge the contribution of Teemu Jantunen to the literature
search and review, that of Gregory Cans to the creation of routines and procedures for the
fusion and analysis of FishBase and MFD databases, and the insightful comments of the
reviewers.
This study results from a collaboration between the MRC and the Technical Body for
Fisheries Management (TAB) who requested and funded the work, and the WorldFish Center
for who the author works.
TECHNICAL ADVISORY
BODY FOR FISHERIES
MANAGEMENT (TAB)
v
vi
Summary
Nature and objective of the study
This report reviews the factors that trigger fish migration in the Mekong River and
other tropical freshwater river systems. It aims to provide a basis for understanding the
consequences of human intervention to the natural flow regime of the river system on fish
migration and thereby on the fisheries of the Lower Mekong Basin.
The report comprises two parts. It begins with a systematic review of the published
information on migration triggers and cues in the Mekong River system and in other
tropical rivers worldwide. This part includes a discussion of other associated issues, such as
spawning triggers, which should also be taken into account when dealing with Mekong fish
migrations and management.
The second part of the report presents a quantitative analysis of the environmental
factors that trigger the migration of Mekong species. This section is based on a merger of
FishBase 2004 with the Mekong Fish Database, and covers the entire population of 768
Mekong species held within FishBase. The merged data set includes biological data and
coded ecological information for all of these species. The results are supplemented by an
analysis of data from a recently published study of migration patterns and hydrological
triggers at the Khone Falls (Southern Lao PDR).
Summary of the findings
Literature review: migration triggers in the Mekong and in other tropical systems
Literature on the migration of tropical freshwater species (as opposed to freshwater
temperate and marine species) is scarce and there are few documented studies that deal
specifically with migration triggers.
The literature cites water-level and current, discharge, precipitation, the lunar cycle, water
colour and turbidity and the apparition of insects as key migration triggers for tropical
freshwater fish worldwide.
· Water level is the environmental parameter most often cited as a migration trigger;
this parameter correlates closely with discharge and water current. Thresholds, or
changes, in water level, discharge or current are known to trigger the migration of 30
Mekong fish species. In the case of many, it is the variation of water level, rather than
particular water-level thresholds, which acts as the trigger.
· The first rainfalls of the wet season also trigger breeding and reproductive migration
in the tropics. In the Mekong River system, 11 species are known to be triggered by
vii
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
early rainfalls. In several river systems, the migrations cued by the first rains are also
associated with the lunar cycle.
· Changes in turbidity, or in water colour, are recorded as migration triggers for nine
species in the Mekong River system. However, these criteria do not discriminate
between changes caused by increasing or decreasing sediment load, and those caused
by blooms of algae or other planktonic organisms.
· The apparition of insects triggers the migration of five Mekong fish species.
The literature also cites the lunar phase as an environmental trigger. The influence of the
moon on several Mekong species is well documented, however it is unclear which lunar
phase is the trigger, as this seems to vary depending upon species and location. However, in
the Mekong River system the migration of large numbers of fish across the Khone Falls at
the time of the second new moon after the winter solstice (at the time of the Chinese New
Year), is well known. Indeed, similar spectacular migrations corresponding to the lunar phase
have also been recorded in Africa, but the importance of the moon in migration, in general,
remains far from clear.
Additional issues highlighted by the literature review
The literature review presented in this report targeted those environmental factors that trigger
migration; however the review also brought to light three other important points, namely:
· Fish density as a complementary biological migration trigger.
· The need to consider spawning triggers in addition to migration triggers. These
reproduction triggers are equally crucial to the sustainability of fish populations and
also concern non-migrant species.
· The multiplicity and interaction of triggers and the role they play at different times
and in different physiological mechanisms, mean that triggers cannot be reduced to a
single parameter such as `discharge'.
Data analysis: quantification of Mekong migration triggers
Status of knowledge on migration of fish species
· The migration status (that is whether a given species is migratory or non-migratory) is
known for only 165 (24%) Mekong fish species.
· There is no information about the migratory behaviour of the remaining 579 (76%)
Mekong species.
· Of the species whose migration status is known, 135 (87%) are migrant species and 30
(13%) are non-migrant species.
viii
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
Status of knowledge on migration triggers of fish species
· The merged data set cites five environmental factors that trigger or cue migration:
i) variation in river discharge; ii) variation in water level; iii) first rainfalls after the dry
season; iv) change in water turbidity or colour; and v) apparition of insects.
· Environmental factors are known to trigger migration of at least 30 (18%) of the 165
migratory species; 12 of these are sensitive to more than one trigger.
· The migration cues of the remaining 135 (82%) migratory species are unknown.
Classification of factors triggering fish migration
Of the species for which migration cues are known:
· ninety per cent respond to variations in water level or in discharge;
· thirty per cent respond to the first rainfalls at the end of the dry season;
· thirty per cent respond to change in turbidity or water colour;
· ten per cent respond to the apparition of insects.
Fish families most sensitive to triggers
The pangasiid family (catfishes) appears to be the most sensitive to migration cues; 11
of the 19 species in this family (58%) respond to one or more environmental trigger. The
next most sensitive families are silurids (3 species, 9% of the species of this catfish family)
and Cyprinids (9 species, 3% of the family). Thus catfish species are the most sensitive to
migration triggers.
Correlations between environmental cues in the Mekong.
Analysis of available qualitative ecological records shows no statistically significant
correlations between any of the environmental triggers. However, a quantitative analysis
of environmental variables of a three-year data set shows that water level and discharge
are closely correlated and should be considered as a single cue. The other environmental
parameters are independent.
Quantitative relationships between discharge and migrations
Eight distinct waves of fish migration occur annually at Khone Falls in southern Lao
PDR. These have been studied and recorded in great detail over a number of years. An
extensive analysis combining six years of fishery data from Khone Falls and corresponding
hydrological measurements from gauges at Pakse (the nearest monitoring station) shows:
· That the most diverse catch is taken when the discharge is low.
ix
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
· That 96% of the fish is caught at discharge rates of between 2000 and 8000 cumecs
(cubic metres per second), and that the most important discharge for fisheries is
between 2000 and 3000 cumecs. Additional results show that five species make almost
half of the catch. These species are all known to be sensitive to hydrological triggers.
· An analysis of the relationship between the abundance of the 47 most common taxa in
catches and discharge shows that 13 taxa are very sensitive to (low) discharge, 17 taxa
are sensitive to discharge, 13 taxa are not sensitive to this variable, and the relationship
is unknown for 4 taxa.
These results all converge to highlight the extreme importance of low water-levels and
discharges to the richness and production of Khone Falls fisheries, and to the Lower Mekong
in general, at a time when fish are dense and fisheries very intensive.
With regard to flow modifications, the most important negative impacts on fisheries will be
those that increase dry season flows (impact on ecology and catchability of fish), and those
that delay the onset of the flood. Flow modifications in the transitional or rainy seasons
are likely to have much smaller impact. The importance of Mekong fisheries (which, at
2.6 million tonnes per year, are 7 times greater than the whole Northern American inland
fisheries sector) to the lives and livelihoods of the basin's inhabitants calls for additional
thorough analyses of the impact of flow modifications on fisheries, in order to integrate this
essential sector in plans and policies for the development of the Mekong's water resources.
KEY WORDS: Mekong River; fish migration triggers; fisheries; Khone Falls
x
1. Introduction
Scope of the study
The objective of this report is to review literature on the factors that trigger, or cue, fish
migrations in tropical freshwater systems, particularly the Mekong River system, in
order to better understand the possible impacts of flow changes induced by water-related
developments (primarily dams) on fish migrations and ultimately on the fisheries of the
Mekong.
The demand for this study was driven by the members of the Technical Advisory Body for
Fisheries Management (TAB)1, who identified a `knowledge gap' on this subject. Despite
the fact that a large portion on the Mekong's fish migrate (particularly the commercially
important species), very little hard information seemed to exist about migration and
specifically the environmental factors that trigger migration. Clarification was needed, and it
was also considered useful incorporate information from studies on other tropical freshwater
systems.
The bibliographic review focused on river fishes and migrations; marine and estuarine
systems were not included. Their exclusion is justified because brackish environments
account for only three per cent of the area of the lower Mekong River system.
Within the freshwater realm, the review concentrated on potadromous fishes, fishes that
migrate entirely within freshwater systems. However, the data analysis also included
anadromous and catadromous Mekong species.
The review was also focused on tropical systems, as major migration triggers in cold and
temperate countries are large-scale seasonal temperature and photoperiod variations (up to
200C and 8 hours of light per day), which do not occur in tropical environments. However,
temperature and photoperiod variations were included in the review of fish migration
triggers.
Methodology
To identify relevant articles, the literature search used two large bibliographic resources
(Cambridge Scientific Abstracts CSA and Aquatic Science and Fisheries Abstracts ASFA)
that cover the majority of journals dealing with fish and fisheries worldwide. Selected articles
were acquired through the Agora, Science Direct and SwetsWise portals.
1 The TAB comprises senior representatives from the fisheries ministries and departments of the four riparian countries,
Cambodia, Lao PDR, Thailand and Viet Nam in the Lower Mekong Basin. SIDA, (Swedish International Development
Assistance) and Danida (the international development agency of the Danish government ) fund to the body through Mekong
River Commission (MRC) which also provides additional logistical and operational support.
1
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
The literature search was supplemented by the resources of WorldFish Center library
in Malaysia, access to the IRD on-line library1 and the use of Library of the Museum
National d'Histoire Naturelle in Paris, which is a source of scientific colonial publications
from Indochina and Africa. Researchers from the IRD (Institut de Recherche pour le
Développement, formerly ORSTOM, France) and from the Tropical Ecology working group
at the Max Planck Institute for Limnology (Germany) provided additional information on
freshwater fish migration in Africa and South America.
In addition to this literature review, the author undertook a systematic screen of the two main
databases that cover all Mekong fish species: the Mekong Fish Database (MFD) produced
by the MRC (MRC 2003), and FishBase, the repository of all published information on
fish maintained by the WorldFish Center (Froese and Pauly, 2000, see www.fishbase.org).
The MFD provides detailed ecological information, while FishBase provides a number of
published life history parameters, such as age at first maturity, food items and estimated life
span, for each species.
This overview confirmed the earlier conclusions of Lucas and Baras (2001), who recognised
that while a large amount of information is available on migrant species in riverine and
marine environments in cold and temperate latitudes, much less literature exists on the
migration of tropical freshwater species, despite the huge diversity of this fish fauna. In the
Mekong Basin for instance, where migrations are known to be an essential ecological feature
(e.g. Pantulu, 1986; van Zalinge et al., 2004), there is no information on migration for 76%
of the species.
In this context of limited knowledge about the migration of tropical freshwater fishes, the
particular issue of migration triggers, if often mentioned in ichthyology, has very seldom
been the object of specific studies. In the Mekong Basin, no specific study of migration
triggers has been attempted, although Baran et al. (2005) do address the migration of 47
species in their broader review of fisheries bioecology at Khone Falls.
1 http://www.bondy.ird.fr/pleins_textes/index.htm. 35,000 documents online, including most of the French ichthyologic
research in West Africa and in South America
2
2. Definitions and Context
Migrations, triggers and cues
According to dictionaries, to migrate consists of passing, usually periodically, from
one region to another for feeding or breeding1. However several definitions of the term
`migration' have been proposed with a particular reference to fishes (e.g. McKeown, 1984;
Sparre and Venema, 1992; Lucas and Barras, 2001). The concept of migration starts
with clear-cut long range journeys between distant localities, but as scientists started
acknowledging that short-distance movements could also be crucial in the life history of a
species, the challenge was to sort out in one sentence the difference between `significant'
moves and random movements of a species.
In their studies of Mekong fish migration, Singhanouvong et al. (1996a) define migrations as
`any purposeful, seasonally regular type of movement of individuals from one ecologically
distinct zone to another'. The most simple, clear and comprehensive definition is probably
that of Northcote (1984), who defines migration as, `movements that result in an alternation
between two or more separate habitats, occur with a regular periodicity, and involve a large
proportion of the population'.
A trigger is commonly defined as a factor that initiates a process or reaction, by analogy to
the metallic part of a gun moved by the finger to fire. In the case of Mekong fish migration,
triggers are understood as environmental factors that provide a sudden signal for fish to start
actively migrating.
Similarly, a cue is `a signal to a performer to begin a specific action'. The term is considered
in this study as a synonym of trigger. Referring to reproductive cues, Helfman et al.
(1997) distinguish predictive cues (that trigger, for example, gonadal maturation before
any migration), synchronizing cues that signal appropriate environmental conditions, and
terminating cues that indicate the end of the appropriate period.
Types of migration
From a spatial perspective fish migration can be lateral, longitudinal or vertical, and the
movements can be either active or passive (in particular in the case of eggs and larval drift).
Longitudinal migrations2 take place up and down the main river-channel whereas lateral
migrations are movements from the main river channel into the floodplain, and back again
when the water recedes (examples in Welcomme, 1979). However, lateral migrations are
1 More generally, the study of the timing of recurring biological events and the causes of their timing is called phenology
(Lieth, 1974).
2 Longitudinal migrations can be either anadromous, i.e. ascending rivers from the sea for breeding, like salmons, or
catadromous, i.e. living in fresh water and going to the sea to spawn, like eels, or potamodromous (migratory within
freshwater). According to Gross et al. (1988), catadromous species predominate at tropical latitudes where rivers
productivity exceeds that of marine waters, and conversely in temperate latitudes.
3
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
often followed by longitudinal migrations within the main river channel (Bao et al. 2001).
As pointed out in Jorgensen et al. (1998), lateral migrations, if less spectacular, are at least as
important as long-distance longitudinal migrations in terms of fish production.
Vertical migrations of adult fish have not been detailed in this study as only two Mekong
species are known to undertake such migrations: Clupeichthys aesarnensis and Corica
laciniata (Warren, 2000).
From an ecological viewpoint, Baker (1978; in McKeown, 1984) identifies two main types
of fish migration: obligatory and facultative. Obligatory migrations refers to movements
following a series of physiological modifications in fishes, these modifications being
themselves triggered by environmental parameters such as moon phase or temperature. This
physiological response to triggers is thus age and size specific. On the contrary facultative
migration refers to movements made response to degrading living conditions, such as de-
oxygenation, food scarcity or high predation pressure.
Why do fish migrate?
Animals migrate because key habitats essential for their survival in terms of
reproduction and food availability are separated in time and space (Poulsen
et al., 2002). For example, in a tropical floodplain river like the Mekong, breeding
migrations upstream ensure that newly hatched fish larvae and juveniles can drift
with the rising flows down to floodplains when they become most productive and
accessible to fishes, i.e. at the beginning of the rainy season (Bao et al., 2001). In this
respect these migrations contribute to increased growth and survival of a species,
and thus to increased productivity of a system (Fernandes and de Merona, 1988). It
is generally considered that these movements have evolved with, and thus are finely
tuned to, the environment within which they occur (Poulsen, 2003). Some authors
also consider those migrations as a way for species to minimize predation on their
juveniles in confined areas; they also allow the dispersion of juveniles all over a
river system, thus maximizing the expansion and survival chances of the species
(Fryer, 1965). Some carnivorous species also follow shoals of their migrating preys,
and thus undertake similar migrations, as highlighted by Daget (1957) in Africa.
Migrations are a remarkable feature of South American fishes. As in the Mekong they
involve commercially important species and stocks, can be observed on a large scale, and
occur over hundreds of kilometres. They have been described in a number of books and
studies (reviews in Welcomme, 1985; Barthem and Goulding, 1997; Winemiller and Jepsen,
1998; Carolsfeld et al., 2003). In contrast, migrations do not appear prominently in the
literature on African freshwater fishes. They are of course mentioned in a number of studies,
in particular in relation to floodplains (e.g. Niger River, Lake Chad), but for instance a
comprehensive review such as that of Lévêque and Paugy (1999) only dedicates a few pages
out of 500 to migration issues. This could be due to a limited number of focused or precise
studies as suggested by Lévêque (1999) despite the large number of potamodromous species
in Africa. The low emphasis put on fish migrations in Africa could also be explained by
partitioning of fish bioecology into specialties dealing with breeding, nutrition, growth or
4
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
competition; multipurpose migrations are partly addressed in each discipline thanks to a high
number of detailed studies, but do not constitute a field of research per se.
Floodplain fish ecology in a nutshell
`In floodplain rivers, floods may increase the size of aquatic environment up to 50%
annually, and also bring in nutrients which stimulate rapid growth of micro-organisms,
invertebrates and plants, giving abundant food and cover for the fish during the high
water season. The high water season is the main feeding and growing time for the fish
which then lay down fat stores to last through the ensuing dry season, when they have
to retreat to the main rivers where there is little food, or remain in floodplain pools.
Mortality is very high as the water recedes.
On the floodplain, losses due to eggs being stranded, deoxygenation and predation are
very great, providing great pressures for rapid development and growth. Many of these
floodplain fishes mature in one or two years.
Both the intensity of flooding (height and duration of flood) and amount of water
retained at low water many affect fish numbers in subsequent years. The dependence
of these fishes on the arrival of floods for spawning and the effects of floods on growth
rates of individual fish, was well shown.'
Lowe McConnell, 1979
Migration in the Mekong River
Poulsen et al. (2002) distinguish three major migration systems in the Lower Mekong Basin:
the Lower Mekong (altitude 0150 masl1), the Middle Mekong (altitude 150200 masl) and
the Upper Mekong (altitude 200500 masl) (Figure 1).
In the Lower Mekong migration system the migrations are basically movements out of
the floodplains and tributaries, including the Tonle Sap, to and up the Mekong River at
drawdown period. A number of species spawn around their dry season refuges usually at the
onset of the monsoon and beginning of water level rise.
In the Middle Mekong migration system, fish move upstream during the wet season and
associated rising waters, and enter the tributaries and their associated flooded areas for
feeding. During drawdown they leave the tributaries and return to dry season refuges
downstream in the Mekong (Poulsen, 2003).
Finally in the Upper Mekong migration system the fish migrate upstream to spawning
habitats during the wet season to return later to their dry season habitats also along the main
river (van Zalinge et al., 2004).
It is important to note that the systems are not closed but are strongly interconnected, and
many species are known to migrate from one system to another.
1 masl = metres above sea level
5
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
Viet Nam
Chiang Saen
Upper
Mekong
System
Lao PDR
Vientiane
Middle
Mekong
Thailand
System
Pakse
Cambodia
Khone Falls
Lower
Tonle Sap Mekong
System
Kratie
Phnom Penh
Viet Nam
0
50 100
200
300 Kms
Figure 1. Migration systems in the Lower Mekong Basin (after Poulsen et al., 2002)
6
3. Review of Migration Triggers
For a migration to be successful it must be initiated at the appropriate time. Thus migration
cues play an essential role in tuning up the response of a species, allowing a coordinated
breeding or feeding strategy among individuals, that will maximize the chances of survival at
the scale of a population1.
`One postulated function of cyclical behaviors is the opportunity that it affords
individuals to synchronize their behaviours with that of conspecifics. Nowhere is such
synchronization more obvious and necessary than in reproduction. Not only must
both sexes aggregate at the same locale to release gametes, but preparatory events of
gametogenesis (gamete production) and secondary sex character development must
also occur with similar timetables that converge on the same small time window.'
Helfman et al., 1997
In this context, a better understanding of the factors that initiate migrations is important
for the sustainable management of capture fisheries, in particular in a system such as the
Mekong where fish production is exceptional (Baran et al., in press) and where migrations
play an important role. The sections below present a synthesis of the literature reviewed.
This synthesis is based on the following sources:
Baird (1998); Baird and Flaherty (2001, 2002, 2004); Baird et al. (2001a, 2003, 2004); Baird
and Phylavanh (1999); Bardach (1959); Chan (2000); Chan et al. (1999); Chomchanta et
al. (2000); Heng et al. (2001); Hortle et al. (2005); Ngor et al. (2005); Poulsen and Valbo-
Jørgensen (2000); Poulsen et al. (2004); Rainboth (1996); Roberts (1993); Roberts and Baird
(1995); Roberts and Warren (1994); Sao-Leang and Dom-Saveun (1955); Schouten et al.
(2000); Singanouvong et al. (1996a and b); Sukumasavin and Leelapatra (1994); Viravong et
al. (1994); and Warren et al. (1998).
Discharge, water level and current
Water level is the environmental parameter most often cited in the literature as a migration
trigger. This parameter is however also correlated to discharge and current speed, as will be
demonstrated in Chapter 4. But the latter variables are less easy to observe and quantify than
water level, which might explain the higher occurrence of water-level in records of migration
triggers. The correlation between these environmental cues leads Poulsen et al. (2004) to
simply say, for a number of Mekong species, that `the arrival of the monsoon season triggers
the fish to migrate'.
1 Hence the notion of involving `a large proportion of the population' highlighted in Northcote's definition of migration.
7
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
Mekong River
In their study of Mekong fish migrations, Poulsen (2000) and Bao et al. (2001) found that
in the case of all species, a change in water level was the most important factor associated
with both longitudinal and lateral migration. Fishers from Khone Falls also say that at the
beginning of the rainy season the change in water level is the single most important factor
determining the catch of migrating fish (Viravong et al., 1994). However, it is also noted that
the dry season migration seems to be less under the influence of changing water levels and
climatic conditions than the May-June migration that proceeds the flood.
Baird et al., (2004) and Singhanouvong et al. (1996a and b) argue that the variation of
water level, rather than a particular water level threshold, is the factor that triggers migration
(this was demonstrated later by Baran and Baird (2003) and Baran et al., (in press)). This
phenomenon has been illustrated in particular with Pangasius krempfi.
According to FishBase and MFD, 26 species are known to be triggered by thresholds or
changes in discharge, water-level or current; they are:
Cyprinidae: Bangana behri; Barbonymus gonionotus; Cyclocheilichthys enoplos; Cyprinus
carpio carpio; Labeo chrysophekadion; Macrochirichthys macrochirus; Parachela
oxygastroides; Paralaubuca typus.
Pangasidae: Pangasius conchophilus; Pangasianodon hypophthalmus; Pangasius krempfi;
Pangasius kunyit; Pangasius larnaudii; Pangasius macronema; Pangasius polyuranodon;
P. sanitwongsei.
Siluridae: Micronema bleekeri; Wallago leerii; Hemisilurus mekongensis.
Others: Hemibagrus filamentus (Bagridae); Tenualosa thibaudeaui (Clupeidae); Botia
modesta (Cobitidae); Lycothrissa crocodilus (Engraulidae); Pristolepis fasciata (Nandidae);
Chitala blanci (Notopteridae) and Osphronemus exodon (Osphronemidae).
Some other species can be added to this list: Pangasius bocourti (Chhuon, 2000),
Puntoplites falcifer and the southern population of Pangasius sanitwongsei (Poulsen et al.,
2004), to some extent Morulius chrysophekadion (Heng et al. 2001) and Henicorhynchus
siamensis (Bao et al., 2001). The latter case is not clear as: i) the taxonomy of the genus
Henicorhynchus is confused (in particular with Cirrhinus); ii) the number of species in this
genus is not fixed; and iii) the identification of most species of the genus is almost impossible
in the field.
In contrast, van Zalinge et al. (2002) report that it is the spawning of Pangasianodon
hypophthalmus, that is triggered by a rapid rise in the Mekong water levels, and
Singhanouvong (1996b) refers to occasions when Pangasius conchophilus has not migrated
despite considerable increases in flow volumes. Finally, Baird et al. (2001a) highlight
considerable uncertainty in the literature about which factors trigger Pangasius macronema
migrations.
8
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
Other tropical rivers
The influence of seasonal water level changes on fish ecology and migrations in South
America has been discussed in a number of publications, in particular by Fernandez and
Merona (1988) and Gercilia et al. (2000) for the Amazon, by Winemiller and Jepsen (1998)
for the Orinoco systems, Sato and Godinho (2004) for São Francisco River, Agostinho et al.
(2004) for Upper Paraná Basin and Watkins et al. (2004) for the Iwokrama forest in Guyana.
Fish movements in these systems are associated with changes in water level during the
cyclical seasonal flooding. Fishes occupy these flooded areas during rainy season, then move
to more permanent water bodies when water recedes (de Oliveira and Garavello, 2003). The
genus Prochilodus provides an alternative example of species undertaking a long distance
spawning migration in the dry season, so that they arrive before, or at the same time as, the
rising flood (Fuentes and Espinach Ros, 1998).
However, in all these cases fish migrations seem to occur gradually as the water-level
changes (either progressing or receding), and no specific thresholds or triggers are identified,
in particular at the scale of the whole fish community. Thus in the Upper Paraná Basin,
Paraguay, migration and spawning occurs over a large period of time, between October and
January for Characiforms and between December and March for Siluriforms. However,
if the flooding is delayed, spawning is also delayed, and most migratory species then start
during February. A failure of fish reproduction was reported in the Upper Paraná as a
consequence of the absence of flooding during the spawning season (Agostinho et al., 2004).
Pavlov et al. (1995) confirm, in their study of ichthyoplankton of the Amazon, that migration
is accelerated by rising waters (however, migration of juveniles is also recorded at lower
water levels because a portion of the adult community spawns all year round).
In Nigeria, Ezenwaji (1998) shows that the migration of the juveniles of the catfish Clarias
albopunctatus is triggered by current speed (no migration below 0.08 ms-1). In the Niger
Inner Delta, Benech et al. (1994) and Benech and Penaz (1995) show that migrations out of
the floodplain back to the mainstream start immediately after the water starts to descend.
In the Murray Darling system of Australia, the results of Reynolds' studies (1983) suggest
strongly that the migration of a number of species is related to changes in water level and
reproductive behaviour. For instance, Golden and Silver perch undergo long-distance
upstream migrations initiated by water-level rises at the onset of major flooding. However,
according to Humphries and Lake (2000), in this system `the generalisation that all river
spawning behaviour is linked to the hydrological regime may be incorrect'. As a matter of
fact, Reynolds (1983) also found that unlike perches, catfishes, which are nest builders and
lay demersal eggs, do not undergo long-distance movement. These observations lead to the
conclusion that long distance migration triggered by a rise in water level is necessary for
non-guarder species, so that their drifting eggs and larvae do not get washed into the sea but
arrive at fertile floodplains at the right time.
9
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
Precipitation
Precipitation, at the onset of the rainy season, seems to trigger breeding and reproductive
migration in the tropics in conjunction with associated rises in water level. Lowe-McConnell
(1987) provides details of this phenomenon.
Mekong River
In the Mekong Basin, heavy monsoon rains begin in MayJune and continue till October.
Fishers along the LaoThai stretch of the Mekong often mention that the first rains after the
dry season trigger fish to migrate upstream (Poulsen, 2000).
According to our review, ten species are known to be triggered by early rainfalls; they are:
Cyprinidae: Barbonymus gonionotus; Cyclocheilichthys enoplos; Mekongina erythrospila;
Paralaubuca typus
Pangasidae: Pangasianodon gigas; Pangasius pleurotaenia; Pangasius polyuranodon
Others: Micronema bleekeri (Siluridae) and Tenualosa thibaudeaui (Clupeidae)
Poulsen et al. (2004) also say that the migration of Micronema apogon (Siluridae) is
triggered by the first rain at the end of the dry season and by water-level changes. These
migrations are under the additional influence of the moon (in Cambodia, migrations out of
floodplains occur on, or immediately before, the full moon).
Other tropical rivers
In the Amazon strong rains, as well as lunar rhythms, seem to be important stimuli for
schooling of Semaprochilodus insignis and S. taeniurus during their spawning season
(Ribeiro; 1983, in Fernandes and de Merona, 1988).
Similarly, the preliminary results of Silva and Davis (1986) from Sri Lanka suggest that nine
species of indigenous fish actively migrate upstream to spawn before the onset of the north
east monsoonal rains.
Pre-monsoon migration dominates in the Ganges, especially with catfish and Cyprinids, but
the peak can vary as snow melt also induces migration (Payne et al., 2003). This suggests
that changes in water level of rivers and lakes, rather than rainfall, is the direct trigger of
migration.
Benech and Dansoko (1994) conclude that in the inner Niger Delta both flooding and raining
can trigger reproduction; however closer analysis shows that flooding triggers species that
breed in running waters, whereas raining triggers species that breed in calm waters.
10
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
Lunar cycle
At particular times of the year, migrations of certain species are known to be in tune with
the lunar cycle. The moon can then, directly or indirectly, act as a fish migration trigger.
Fish may sense the lunar cycle through gravitational force, visual cues or indirectly, via
tidal inferences. As Helfman et al. (1997) note, the new moon is the primary time when
elvers (juvenile eels) migrate upstream and maturing adult eels migrate downstream, but the
underlying mechanisms of this periodicity remain unknown.
Mekong River
Blache and Goosens (1954) provide the first records in Cambodia of migrations that are
triggered by the moon. These authors also showed that falling atmospheric pressure and
storms could disrupt the migration process.
According to Poulsen (2000), the lunar phase influences the migratory behaviour of many
Mekong fish species. This was demonstrated previously at Khone Falls (e.g. Warren et al.,
1999; Baird and Flaherty, 2001) and in the Tonle Sap (Lieng et al., 1995) where during
the migration period (November to March) the catches of the dai fishery (in particular
those of Henicorhynchus spp.1) vary strongly with lunar periodicity. Deap (1999) found
for this fishery a peak period of 46 days before full moon and a low period during the
rest of the month. The full moon is said to initiate the migration of certain fish such as
Morulius chrysophekadion, Paralaubuca typus, Belodontichthys dinema (Heng et al.,
2001) Tenualosa thibaudeaui, and Cirrhinus microlepis (Phallavan and Ngor, 2000). The
migration of Pangasianodon hypophthalmus, which is triggered mainly by variations of
water level, is said by Poulsen et al. (2004) to be also under lunar influence, since migration
of this fish occur normally just before, and during, the period of the full moon. At Khone
Falls, Baird et al. (2003) demonstrate that between January and March there is a very clear
correlation between overall catches of a fishery2 and the new moon. The bulk of the catch
(78.6%) at this time comprises small cyprinids, Henicorhynchus lobatus3 and Paralobuca
typus. Conversely, Baird and Flaherty (2001), working on a different fishery4, and thus on a
diversity of medium-sized cyprinids, conclude that no significant correlation can be found
between peak catches and lunar phases.
Chomchanta et al. (2000) highlight the crucial role of the new moon of the second lunar
cycle after the winter solstice (the time of the Chinese New Year) in the dry season
migrations. The spectacular intensity of the phenomenon was first reported by Roberts
(1993) and later confirmed by Roberts and Warren (1994) and Viravong et al. (1994). The
latter also note large movements of fish during the first lunar cycle after the winter solstice.
However, the importance of new moon of the second lunar cycle as a migration cue, if
striking at Khone Falls, was not witnessed further up or down the Mekong at Pakse or at
Stung Treng (Roberts and Warren, 1994).
1 Representing 40% of the yield according to Baran et al. (2001).
2 The `tone' or fence-filter trap fishery.
3 Recorded as Cirrhinus lobatus or under Henicorhynchus spp. in other studies.
4 4-9 cm meshed gillnet fishery.
11
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
Also, over the years, migratory activity at this time seems to be lowest when water
temperatures were highest, and vice versa (Viravong et al., 1994). In Cambodia, Bao et al.
(2001) observe that in the Tonle Sap Henicorhynchus siamensis migrates just before full
moon, but a little upstream the Mekong near Kratie it migrates during the full moon, and
further up at Sambor, it migrates immediately after the full moon. This probably reflects the
time needed for the species to swim upstream (Baird et al., 2003), and emphasises the need
to clearly identify the starting location when evaluating with migrations triggered by the
moon (and this certainly also applies to the other cues).
Among the species apparently triggered by the Chinese New Year new moon are
Labiobarbus leptocheilus, Botia modesta, Scaphognathops stejnegeri and S. bandanensis
(Roberts and Warren, 1994), although Singhanouvong et al. (1996a) conclude from their
observations that S. stejnegeri and S. bandanensis, unlike other species, migrate during
the period from the full moon to the last quarter. Warren et al. (1998), acknowledging the
contradicting conclusions of Welcomme (1985), Lieng et al. (1995) and themselves, suggest
that response to lunar influence might be site-specific, the phase when the moon is obscured
being preferred in confined environments where predation pressure is high.
Other tropical rivers
In the Lago do Rei, South America, the migratory movement of fishes out of the lake
coincides with the maximum flood-peak that occurs between the new moon and the full
moon (Fernandes and de Merona, 1988). However, no long-term trend in migration and lunar
cycle was detected in their study.
In Africa, Daget (1949, 1952, 1957) showed that the Tineni (Brycinus leuciscus1, Characid)
undertake massive migrations out of the floodplain at the end of the rainy season. These
slow and long migrations (1 to 1.5 km/h sometimes over 400 kilometres), even if they
are triggered by receding waters, are also driven strongly by the lunar phase, with three
migration pulses corresponding to the October to December moons. In fact, each time
the peak of migration occurs during the first half of the lunar month. Interestingly, Daget
observed that B. leuciscus congregate and migrate cohesively under the moonlight, and that
the shoals disaggregate at the end of each lunar month; unlike the moonlight, the sunlight
does not affect the fish migration. He also noted that a slight rise in water level will suffice
to bring migration to a halt. Supplementing Daget's results with their studies in Lake Chad,
Benech and Quensiere (1983) reach the general conclusion that the influence of the moon
is not predominant, that it plays only a secondary role after hydrology2, and that only a few
species are regularly and repeatedly influenced by the moon.
1 Formerly Alestes leuciscus.
2 This was confirmed in the inner Niger Delta by Benech et al. (1994, 1995), who conclude that the moon only influences
lateral migrations that were originally triggered by hydrological variations.
12
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
Other migration triggers
Apparition of insects
Poulsen (2000), found that fishers working along the upper stretch of Mekong River
believe the appearance of certain insects, in particular dragonflies and mayflies, is one of
the most important indications that fish are about to begin migrating. The small pangasid
Pangasius pleurotaenia, is a good example as it appears in high numbers when these insects
(dragonflies in particular) are abundant; on these occasions fish are observed appearing at the
surface and feeding on the insects.
Data held in FishBase and MFD, shows that the migration of Pangasius macronema and
Pangasius polyuranodon (Pangasidae) and Paralaubuca typus (Cyprinidae) correlates with
the apparition of insects. Upstream of Khone Falls the migration of Pangasius conchophilus
also coincides with blooms of insects (Chhuon, 2000).
Water colour and turbidity
A change in turbidity or in the water colour is mentioned as a migration cue in the Mekong
by several authors. It should be noted that sediment load is synonymous of turbidity but not
necessary of water colour, as the latter term can also include algal developments such as that
of green algae noticed annually in Khone Falls.
According to FishBase and MFD, nine species are known to be triggered by a change in
water colour; they are:
Cyprinidae: Bangana behri; Cyclocheilichthys enoplos; Labeo chrysophekadion; Mekongina
erythrospila; Paralaubuca typus
Pangasidae: Pangasianodon gigas; Pangasius bocourti; Pangasius polyuranodon
Others: Tenualosa thibaudeaui (Clupeidae)
However, the notion of change in turbidity or water colour is imprecise as it does not
discriminate between increasing sediment load at the beginning of the rainy season,
decreasing sediment load in the dry season and algal or planktonic development.
Temperature and photoperiod
In their extensive review of fish diversity, Helfman et al. (1997) note that seasonality among
tropical freshwater fishes is defined more by rainfall than by temperature, as seasonal
temperature fluctuations are not very strong. Pavlov et al. (1995) also note that there is
no relationship between migration of Amazon ichthyoplankton and water temperature.
According to McKeown (1984), what seems to be more important than the length of the
photoperiod is the change from light to dark.
In an observation that applies to most tropical river systems, Daget (1957) concludes that
migration responses to `sidereal and meteorological influences' in the Niger River are very
13
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
specific and that species belonging to the same genus can have very different responses.
This shows that a significant analysis of migration triggers must go beyond a collection of
anecdotal observations, and requires a numerical approach, as detailed for the Mekong River
in the following section.
Issues raised by the literature review
The literature review brought to light a number of associated issues that may be of
importance in the study of the Mekong fish migrations and for the sustainable management
of the fishery resources.
Importance of fish density as a complementary biological trigger
Helfman et al. (1997) stress that variability of migration cues can arise from secondary
parameters such as water quality, presence of predators or social interactions, which can
contribute to blur the effect of triggers. According to Singhanouvong et al. (1996b), Khone
Falls is a bottleneck where fish gather in the mainstream below the rapids before undertaking
the ascent of the falls. Singhanouvong et al. hypothesize that `the migratory response to both
absolute and changes in flow-volume also depends on the overall stage of the migration', the
fish undertaking their migration only if `there are sufficient numbers of fish waiting below
the fault line'. Baird and Flaherty (2001) support this theory, which also echoes Daget's work
in Africa.
Importance of spawning triggers in addition to migration triggers
In their review of Amazon catfishes and their migrations, Barthem and Goulding (1997)
provide details of large scale migrations, often over hundreds of kilometres, but do not
mention clear migration triggers. However, the first flood is mentioned as a spawning trigger
for some catfish species of the Paraguay Basin (spawning taking place one day after the first
heavy floods). `Most of the species depend absolutely on the cues associated with flooding,
for it is these cues that trigger reproduction' (Harvey and Carolsfeld, 2003).
The role of first strong rains as a recurrent trigger of spawning for African river fishes is
illustrated by Lowe-McConnell (1987), Ochumba and Manyala (1992) and Ezenwaji (1998).
Likewise, Fuentes and Espinach Ros (1998) show that rising water levels and temperature
act as spawning synchronizers in South America. In the inner Niger Delta, Benech and
Dansoko (1994), using flooding/production correlations, demonstrated that intense flooding
induces a higher breeding effort.
However, a clearer distinction between migration triggers and spawning triggers, if necessary
on the scientific ground, might not be of great use to river managers. Scott (1979) stresses
the diversity of the physiological processes and stages in fish reproduction that are sensitive
to external cues, and the multivariate nature of spawning triggers. According to him, `it
seems to be only the final stage of the reproductive cycle, spawning, which is associated with
flooding, and the questions remains of how such species regulate the earlier stages of their
cycle so that they are physiologically ready for spawning when the flood comes'.
14
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
Finally, the role of environmental triggers vis-à-vis reproduction highlights the need to
consider the role of these triggers on the reproduction of fishes that do not migrate. The
alteration of some environmental factors, for instance through hydropower development or
river management, may also influence reproduction process or success even among non-
migrant species. This issue is not explored in this current study.
Multiplicity of triggers
A number of tropical species migrate in floodplains to spawn, whereas others, like large
characids, migrate upriver to headwaters in anticipation of seasonal rains. In tropical rivers
throughout the world, predatory species often spawn earlier than their prey, thereby assuring
a food source for their offspring (Helfman et al., 1997). This diversity of requirements helps
to explain the number of migration triggers that come into play for different species and in
different time windows.
Singhanouvong et al. (1996b) were the first authors to provide a summary of the factors
likely to trigger migrations and to present a critical analysis of possible migration cues in
the Mekong Basin. According to them, water level, variations in water level, flow volume
and local rainfalls are important stimuli at Khone Falls, whereas water velocity, temperature
and turbidity are possible stimuli. The lunar phase and cloud cover play a role when in
conjunction with water level and flow volume, but pH, oxygen concentration, water hardness
and alkalinity have a negligible effect.
The bi-directional migration movements across Khone Falls (upstream and downstream)
`appear to be dependant on local hydrological conditions, [...] and triggered by channel
flow-volume, although water depth and current velocity might be the actual factors
involved'.
The diversity of triggers also reflects the interactions among environmental factors and the
fact that fish production cannot be summarized nor controlled by a single parameter. The
complexity highlighted in the various studies of migrant species also expresses a diversity
of physical and ecological processes occurring at different spatiotemporal scales, and
interactions among scales. For instance, Labbe and Fausch (2000) showed that for darter
(a small American perch), spring rains trigger reproduction but also increase groundwater
levels, which in turn connects stream reaches. This increased connectivity allows the darter
`metapopulation' to persist at landscape scale. As Poulsen (2000) puts it, the high fisheries
productivity of the Mekong Basin is not so much determined by a certain amount of water
containing a certain number of fish but rather by the annual rise and fall of the Mekong
waters. Fausch et al. (2002) conclude that, `to provide useful information for managers,
stream fish ecologists will need to embrace the complexity of these ecological systems at
multiple scales, not force simplicity upon them'.
15
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
16
4. Quantitative Analysis of Mekong Fish Migration
Triggers
The two main repositories of information about fishes, and Mekong fishes in particular,
are FishBase and the Mekong Fish Database (MFD, 2003). FishBase is based on a strict
academic approach and was used as a taxonomic reference for Mekong species; MFD
containing much more than FishBase about Mekong fishes (in particular unpublished
information) was used as the main reference (supplemented by FishBase) for information on
the ecology and migrations of species.
FishBase (web version, www.fishbase.org, June 2005) holds records on 768 species that have
been clearly identified and documented as living in the Mekong Basin1. These species belong
to 62 families.
Methodology
The analyses done and detailed in this section are based on the following approach:
· creating a matrix of life history parameters for all Mekong species from FishBase;
· adding bioecological information from MFD to the matrix;
· adding complementary information from FishBase--in particular about those species
on which there is information on migration triggers or cues;
· identifying migration triggers and cues in the enlarged matrix;
· coding migration triggers and cues;
· analysing the matrix--results of these analyses are presented below.
The FishBase team, on the author's request, created a specific module2 to generate a
matrix of all Mekong species and a number of life-history parameters of these species. The
quantitative information available in this matrix is summarised in Table 1. This table details
the life-history variables proposed by FishBase matrix, the number and proportion of species
in the table for which a value is available for a given variable, and ultimately whether this
variable was kept for further analyses. The variables selected are detailed in Table 1 below
Among the reasons for putting this information together are:
· the need for an overview of available knowledge on Mekong fish species;
1 MFD, with less selective criteria, identifies 924 species.
2 See: www.fishbase.org, Information by ecosystem / Species ecology matrix.
17
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
· the need, given the high diversity of species and of specific strategies, for a
quantitative approach to species ecology;
· the need to identify of guilds (= groups or clusters) of species that behave similarly
and will respond similarly to environmental modifications.
This approach was used by Winemiller (1989) who, working on Venezuelan floodplains,
quantitatively analyzed 10 life history traits from 71 species. He was able to recognise three
major life strategies and corresponding guilds of species, and could demonstrate that seven
life history traits correlated significantly to body length.
Table 1. Life history variables detailed for Mekong species
Variable
Abbreviation Meaning
Measured or
Number of
(unit)
calculated
records (0)
available and
% of 768
species
Maximum length
Lmax (cm)
Maximum length ever reported Measured
716; 93%
for the species in question
Life span
tmax (year)
Approximate maximum age
Calculated
762; 99%
that fish of a given population
(estimated
would reach
from Linf., K
and to.)
Age at first maturity
tm (year)
Average age at which fish of a Calculated
631; 82%
given population mature for the (estimated
first time
from Linf., K
and to.)
Length at maturity
Lm (cm)
Average length at which fish of Calculated
762; 99%
a given population mature for
(estimated
the first time
from Linf.)
Length for max. yield
Lopt (cm)
Length class with the highest
Calculated
762; 99%
biomass in an unfished
(estimated
population
from Linf.)
Trophic level
Rank of a species in a food
Calculated
390; 51%
web, calculated from food
items, weighted by the
contribution of the various food
items to the diet.
For each species in the life history matrix, information on migration was automatically
extracted from the MFD in MS Access format. For species listed in FishBase, but not present
in the MDF, a synonyms correspondence table was used to search for all possible synonyms,
and the relevant information was then extracted from the synonym species.
In the resulting expanded matrix, species were sorted according to the `migration' criteria
present in MDF. Information on migration was available for only 189 of 768 Mekong fish
species listed in the database1.
A multivariate approach of the available data was performed following the example of
Benech and Dansoko (1994). However, the nature of variables (both qualitative and
quantitative) requires sophisticated statistical methods and the limited number of species
1 In either fields `Migration Type' or `Migration' of the database.
18
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
in the matrix on which migration data was available, did not justify the use of complex
analyses or permit the author to draw strong quantitative conclusions. As a result, a simple
presentation of the main trends was preferred, and is discussed below.
Migration and triggers of the 768 Mekong fish species
Status of knowledge on migration of fish species
The table below summarises the information currently known about migration of fishes
basinwide:
Table 2. Information available on migration of Mekong species
Number of species
% of total species
Migrant
165
21
Non-migrant
24
3
No information
579
76
Thus:
· the migration status (whether the fish is migratory or non-migratory) is known for only
189 (24%) of the 768 Mekong fish species;
· there is no information on migration status of three-quarters of Mekong species;
· of the species whose migration status is known, 165 (87%) are migrant species and
24 (13%) are non-migrants.
Status of knowledge on migration triggers of fish species
Information was read, synthesized and coded for each of the 189 species for which
information on migration is available.
· The databases list five environmental factors that act as migration triggers:
variation in river discharge;
variation in water level;
first rainfalls after the dry season;
change in the turbidity or colour of the water;
apparition of insects, in particular dragonflies.
In principle, all these factors are somehow correlated, as the water level varies with
the discharge, high turbidity is correlated to rainfall (due to runoff) and thus to
19
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
high discharge, and insects appear in the air after the first rainfalls. The correlations
between these variables on the basis of their trigger effect will be analysed in a next
section.
· Thirty of the 165 migratory species are known to be sensitive to an environmental
migration trigger. Of these, 12 species are sensitive to more than one trigger.
These thirty species are:
Pangasiidae (shark catfishes)
Pangasianodon gigas, Pangasianodon hypophthalmus, Pangasius polyuranodon,
Pangasius macronema, Pangasius bocourti, Pangasius conchophilus, Pangasius
krempfi, Pangasius kunyit, Pangasius larnaudii, Pangasius pleurotaenia, Pangasius
sanitwongsei (11 species)
Cyprinidae (minnows or carps)
Paralaubuca typus, Cyclocheilichthys enoplos, Bangana behri, Barbonymus
gonionotus, Labeo chrysophekadion, Mekongina erythrospila, Cyprinus carpio
carpio, Macrochirichthys macrochirus, Parachela oxygastroides (9 species)
Siluridae (sheatfishes)
Hemisilurus mekongensis, Micronema bleekeri, Wallago leerii (3 species)
Bagridae (bagrid catfishes)
Hemibagrus filamentus
Clupeidae (herrings, shads, sardines, menhadens)
Tenualosa thibaudeaui
Cobitidae (loaches)
Botia modesta
Engraulidae (anchovies)
Lycothrissa crocodilus
Nandidae (Asian leaffishes)
Pristolepis fasciata
Notopteridae (featherbacks or knifefishes)
Chitala blanci
Osphronemidae (gouramies)
Osphronemus exodon
20
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
The summary information, extracted from FishBase and the Mekong Fish Database,
on the 30 species sensitive to migration triggers is presented in Annex 1.
Migration cues have been documented for 30 (18%) of the migratory species. Migration cues
are unknown for the remaining 135 species (82%) (Figure 2).
Migrant -- triggers known
(4%)
30
Migrant -- but no trigger (or no information about triggers)
(17%)
135
24
Non-migrant species
(3%)
579
No information about migration
(76%)
Figure 2. Migratory behaviour and sensitivities to triggers among 768 Mekong species
For the 76% of fish species for which migration patterns are not documented, there
are undoubtedly a number of migrant species, and in particular species sensitive to
environmental cues which trigger migration, but these have yet to be identified.
Classification of factors triggering fish migration
An analysis of the factors that trigger migrations shows (Figure 3) that 80% of species for
which migration cues are documented respond to a variation in water level. This variation
can be either when the water is rising (e.g. Pangasius krempfi) or when it is receding
(Pangasius polyuranodon in northern Cambodia), or in both cases (Pangasius larnaudii,
P. pleurotaenia, most white fishes migrating temporarily into floodplains).
Water level
a
tion
Water color
ing migr
Rainfall
igger
Discharge
t
ors tr
Insects
F
ac
0
10
20
30
40
50
60
70
80
Percentage of those fish species for which triggers are known
(the number of species is given in the bars)
Figure 3. Percentage of species sensitive to migration cues
that respond to a given triggering factor
21
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
The second and third most frequent triggering factors are a change in water colour and the
first rainfalls at the end of the dry season, both affecting nine (30%) of the species sensitive
to environmental cues. Discharge and the apparition of insects each count for three (10%)
species known to be sensitive to environmental cues.
As statistical analysis shows (see the section below--Correlation between environmental
variables) that water level and discharge are closely correlated parameters, and should
be considered as a single environmental cue. This means that 90% of species sensitive to
migration cues are sensitive to variations, or thresholds, in water level/discharge.
Fish families most sensitive to triggers
From a taxonomic viewpoint, the pangasiid catfishes stand out as the family most sensitive
to migration triggers. Eleven (58%) of the 19 species of the family that live in the Mekong
respond to environmental triggers. Silurids (another catfish family) follow next with three
species (9% of the species belonging to the family), then cyprinids with nine species,
representing 3% of the 285 species of this family found in the Mekong (Figure 4).
Thus among the 18% of migratory fish for which the triggers have been documented,
catfishes, with 15 species listed, are by far the group most sensitive to triggers, particularly
bearing in mind that many of these species are sensitive to several triggers at once (see
Annex 1 for details).
Seven other families
represented by one
species only
Pangasiidae 11 species
(out of 19)
Siluridae 3 species
(out of 32)
Cyprinidae 9 species
(out of 285)
Figure 4. Species sensitive to migration triggers, per family
Correlation between migration cues
Correlation between triggers of fish migrations
Of the thirty species sensitive to migration cues, twelve species are sensitive to at least two
triggering factors, and four species are known to respond to either three or four cues. The
fact that triggering factors (discharge, water level, etc) may be inter-correlated has been
22
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
highlighted above. In order to clarify the interactions between triggering factors, a principal
component analysis was performed on the table of triggers to calculate the correlation
coefficient between all factors (30 rows = species, 5 columns = triggers; Table 3).
Table 3. Correlation coefficient between migration triggers (from the Annex 1)
Discharge
Water level
Rainfall
Water colour
Insects
Discharge
1
Water level
-0.389
1
Rainfall
-0.218
-0.218
1
Water colour
-0.218
-0.218
0.524
1
Insects
0.259
-0.111
0.267
0.267
1
The correlation matrix shows that according to this data set based on qualitative ecological
records, none of the triggers correlates significantly with another. Thus according to these
data, variation in discharge, water level and water colour, the apparition of insects and early
rainfalls are not triggering factors systematically recorded together.
Correlation between environmental variables
As noted previously, changes in water levels, turbidity and the first rains anticipate the arrival
of the annual monsoon. Some fishers say that it is the combination of the three factors that
triggers migration (Poulsen, 2000). The possibility that a combination of different factors
triggers migrations has also been documented in South America. Sato and Godinho (2004),
working in the São Francisco River, note that fishes migrate mainly from October to January,
when the water levels tend to rise, temperatures are higher and the days are longer.
In order to assess the possible correlations between environmental variables mentioned in
the previous sections, we combined hydrological data from the MRC with environmental
data from the Department of Hydrology in Lao PDR (Ministry of Water Resources and
Meteorology). Data from Pakse (the gauge station closest to the Khone Falls) over the
period from July 1995 to June 1998 (n = 36 months) was used. The matrix of correlation
coefficients is given in Table 4.
Table 4. Correlations between environmental parameters in Pakse
Temperature
Rainfall
Water level
Discharge
Temperature
1
Rainfall
0.13
1
Water level
0
0.25
1
Discharge
0
0.26
0.99
1
This analysis, although limited in the time covered, shows clearly that in Pakse:
· discharge correlates strongly to water level;
· other environmental parameters do not correlate.
23
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
Although data on total suspended solids was available, correlations between turbidity and
other environmental variables has not been calculated here because of a bias: the relationship
between discharge and water quality varies between the rising and falling stages of the flood.
The phase of the moon, which is also available in data set, does not, of course, correlate with
any of these environmental factors.
These results show that in further analyses of environmental cues, water level and discharge
should be considered as a single trigger, and other parameters can be considered as
independent.
Quantitative relationships between discharge and migrations
Baran et al. (2005) present the most detailed study of the relationship between triggers and
discharge in the Mekong Basin. This study, which was based on six years of monitoring data
recorded from a multi-gear fishery situated just below Khone Falls in Southern Lao PDR
(Figure 1), reviews the response of fish species to hydrology. The study combined daily
fishery records complied by I.G. Baird with MRC daily hydrological records collected at
Pakse, located about 130 km upstream of Khone Falls.
A general description of the migration patterns in Khone Falls is given below, then the results
of Baran et al. (2005) are reviewed and deepened.
Overview of migration patterns at Khone Falls
Baird (2001) provides a precise description of the succession of migrant fishes below Khone
Falls. The following list is a summary of his observations:
· The fishing year begins at the end of the monsoon season, in late September or
October. Villagers then target short distance migrators (`black fishes') that move from
seasonal streams and wetlands back to perennial water bodies.
· By the end of October, fish begin migrating up the Mekong River from Cambodia,
starting with medium and large-sized cyprinids.
· Between November and early January, three spawning cyprinids are abundant in
catches, in particular Probarbus jullieni (65% of the catch of large-meshed gillnet
fishery). These species are supplemented by about 30 other species of large fishes,
mostly cyprinids.
· Around mid-December, a wave of small cyprinids arrives, starting with
Henicorhynchus spp.
· They are followed by a wave of medium-sized migrating cyprinids, comprising about
100 species, whose fishing season lasts from December to January.
· Early February important schools of small cyprinids begin arriving at Khone
Falls from Cambodia. Their movements are closely associated with lunar cycles.
24
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
Henicorhynchus spp. and Paralaubuca typus are the two most abundant taxa, making
up to 80% of the catch. The migration generally ends around March.
· In April, schools of large sized Cirrhinus microlepis (cyprinds) migrate up-river from
Cambodia, at the height of the dry season.
· By the end of April, still during the dry season, the first schools of small migratory
catfishes arrive from Cambodia.
· By May, when the monsoon rains begin to fall and the Mekong River rises
dramatically, many species of medium to large pangasid catfishes and other large
cyprinids arrive from Cambodia. This is a time of intensive fishing targeting in
particular large pangasiids.
· Between June and July the catch is made of small and medium-sized cyprinids that
migrate downstream from Lao PDR to Cambodia.
· By July, the water levels are high and fishing becomes difficult. It is thus difficult
to determine possible migratory patterns at this season, but it is believed that most
species then migrate downstream.
These patterns are illustrated in Figure 5.
The combination of fishery catch data with hydrological records by Baran et al. (2005)
shows that:
· the highest species diversity in catches correspond to the lowest water levels;
· as much as 96% of the Khone fishery yield is made of fish taxa sensitive to discharge.
Representativeness of the Khone Falls studies
The fact that almost the entire catch, in terms of biomass, is made of fish taxa sensitive
to discharge highlights the dramatic impact that flow modifications may have on the fish
harvest at Khone Falls and possibly in the whole Lower Mekong Basin. However, one might
question the representativeness of conclusions drawn from this study in the Khone Falls, and
the validity of inferring from this area to the whole LMB.
Representativeness of sampling
The fact that the overall catch results from 32 distinct fishing methods targeting all species
and operated during six years at all hydrological levels ensures an extensive sampling (of
666,000 individual fishes), much superior to any `scientific' sampling that would necessarily
be more limited in time and in scope.
This sampling however is driven by fishers, not by scientists, which may create a bias
towards valuable commercial species. In fact, fishers deploy an exceptional diversity of gears
and strategies (detailed in particular in Roberts and Baird, 1995; Claridge et al., 1997) to
maximize their yield: they catch all species and all sizes, fish year round and harvest resident
25
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
S
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Figure 5. Major migration patters just below Khone Falls (figure based on Baird, 2001)
26
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
fishes as well as migrant ones (although migrants form the bulk of the catch). Thus the bias
in sampling is minimal, certainly not higher than that of a more limited scientific sampling
using only a few gears.
However, it should be acknowledged that in this particular stretch of the river system, the
share of migrant fishes could be slightly overestimated when picturing the local river fish
community, as not all species force their way through this natural bottleneck where they get
caught.
Spatial representativeness
The results detailed in the above study and below are specific to Khone Falls, i.e. of the
middle part of the lower Mekong River. The corresponding fish community includes a
number of species that are found upstream but not far downstream, and this community is
significantly different from that found, for instance, in the Delta.
However, the species that make the bulk of the catch are the same as in Cambodia as
they migrate between both countries (see MRC 2001 for details). The top-ten species in
the Khone Falls fisheries are detailed in Table 5; a comparison with the top-ten catches
in Cambodian fisheries (van Zalinge et al., 2000) shows that the four taxa common to
both fisheries (namely Henicorhynchus spp., Paralaubuca typus, Pangasius spp. and
Cyclocheilichthys enoplos) constitute 36% of the total catch in Cambodia, i.e. about 245,000
tonnes in this country alone, according to van Zalinge et al. (2004).
Thus, the conclusions drawn from the Khone Falls study largely apply to Cambodia. If
Lao PDR contributes only about 7% of the Mekong fish harvest, Cambodia contributes,
with 682,150 tonnes, one fourth of this total harvest. Therefore, the conclusions regarding
the relationship between discharge and migrations drawn from the Khone Falls area, i) are
significant to a large share of the catch of the whole Mekong, and ii) apply to more than two
hundred thousand tonnes of fish contributing to food security basinwide.
Catches, species richness and discharge: global patterns
We propose below a detailed analysis of the relationship between daily catches in Khone
Falls and corresponding discharge levels. All gears have been lumped together (the reasons
why this is the least dissatisfying option are detailed in Baran et al. 2005, chapters 4 and 6),
species have been identified, biomasses expressed in kilograms, and discharge levels (cubic
metres per second, i.e. cumecs) have been rounded to the closest thousand. Figure 6 shows
the response of the Khone Falls fish community, in terms of biomass and species richness, to
the various discharge levels.
27
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
33000
32000
31000
30000
29000
28000
27000
26000
25000
24000
23000
22000
21000
20000
19000
18000
17000
16000
15000
14000
13000
12000
11000
10000
9000
8000
7000
6000
5000
4000
3000
2000
1000
100 90 80 70 60 50 40 30 20 10 0
0
10
20 30 40
50
60 70
80 90 100
% of total biomass harvested
Discharge
% of total number of dominant taxa
(cumecs)
Figure 6. Percentage of the total biomass and of the dominant taxa harvested at each discharge level.
(Out of 201 fish species recorded in Khone Falls, 110 taxa are defined as dominant.)
This figure shows that:
· the discharge levels that correspond to the most diverse catch are the low discharge
levels, in particular between 2000 and 8000 cumecs;
· very low discharges, between 2000 and 4000 cumecs, correspond to the maximal
diversity in catches, with more than 90% of all dominant taxa recorded caught at these
discharge levels;
· almost ALL the biomass (precisely 95.7% of it) is caught between 2000 and 8000
cumecs.
These results highlight the extreme importance of low water-levels and discharges in the
richness and production of the Khone Falls fisheries.
28
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
Catches and discharge: detailed patterns
In Figure 7, the global patterns outlined above are detailed for each of the 110 taxa dominant
in catches.
a
r
v
e
s
t
e
d
h
a
s
s
m
i
o
B
110 dominant taxa
0
1000
5000
10000
15000
20000
25000
30000
35000
Discharge (cumecs)
Figure 7. Catch of 110 dominant taxa in Khone Falls, in relation to discharge levels
This figure, intentionally simplified to illustrate major patterns, shows that most catches are
realized at low discharge levels (between 2000 and 8000 cumecs), with scattered but very
minor catches are higher rates, and that the bulk of this catch is actually due to a few species
or taxa. There results also show how sharp the migration peaks of these dominant species can
be.
The catch of the top-ten species as a function of discharge is detailed in Table 5.
Table 5. Top-ten species or taxa in Khone Falls catches, and percentage of the total catch of this
species/taxon for a given discharge range
Discharge ranges (cumecs) and corresponding % of the specific catch
Species or taxon
% of the
1000-
6000-
11 000-
16 000-
21 000-
26 000-
31 000-
overall catch
5000
10 000
15 000
20 000
25 000
30 000
35 000
Henicorhynchus spp.
20.0
94
5
1
0
0
0
0
Pangasius krempfi
14.4
76
23
1
1
0
0
0
Pangasius conchophilus
11.8
70
29
1
0
0
0
0
Paralaubuca typus
11.7
100
0
0
0
0
0
0
Pangasius macronema
8.1
100
0
0
0
0
0
0
Probarbus jullieni
4.5
93
7
0
0
0
0
0
Cosmochilus harmandi
4.5
83
15
2
1
0
0
0
Scaphognathops bandanensis
3.5
99
1
0
0
0
0
0
Labiobarbus leptocheilus
1.8
95
4
1
0
0
0
0
Botia modesta
1.5
100
0
0
0
0
0
0
29
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
This table, which complements Figure 7, confirms how the bulk of the catch is correlated to
low discharge levels, and shows that this relationship is due to a handful of species only.
Among the top-ten species listed in Table 5, five (Pangasius krempfi, Pangasius
conchophilus, Paralaubuca typus, Pangasius macronema and Botia modesta) have been
identified earlier in this review as triggered by thresholds or changes in discharge, water level
or current. These five species alone represent 47% of the total annual catch at Khone Falls.
Response of 47 fish taxa to discharge
Baran et al. (2005) give individual relationships between abundance and discharge for 53
taxa. Data being gathered on a daily basis over six years, abundance is expressed in terms of
biomass of a taxon in daily catches of the overall multi-gear fishery, and discharge is taken
from gauge records measured at Pakse, located about 130 km upstream of Khone Falls.
Out of the 53 taxa studied, 6 were discarded as being rare and 47 have been kept as
`dominant taxa', for which at least 50 individuals were caught per year. Out of these 47
dominant taxa, the sensitivity to discharge is nil or unclear for 14 of them: Hemipimelodus
borneensis, Hemisilurus mekongensis, Kryptopterus spp., Lobocheilos melanotaenia,
Opsarius spp., Osteochilus spp., Pangasius pleurotaenia, Pangasius polyuranodon,
Pristolepis fasciata, Pseudomystus siamensis and Rasbora spp. The sensitivity to discharge
can be thus quantified for 36 remaining fish taxa, as detailed in Annex 2 and illustrated in
Figure 8. These results show that four groups of species can be identified:
· very sensitive species caught during an extremely narrow discharge range (e.g.
Paralaubuca typus);
· sensitive species whose peak abundance corresponds to a narrow discharge range
(e.g. Pangasius macronema), but that can also be caught, in lesser abundance, at other
discharge values;
· non-sensitive species, whose catches do not exhibit a peak in relation to discharge;
· species whose response or lack or response vis-à-vis discharge cannot be
characterised.
These results also show that low discharge levels (between 1000 and 5000 cumecs) are
important to several species, and correspond to the peak in the catch of these species.
30
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
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Figure 8. Discharge and fish migration at Khone Falls. Lines represent the distribution range of a
taxon in catches as a function of discharge, and yellow boxes highlight peaks within the
distribution range
31
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
32
5. Conclusions
Migration and triggers
The first point highlighted by the literature review is that relatively few studies focus
specifically on the migration of tropical freshwater species. In the 40 publications on this
topic the Mekong figures prominently as a river where fish migrations play an essential role
in the ecology of the system and sustainability of the fisheries.
Fish migration triggers have only been addressed in a handful of publications worldwide, and
the very dominant share of Mekong-related publications among those dealing with this topic
confirm the special importance of migration triggers in the Mekong Basin.
However, this study also shows that given the rich biodiversity of the Mekong River (768
fish species documented scientifically), knowledge of migration patterns is available for only
24% of the fish species. Lack of information about the possible migratory behaviour of three-
quarters of Mekong species calls for an increased effort in bioecological studies in order to
better assess the behaviour, requirements and response to environmental changes of the large
majority of Mekong fish species.
The literature review identified five major migration triggers in tropical rivers worldwide,
as well as in the Mekong; they are i) discharge, water level and current; ii) rainfalls at the
end of the dry season; iii) changes in water colour and turbidity; and iv) the apparition of
insects. The fifth possible trigger, the moon, is also often considered important, but its role
(in particular that of a given lunar phase) remains unclear.
Of the 165 Mekong fish species known to migrate, migration cues have been documented for
30 species; i.e. for 18% of them. Migration cues are unknown for the remaining 82% of these
migrant species.
Water-level is the environmental parameter most often cited in the literature as a migration
trigger. Literature and our data show a close correlation between this parameter and
discharge and water current, and that they are best considered as a single trigger.
In the Mekong Basin, 26 species are known to be triggered by thresholds or changes in
discharge, water level or current.
The first rainfalls marking the end of the dry season also trigger breeding and reproductive
migration of fishes in the tropics, and of nine species in the Mekong Basin.
The migration of nine Mekong fish species is known to be triggered by a change in turbidity
or in the water colour. The latter terms include increasing sediment load, decreasing sediment
load and algal or planktonic development.
33
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
Finally, the migration of three Mekong fish species is triggered by the apparition of insects,
in particular at the beginning of the rainy season.
Ninety per cent of Mekong fish species for which migration cues are documented respond
to a variation in water level or in discharge. The second and third most frequent triggering
factors are a change in water colour and the first rainfalls at the end of the dry season, both
affecting 30% of species sensitive to migration cues. The apparition of insects is a trigger for
10% of species known to be sensitive to environmental cues. As mentioned above, the moon
is widely acknowledged as playing a role in the migration of several Mekong species, but is
it unclear whish phase of the moon is a trigger, as this seems to vary depending upon species
and locations. In the Khone Falls area, spectacular migrations occur at the new moon of the
second lunar cycle after the winter solstice, the time of the Chinese New Year.
Data analysed showed that the pangasiid family (catfishes) is the most sensitive to migration
triggers, that affect 58% of the families 19 species. The other sensitive families are silurids
(another catfish family, with 3 species, i.e. 9% of its species) and cyprinids (9 species; i.e.
3% of the family). Thus among documented species, catfishes, with 15 species, are by far the
group most sensitive to migration triggers. Catfishes are highly appreciated on markets, as
attested by their market price that is among the top three highest prices per kilogram (US$2
to 2.16/kg in Cambodia according to Rab et al., 2005). Catfishes also play a major role in the
regional aquaculture sector, the fingerlings being caught in the wild to be raised in cages1. So
beyond capture fisheries catches, a modification of triggers and of the reproductive success
of catfishes might result in diminished supply for the whole aquaculture sector in Cambodia
and southern Viet Nam.
Among the ten dominant taxa in Cambodia listed by van Zalinge et al. (2000), four are
sensitive to migration cues: Cyclocheilichthys enoplos, Pangasius spp., Barbonymus
gonionotus and Paralaubuca typus. They represent 18% of the total catch and 14% of the
commercial value respectively, not to mention Henicorhynchus sp. (21 additional per cent of
the catch) that is sensitive to water current and lunar phase without being listed in this review
as a species whose migration is clearly triggered.
Discharge, catch and diversity
The current study, based on Khone Falls fisheries and hydrological data whose
representativeness has been demonstrated, shows clearly that 96% of the total fish biomass
harvested year round in the Khone Falls is harvested between 2000 and 8000 cumecs
(discharge measured in Pakse). The most `productive' discharge levels are 2000 and
3000 cumecs, as they total more than 60% of the annual yield.
This dependence of catch on low, dry season, discharge levels is due, i) to the multiple fish
migration waves that occur during low water levels (see Figure 5); ii) to the dominance of
1 In Cambodia, the aquaculture production of species whose cycle is mastered represents less than 5.5% of the total freshwater
fish production. Ninety four per cent of the fish production thus originates from capture fisheries and from wild fingerlings
including catfishes- grown in cages.
34
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
a few fish taxa in catches, most of these taxa being sensitive or very sensitive to discharge
levels (see Figure 8), and iii) to the catchability of fish at these discharge levels.
The results also show that the diversity in catches is also higher at low discharge levels
(in particular between 2000 and 8000 cumecs) and decreases progressively as discharge
increases. However in tropical fish communities, the biodiversity is made of a small number
of very dominant species and a long tail of rare species. Thus the impact of dry season flows
alterations will be much more dramatic for fishers and food security than an ecological
analysis encompassing all species might tell.
The crucial importance of the dry season low discharge levels for the ecology of dominant
fish species and, in particular, for the overall fishery harvest and food security should be
strongly emphasized. This importance makes it a priority area of research to better inform
development options. The water allocation rules being developed by the MRC and the
Mekong riparian countries should also integrate the information regarding fisheries and its
dependence upon low discharge levels.
Consequences of flow modifications on fish
A recurrent question is that of the consequences of flow modifications, due to damming or
water abstraction, on fish migrations and ultimately on fish production. Several conclusions
can be drawn from the above results.
Dry season flow modifications
Dry season flow modifications will be those having the strongest impact on fish ecology and
catches.
· Increased dry season flows (possibly due to water released by dams)
The biological implications of increased dry season flows would be that dry
season migration thresholds or cues are never reached, thus possibly inhibiting
the migration of species sensitive to these low flows. These species and their level
of sensitiveness are detailed in Figure 8. As most migrations occurring in the dry
season have a reproductive purpose, the biological impact of increased dry season
flows might be on reproduction success.
The fisheries implications of increased dry season flows would be that most
gears designed to catch species migrating at low water levels cannot be operated
any longer or are less efficient at higher water levels, hence a loss of catch and
productivity even in presence of fish.
· Decreased dry season flows
A reduction of dry season flows, for instance due to water abstraction for irrigation,
would have dramatic consequences if the discharge in Pakse goes below 2000 cumecs,
35
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
as shown by Figures 6, 7 and 8 where no catches are recorded for such low discharge
levels.
· Delayed flood onset
Delayed flood onset might have a significant negative impact on the fish abundance
as this factor is playing a strong trigger role in the migration of a majority of
commercially important species. Several reports based on fishers' observations have
also documented a positive relationship between an early flood and a productive
fishing year, as early floods generally correspond to long flooding periods that allow a
longer growth period for fish and thus a bigger annual fish stock. Welcomme and Halls
(2003) also highlights the fact that in a system where the upstream movement of adult
compensates the downstream drift of larvae, a natural or artificial variation of the flow
regime is likely to result in a very different distribution of fry and thus in a fluctuating
production in downstream regions, as illustrated by the Prochilodus genus in the
Parana River (South America).
Impact on fish migration and production
The literature review did not identify any study having focussed specifically on the
impact of dams on migration triggers. However the broader issue of the impact
of dams on fish migration and production has been detailed in several extensive
reviews (including Jackson and Marmulla, 2001, Welcomme and Hall 2003 or
Arthington et al. 2004). Among the most documented assessments of the impact of
dams on floodplains rivers are those from the Niger Basin (Laë, 1994; 1995; 1997;
Laë et al., 2003). These results show a close correlation, over 25 years, between
the decline of catches and that of annual average discharges (the latter being due
to a combination of droughts and dam construction). In the Niger Central Delta, a
reduction of 75% of the area of floodplain resulted in a 50% loss of the fish harvest,
the two dams of the system contributing 10% of these losses (Laë, 1992). However
these results also highlighted the fact that a declining natural fish production can be
blurred by an increased concentration of fishes (hence a higher catchability) and an
increased fishing efficiency due to new gears and fishing methods.
The increased production of reservoir fish following the creation of a dam is
often cited as a natural compensation for the loss of capture fish. However out
of 160 families living in freshwater, only 17 are fully lacustrine or able to live in
lakes at all stages of their life cycle (Fernando and Holcik, 1982), most species
having to return to free-flowing rivers to breed. An analysis of the information
available in the Mekong Fish Database shows that in the Mekong Basin, nine
species only are known to breed in reservoirs such as those that could be created
behind dams: Cirrhinus jullieni; Cirrhinus molitorella; Clupeichthys aesarnensis;
Cyclocheilichthys apogon; Hampala macrolepidota; Oreochromis niloticus
niloticus; Oxyeleotris marmorata; Pristolepis fasciata and Puntioplites proctozysron.
36
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
Rainy season flow modifications
The impact of rainy season flow modifications would be minor compared to dry season flow
changes.
· Decreased flood peaks in the rainy season might slightly improve the catchability of
fish but not necessarily in a significant way, the harvest being very limited anyway in
that season (the bulk of the catch is made in the dry season).
· Delayed flood peaks might not have a major impact on the catch and food security,
as flooding is mainly a period of growth for fish, without noticeable migration nor
breeding patterns.
Methodological points
Most of the literature in the above review uses information supplied by fishers. This fact has
two consequences.
The first consequence is that information is biased towards commercially dominant species,
i.e. the relatively small fraction of species numerically abundant that are typical of tropical
fish assemblages (Gaston, 1994). Given this bias, only a fraction of the full biodiversity
spectrum is considered (i.e. large knowledge gap from a conservation perspective).
Conversely the knowledge gathered is the one that matters most to the livelihood and food
security of riparian communities.
The second consequence is that the documented migration triggers and cues are mainly those
fishers can, and choose, to record. This approach based almost exclusively on traditional
ecological knowledge ignores the variables that are not visually perceptible, and results in
the total absence of information on some variables, for instance chemical, that are known to
cue migrations in temperate rivers.
Thus changes to water chemistry and sediment load, among other factors that might result
from development activities, may also be important but remain totally undocumented.
The literature review has also highlighted three points to be considered in future studies:
· The importance of fish density as a complementary biological trigger of migrations,
fish undertaking their migration only if there is a sufficient density of individuals in a
given place.
· The importance of spawning triggers in addition to migration triggers, as reproduction
is a phenomenon that itself requires appropriate coordination among individuals and
thus coordinating triggers. These reproduction triggers are crucial to the sustainability
of a fish population and also concern non-migrant species.
· The multiplicity of triggers, that play a role at different times in different physiological
mechanisms; they result from a sum of ecological processes and cannot be reduced to
a single parameter such as discharge at time t.
37
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
Despite a remaining share of uncertainty this study highlights, with quantitative arguments,
the possible impact of water level/discharge alterations on fish migrations in a river
system that generates 2.6 million tonnes of capture fish per year (van Zalinge et al., 2004)
and produces four times as much fish per square kilometer than the North Sea (Jensen,
2001), Cambodia alone harvests more freshwater fish than the whole Northern America.
Subsequently, basin development plans should definitely include a strong component
focusing on the impact of flow modifications on inland fisheries, so that trade-offs are
adequately assessed and that sound development options are chosen, integrating the fact
that millions of basin dwellers currently depend on Mekong aquatic resources for their food
security and livelihood.
38
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Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
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Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
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48
Annex 1. Life history, migration and triggers of Mekong
fish species
49
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
Life history parameters, migration pattern and migration triggers of the 30 Mekong fish
species known to have environmental migration cues (after FishBase 2005 and Mekong Fish
Database 2003)
Y
es
Y
es
Y
es
Insects
ater
Y
es
Y
es
Y
es
Y
es
Y
es
Y
es
Y
es
Y
es
W
colour
T
rigger
Y
es
Y
es
Y
es
Y
es
Y
es
Y
es
Y
es
Y
es
Rainfall
Migration
ater
Y
es
Y
es
Y
es
Y
es
Y
es
Y
es
Y
es
Y
es
Y
es
Y
es
Y
es
W
level
ge
Y
es
Y
es
Y
es
Dischar
season
Migration
Dry and
Rainy
Dry and
Rainy
Dry and
Rainy
Dry and
Rainy
Dry and
Rainy
Dry and
Rainy
Dry and
Rainy
Dry and
Rainy
Dry and
Rainy
Dry and
Rainy
Dry and
Rainy
Dry and
Rainy
Dry and
Rainy
Dry and
Rainy
Dry and
Rainy
2
2
2
2
T
rophic
level
3.3 s.e.
0.43
3.2 s.e.
0.43
2.4 s.e.
0.13
3.3 s.e.
0.44
4.5 s.e.
0.80
2.3 s.e.
0.17
3.2 s.e.
0.40
3.4 s.e.
0.43
3.7 s.e.
0.56
3.0 s.e.
0.32
Length
yield
(cm)
52.9
1
1.4
48.8
19.4
39.4
26.3
52.9
59.7
29.3
39.4
19.4
16.1
80.2
98.5
for max.
205.1
(cm)
44
1
1.8
41.1
18.5
34.1
24.1
44
48.8
26.5
34.1
141.4
18.5
15.7
62.9
75.2
Length at
maturity
Age
0
0
0
0
0
5
at first
2.2
6.4
1.8
4.7
3.4
6.2
4.5
3.5
5.3
maturity
(year)
Life
8
span
19
24
27
(year)
31.6
28.7
7.3
20.4
14.2
2.6
28.7
3.4
7.3
2.6
13.5
Max.
80
18
74
30
60
80
90
45
60
30
25
length
(cm)
40.5
300
120
120
Common name
Shark catfishes
Minnows or carps
Minnows or carps
Herrings, shads,
sardines, menhadens
Minnows or carps
Minnows or carps
Sheatfishes
Minnows or carps
Minnows or carps
Sheatfishes
Shark catfishes
Shark catfishes
Loaches
Featherbacks or
knifefishes
Minnows or carps
Family
Pangasiidae
Cyprinidae
Cyprinidae
Clupeidae
Cyprinidae
Cyprinidae
Siluridae
Cyprinidae
Cyprinidae
Siluridae
Pangasiidae
Pangasiidae
Cobitidae
Notopteridae
Cyprinidae
ospila
onema
onema bleekeri
Scientific Name
Pangasius polyuranodon
Paralaubuca typus
Cyclocheilichthys enoplos
T
enualosa thibaudeaui
Bangana behri
Barbonymus gonionotus
Hemisilurus mekongensis
Labeo chrysophekadion
Mekongina erythr
Micr
Pangasianodon gigas
Pangasius macr
Botia modesta
Chitala blanci
Cyprinus carpio carpio
50
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
Insects
ater
Y
es
W
colour
T
rigger
Y
es
Rainfall
Migration
ater
Y
es
Y
es
Y
es
Y
es
Y
es
Y
es
Y
es
Y
es
Y
es
Y
es
Y
es
Y
es
Y
es
W
level
ge
Dischar
season
Rainy
Migration
Dry and
Rainy
Dry and
Rainy
Dry and
Rainy
Dry and
Rainy
Dry and
Rainy
Dry and
Rainy
Dry and
Rainy
Dry and
Rainy
Dry and
Rainy
Dry and
Rainy
Dry and
Rainy
Dry and
Rainy
Dry and
Rainy
Dry and
Rainy
2
T
rophic
level
3.7 s.e.
0.59
3.7 s.e.
0.57
2.7 s.e.
0.29
3.2 s.e.
0.47
2.7 s.e.
0.34
3.1 s.e.
0.46
2.8 s.e.
0.38
3.3 s.e.
0.52
2.4 s.e.
0.26
4 s.e.
0.66
3.3 s.e.
0.43
3.1 s.e.
0.45
4.1 s.e.
0.69
87
87
Length
yield
(cm)
32.7
19.4
66.5
39.4
80.2
80.2
80.2
46.2
22.7
12.8
12.8
for max.
205.1
121.5
(cm)
29
18.5
53.5
34.1
62.9
62.9
67.5
62.9
39.2
67.5
21.2
141.4
12.9
12.9
90
Length at
maturity
Age
0
0
0
0
0
0
0
0
0
0
0
at first
2.3
1.5
6.8
2.4
maturity
(year)
Life
span
32
24
(year)
9.9
5.9
10.5
17.5
10.8
2.6
3.4
2.7
2.4
7.3
8.8
4.4
4.4
Max.
50
30
60
35
20
20
length
(cm)
100
120
120
130
120
70.2
130
300
180
Common name
Bagrid catfishes
Anchovies
Minnows or carps
Gouramies
Shark catfishes
Shark catfishes
Shark catfishes
Shark catfishes
Shark catfishes
Shark catfishes
Shark catfishes
Shark catfishes
Minnows or carps
Asian leaffishes
Sheatfishes
Family
Bagridae
Engraulidae
Cyprinidae
Osphronemidae
Pangasiidae
Pangasiidae
Pangasiidae
Pangasiidae
Pangasiidae
Pangasiidae
Pangasiidae
Pangasiidae
Cyprinidae
Nandidae
Siluridae
oides
otaenia
ocodilus
empfi
ochirichthys
ochirus
onemus exodon
allago leerii
Scientific Name
Hemibagrus filamentus
L
ycothrissa cr
Macr
macr
Osphr
Pangasius bocourti
Pangasius conchophilus
Pangasianodon
hypophthalmus
Pangasius kr
Pangasius kunyit
Pangasius larnaudii
Pangasius pleur
Pangasius sanitwongsei
Parachela oxygastr
Pristolepis fasciata
W
51
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
52
Annex 2. Response of fish taxa to discharge at Khone
Falls
53
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
Response of fish taxa to discharge at the Khone Falls (modified after Baran et al., 2005)
Taxon in the Khone Falls
Family
Abundance in relation to
Sensitivity to
fisheries database
discharge
discharge
Parambassis wolffi/spp.
Ambassidae
Peak between 1,500 and 4,000
High
cumecs
Bagrichthys spp.
Bagridae
Sharp and dense peak between 1,500 High
and 3,000 cumecs
Tenualosa thibaudeaui
Clupeidae
Species caught between 2,000 and
High
5,000 cumecs
Botia modesta
Cobitidae
Very sharp and dense peak between High
2,000 and 3,000 cumecs
Botia spp
Cobitidae
Very sharp and dense peak between High
2,000 and 3,000 cumecs
Bangana behri
Cyprinidae
Peak between 2,000 and 4,500
High
cumecs
Barbodes altus
Cyprinidae
Species caught between 2,000 and
High
8,000 cumecs, with a peak between
2,000 and 3,000 cumecs
Cirrhinus microlepis
Cyprinidae
Species caught between 1,500 and
High
7,000 cumecs, concentrated between
2,000 and 4,000 cumecs
Crossocheilus reticulatus
Cyprinidae
Species caught between 1,500 and
High
19,000 cumecs, with a sharp peak
between 1,500 and 2,500 cumecs
Crossocheilus siamensis
Cyprinidae
Species caught between 1,500 and
High
10,000 cumecs, with a sharp and
intense peak between 1,500 and
2,500 cumecs (but 10 data points
only)
Cyclocheilichthys enoplos
Cyprinidae
Species caught between 1,500 and
High
20,000 cumecs, with a sharp peak
around 3,000 cumecs
Garra fasciacauda
Cyprinidae
Sharp peak between 2,000 and 3,000 High
cumecs, but few data points
Henicorhynchus spp.
Cyprinidae
Pattern species-specific
High
Hypsibarbus malcolmi
Cyprinidae
Species caught between 1,500 and
High
11,000 cumecs, with a peak between
3,000 and 5,000 cumecs
Labeo erythropterus
Cyprinidae
Species caught between 1,500
High
and 33,000 cumecs, concentrated
between 2,000 and 4,500 cumecs
Labiobarbus leptocheilus
Cyprinidae
Species caught between 1,500 and
High
21,000 cumecs, with a concentration
between 1,500 and 3,000 cumecs
54
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
Taxon in the Khone Falls
Family
Abundance in relation to
Sensitivity to
fisheries database
discharge
discharge
Mekongina erythrospila
Cyprinidae
Very sharp and dense peak between High
1,500 and 3,500 cumecs, in
particular around 2,000 cumecs
Mystacoleucus spp.
Cyprinidae
Peak between 2,000 and 5,000
High
cumecs
Paralaubuca typus
Cyprinidae
Species caught between 1,500 and
High
5,500 cumecs, with a sharp and
intense peak around 2,000 cumecs
Puntioplites falcifer
Cyprinidae
Species caught between 1,500 and
High
7,000 cumecs, with a concentration
between 2,000 and 5,000 cumecs
Scaphognathops bandanensis Cyprinidae
Species caught between 1,500 and
High
21,000 cumecs, with a peak around
3,500 cumecs
Thynnichthys thynnoides
Cyprinidae
Species caught between 2,500 and
High
12,500 cumecs, with a sharp peak
between 2,000 and 3,000 cumecs
Gyrinocheilus pennocki
Sharp and dense peak between 1,500 High
and 3,500 cumecs
Pangasius krempfi
Pangasiidae
Species caught between 1,500
High
and 20,000 cumecs, concentrated
between 2,000 and 9,500 cumecs
Pangasius macronema
Pangasiidae
Species caught between 1,500 and
High
26,000 cumecs, with a sharp and
intense peak between 3,000 and
4,000 cumecs
Laides hexanema/spp.
Schilbeidae
Species caught between 2,000 and
High
5,500 cumecs
Hemibagrus nemurus
Bagridae
No clear pattern but concentration
Medium
around 1,500 - 9,000 cumecs
Cosmochilus harmandi
Cyprinidae
Species caught between 1,500
Medium
and 13,000 cumecs, concentrated
between 1,500 and 6,000 cumecs
Morulius chrysophekadion/
Cyprinidae
Species caught between 1,500
Medium
spp.
and 26,000 cumecs, concentrated
between 1,500 and 5,000 cumecs
Probarbus jullieni
Cyprinidae
Species caught between 1,500 and
Medium
8,000 cumecs, with a peak between
3,000 and 5,000 cumecs
Sikukia gudgeri
Cyprinidae
Species caught between 2,000 and
Medium
22,500 cumecs, with a sharp peak
between 2,000 and 3,000 cumecs
55
Fish migration triggers in the Lower Mekong Basin and other tropical freshwater systems
Taxon in the Khone Falls
Family
Abundance in relation to
Sensitivity to
fisheries database
discharge
discharge
Pangasius bocourti
Pangasiidae
Species caught between 2,000 and
Medium
31,000 cumecs; no clear pattern
Pangasius conchophilus
Pangasiidae
Species caught between 1,500
Medium
and 25,000 cumecs, concentrated
between 3,000 and 6,000 cumecs
Pangasius larnaudiei
Pangasiidae
Species caught between 1,500
Medium
and 22,000 cumecs, concentrated
between 2,500 and 10,000 cumecs
Bagarius spp.
Sisoridae
Species caught between 1,000 and
Medium
13,000 cumecs, rather around 5,000
cumecs
Hemipimelodus borneensis
Ariidae
No clear pattern
Nil
Pseudomystus siamensis
Bagridae
No pattern
Nil
Lobocheilos melanotaenia
Cyprinidae
Species caught between 1,500 and
Nil
24,000 cumecs; no clear pattern
Pristolepis fasciata
Nandidae
No pattern
Nil
Pangasius pleurotaenia
Pangasiidae
No pattern
Nil
Pangasius polyuranodon
Pangasiidae
No pattern
Nil
Hemisilurus mekongensis
Siluridae
No pattern
Nil
Kryptopterus spp.
Siluridae
No pattern
Nil
Opsarius spp.
Cyprinidae
Insufficient data
Unknown
Osteochilus spp.
Cyprinidae
Pattern species-specific
Unknown
Rasbora spp.
Cyprinidae
Unclear response, probably species- Unknown
specific
Glyptothorax spp.
Sisoridae
Species caught between 2,000 and
Unknown
5,000 cumecs, but few data points
56
For further information please contact
Mekong River Commission
P.O. Box 6101, Vientiane 01000, Lao PDR.
Telephone: (856) (21) 263 263 Facsimile: (856) (21) 263 264
Email: mrcs@mrcmekong.org
Website: www.mrcmekong.org
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