GUINEA CURRENT LARGE MARINE
ECOSYSTEM PROJECT
REPORT OF THE REGIONAL WORKSHOP ON METHODS FOR THE
ESTIMATION OF MAXIMUM SUSTAINABLE YIELD
ACCRA, GHANA 22 26 AUGUST, 2005
Report of the Workshop on Methods for the Estimation of MSY, Accra, Ghana 22nd 26th August 2005
TABLE OF CONTENTS
Page
1. OPENING CEREMONY.......................................................................................................3
2. ORGANISATION OF THE SESSION .................................................................................4
3. ESTIMATION OF MSY FROM RESOURCE SURVEYS..................................................5
3.1. Introduction.........................................................................................................................5
3.2. Gulland's (1983) Formula...................................................................................................5
3.3. Cadima's Formula...............................................................................................................6
Exercise 1...........................................................................................................................7
4. ESTIMATION OF MSY FROM SURPLUS PRODUCTION MODELS ............................8
4.1. Introduction.........................................................................................................................8
4.2. The Concept of Surplus Production....................................................................................8
4.3. Alternative Models and their Properties .............................................................................9
Exercise 2.........................................................................................................................11
5. ESTIMATION OF MSY USING ANALYTICAL MODELS............................................13
5.1. Introduction.......................................................................................................................13
5.2. Concepts in Analytical Models.........................................................................................13
5.3. Beverton and Holt's Yield-Per-Recruit Model .................................................................14
5.4. Estimations Procedures.....................................................................................................15
Exercise 3.........................................................................................................................15
6. ESTIMATION OF THE STATUS OF THE FISHERIES USING THE LENGTH-
BASED METHOD ................................................................................................16
Exercise 4.........................................................................................................................16
7. SUMMARY OF SOLUTIONS TO THE EXERCISES ......................................................17
8. RECOMMENDATIONS.....................................................................................................17
9. ADOPTION OF THE REPORT..........................................................................................18
10. CLOSING CEREMONY...................................................................................................18
11. REFERENCE LIST ...........................................................................................................19
ANNEX 1: SPEECHES DELIVERED AT THE OPENING CEREMONY .........................21
WELCOME ADDRESS BY THE REGIONAL DIRECTOR OF THE GCLME
PROJECT, PROF. CHIDI IBE..............................................................................21
ANNEX 2: LIST OF PARTICIPANTS..................................................................................26
ANNEX 3: WORK PROGRAMME .......................................................................................30
ANNEX 4 : LIST OF DOCUMENTS .....................................................................................31
ANNEX 5: SOLUTIONS TO THE EXERCISES...................................................................32
2
Report of the Workshop on Methods for the Estimation of MSY, Accra, Ghana 22nd 26th August 2005
1. OPENING CEREMONY
The GCLME Regional Workshop on Methods for the Estimation of Maximum Sustainable
Yield was held in Accra Ghana, at the conference room of the Regional Coordination Unit,
from 22nd to 26th August, 2005. The opening ceremony was called to order at 10.25 a.m.
Fourteen member countries of the GCLME project took part in the Workshop namely:
Angola, Benin, Cameroon, Congo, Democratic Republic of Congo, Gabon, Ghana, Guinea,
Guinea Bissau, Liberia, Nigeria, Sao-Tome e Principe, Sierra Leone and Togo (list of
participants in Annex 2).
The occasion was chaired Mr. S.K. Quaatey, Deputy Director of the Marine Fisheries
Research Division of the Ministry of Fisheries, Ghana. In his opening remarks, the chairman
welcomed the participants and recalled the processes leading to the Workshop. He explained
that the GCLME Stock Assessment Survey from 4th June to 19th July, 2005 generated large
amounts of data and therefore, this Workshop should be viewed as a follow up to the survey,
as it will provide methods for analysis of the data from the survey.
The ceremony proceeded with speeches by the key speakers for the day. The first by the
Regional Director of the GCLME Project, Prof. Chidi Ibe, read on his behalf by Dr. Jacques
Abe, Environment Expert, GCLME, the second by Mr. E.O. Nsenkyire, Chief Director,
Ministry of Environment and Science, Ghana, representing the Hon. Minister for
Environment and Science, Ms. Christine Churcher and the third by the Hon. Deputy Minister
for Fisheries, Mr. Daniel Dugan (see Annex 2 for full versions of speeches delivered).
The Regional Director outlined the trends in world fish production which, he said, had
increased from 19 million tonnes in 1950 to more than 134 million tonnes in 2002, with
marine capture fisheries contributing up to 70% of the total production. He however
expressed concern about the status of overexploitation of African regional fisheries and
suggested activities to restore the health of the sector. With regards to measures for the
rehabilitation of depleted fish stocks, he underscored the realization of the GCLME Fishery
Resources Survey as the first step in that direction and noted, with satisfaction, the
importance of the Workshop in terms of providing methods for the determination of the
Maximum Sustainable Yield (MSY) of the resources. He then urged participants to work
3
Report of the Workshop on Methods for the Estimation of MSY, Accra, Ghana 22nd 26th August 2005
seriously during the course of the Workshop so as to give useful recommendations for the
benefit of the GCLME region.
The representative of the Minister for Environment and Science, Mr. E.O. Nsenkyire,
delivered the keynote address. He stressed the importance the Government of the Republic
of Ghana attaches to the fisheries sector and pointed out some instruments the Government is
putting in place for proper management of the fisheries resources, especially the National
Fisheries Policy Plan under preparation. He noted the important role that UN agencies play
in the country by assisting with the setting up of pilot projects, the strengthening of fish
farmers' organizations, and highlighted the importance of the transboundary approach as a
prerequisite to resolving the multisectorial problem of fisheries overexploitation.
In his statement, the Hon. Deputy Minister for Fisheries outlined the important role the
fisheries sector plays in socio-economic development and reiterated the problems faced by
the industry. The Workshop on Maximum Sustainable Yield, he said, would give an
indication of the current status of marine fisheries resources in the GCLME region and also,
suggest management options. He pledged the Ministry's support towards sustainable
management of the marine fisheries resources in the region and wished all participants
fruitful deliberations.
2. ORGANISATION OF THE SESSION
Participants examined in detail and adopted unanimously the draft agenda for the Workshop.
Ms. Maria Esperanca Pires dos Santos from Angola was elected by consensus as Chairperson
for the day. The session began with the introduction of participants to the assembly and the
designation of two rapporteurs for the Workshop, Dr. Reynold Johnson and Dr. Seisay, both
from the Republic of Sierra Leone. Participants were assigned to four Working Groups
according to the four themes of the Workshop: 1) Estimation of MSY from Resource
Surveys, 2) Estimation of MSY from Surplus Production Models, 3) Estimation of MSY
from Analytical Models and 4) Estimation of the Status of the Fisheries from the Length-
based Method. Each Working Group identified a rapporteur and a chairman.
4
Report of the Workshop on Methods for the Estimation of MSY, Accra, Ghana 22nd 26th August 2005
3. ESTIMATION OF MSY FROM RESOURCE SURVEYS
3.1. Introduction
Data from Fishery Resource Surveys can be used in Stock Assessment in two main ways:
first, for monitoring, that is, to provide at regular intervals (e.g. annually) indices of stock
abundance; second, to produce estimates of absolute abundance, possibly, at an instance of
time, and most usually in advance of intense exploitation (Gulland, 1983). Surveys that can
produce absolute abundance are important as these estimates, in combination with data of
total catch, fishing and natural mortality rates, can provide preliminary approximations to the
potential yield from the stock.
3.2. Gulland's (1983) Formula
Fishery Resource Surveys provide estimates of total biomass of important species. Although
these estimates are important parameters, they seldom interest the fishery manager, planner
and/or the fisherman. In fact, what they want to know is how much can be caught each year
from the fishery, this entity being obviously related to the biomass, or standing stock. For a
given biomass, the sustainable yield from a long-lived species will be less than that of a
short-lived species. Gulland (1983) suggested that, for surveys of unexploited stock, the
sustainable yield may be estimated by an expression of the form:
Ymax. = aMB (1)
where B is the unexploited biomass, and M the natural mortality rate. Theoretical
considerations suggest that the a value is likely to be around 0.5 (Gulland, 1983) or
somewhat less, so that a well-suited expression for the sustainable yield is:
Ymax. = 0.5MB (2)
Application of this method requires estimates of the natural mortality M, for which there are
many of estimation. However, for length growth data, the most commonly used in the tropics
is the Pauly's (1980) empirical equation as follows:
5
Report of the Workshop on Methods for the Estimation of MSY, Accra, Ghana 22nd 26th August 2005
log 10 M = - 0.0066 0.279 log10 L+ 0.6543 log10 K + 0.463 log10 T (3)
and for weight growth data by:
log 10 M = - 0.2107 0.0824 log10 W+ 0.6757 log10 K + 0.4687 log10 T (4)
where M is natural mortality in a given stock, L the total infinite length in centimeters, W
(live weight in grams) being the asymptotic sizes of that stock, K the curvature parameter of
the VBGF (von Bertalanfy Growth Function) expressed in annual basis and T the annual
mean habitat temperature in oC of the water in which the stock in question lives.
3.3. Cadima's Formula
When a fishery already exists, there is a fishing mortality which should be added on the
Gulland equation. Cadima proposed a suitable modification of the formula in equation (2), in
the form:
Ymax. = 0.5ZB (5)
where Z is the total mortality rate, and Z = (M+F), with F being the fishing mortality rate.
Replacing Z in equation (5) by F and M, we obtain:
Ymax. = 0.5 (F+M) B or Ymax. = 0.5 (Y+MB) (6)
with Y= FB, the catch from the fishery.
Recent studies suggest that the value of 0.5 overestimates the potential yield of the fisheries.
A more conservative estimate will be to put the value of a at around 0.3.
6
Report of the Workshop on Methods for the Estimation of MSY, Accra, Ghana 22nd 26th August 2005
3.4. Limitations
Apart from the operational questions of cost and survey design, common to monitoring
surveys, fish trawl surveys aimed at producing estimates of absolute abundance have to face
questions of possible bias. The first bias can come from the assumption behind the methods
used. For instance, the Swept Area Method assumes that fish populations are homogenously
distributed on the sea floor, a situation that seldom exists in nature. Also, the estimates of the
pelagic abundance, based on the acoustic method, can sometimes be seriously biased, as the
target strength value is very difficult to estimate precisely in most fish species.
Exercise 1
Table 1 gives the results from a survey carried out by the RV Dr Fridtjof Nansen in the
Eastern Gulf of Guinea (Nigeria, Cameroon and Sao Tome e Principe) 11 June-13 July 2004
(using the Swept Area Method). Assuming the natural mortality value of 0.5, estimate MSY
of total demersal species and for each group. How does the MSY for each group differ from
that of the total biomass? Delegates from Nigeria, Cameroon and Sao-Tome are to provide
the estimates of fish production in their respective countries.
Table 1. Swept Area Biomass Estimates in Tonnes from RV Dr Fridtjof Nansen (2004)
Group/Species Nigeria
Cameroon
Principe Sao-Tome
Sparidae
11 481
1 616
1 496
370
Pomasyidae
10 957
10 830
0
18
Sciaenidae
8 857
2 616
0
0
Serranidae
1 304
249
0
119
Lutjanidae
83
350
0
137
Sharks
2 878
180
0
6
Rays
819
140
0
4
Sphyraenidae
7 387
925
0
0
Cephalopods
10 857
797
101
61
Carangidae
39 253
3 103
24
20
Total 93876
20806
1
621
735
7
Report of the Workshop on Methods for the Estimation of MSY, Accra, Ghana 22nd 26th August 2005
4. ESTIMATION OF MSY FROM SURPLUS PRODUCTION MODELS
4.1. Introduction
Surplus Production Models, Biomass Dynamic Models or Production Models are the simplest
Stock Assessment Models that are commonly used. The use of Biomass Dynamic Models has
a somewhat checkered history (Hilborn and Walters, 1992); they form the major assessment
tool for many fisheries, particularly in the assessment made by several tuna agencies such as
the International Commission for the Conservation of Atlantic Tunas ICCAT, the Inter-
American Tropical Tuna Commission IATTC, as well as agencies dealing with regular
finfish, such as the International Commission for South East Atlantic Fisheries ICSEAF.
Despite their relatively wide use, Biomass Dynamic Models have been rated second-class to
Age-structured Analysis. However, it is important to notice the difficulties in aging many
fishes, particularly tropical ones; Age-structured Analysis is therefore not practical in these
fisheries. Furthermore, tropical fisheries catch consists of many species, and the catch data
are difficult, if not impossible, to make species specific. In these circumstances therefore,
treating the entire catch as a Biomass Dynamic Pool may be more appropriate than trying to
look at single species dynamics.
Finally, in cases where both Age-structured and Biomass Dynamic Models have been
applied, the answer has often come out the same (Quinn et al. 1985). Thus, even though
Biomass Dynamic Models remain second-class in the hierarchy of fisheries models, they are
still an important and occasionally indispensable tool. Every fishery biologist should be
familiar with their strengths and limitations (Hilborn and Walters, 1992).
4.2. The Concept of Surplus Production
The changes in a population's biomass, ignoring immigration and emigration from one time
(t) to the next (t+1) can simply be written as:
Bt +1 =Bt + recruitment + growth - natural mortality - catch (8)
8
Report of the Workshop on Methods for the Estimation of MSY, Accra, Ghana 22nd 26th August 2005
One can see from this equation that the two sources of increase in the population are
recruitment of new individuals (birth) and gain in weight by individuals already present in the
population. Catch and natural mortality constitute the two sources of loss. In the absence of
fishing and by combining recruitment and growth into a single term called production,
equation (8) becomes:
Bt +1 =Bt + production - natural mortality catch (9)
If production is greater than natural mortality, the population will grow; conversely, if it is
less than natural mortality, the population will decline. The term Surplus Production is
generally used to represent the difference between production and natural mortality. Surplus
Production represents therefore, the amount by which the population biomass will increase in
the absence of fishing, or the amount of catch that can be taken while maintaining the
biomass at a constant size.
4.3. Alternative Models and their Properties
Schaefer's (1954) model
The first widely used Biomass Dynamic Model was formulated by Schaefer (1954) based on
earlier work by Graham (1935). Schaefer's model is normally written as:
dB
B
= rB 1-
- C (10)
dt
K
where B is the biomass of the stock, r is an intrinsic rate of population growth, K is a
parameter which corresponds to the unfished equilibrium stock size, and C the catch rate. It is
assumed that C is proportional to the stock size and to fishing effort as:
C = qEB (11)
where B is the stock biomass, E the fishing effort, and q a parameter describing the
effectiveness of each unit of fishing effort.
9
Report of the Workshop on Methods for the Estimation of MSY, Accra, Ghana 22nd 26th August 2005
Schaefer's model for C assumes a steady state system, i.e., the stock remains constant from
one year to the next due to constant parameters of recruitment, growth and mortality. At
equilibrium, the relationship between catch per unit effort c/f and effort f is linear (constant
catchability coefficient). Therefore, from catch and effort data it is possible to predict the
long-term yield as:
MSY = a2 /4b (12)
and
fopt. = a/2b (13)
where MSY is the Maximum Sustainable Yield from the fishery, fopt., the fishing effort at
MSY, a and b intercept and slope from the linear regression of the catch per unit effort, c/f
and effort f.
Pella and Tomlinson's (1969) Model
Pella and Tomlinson (1969) considered the extension of the Schaefer's model:
dB
r
= rB -
Bm - C (14)
dt
K
When m is equal to 1, Pella and Tomlinson's model is similar to the Schaefer's model. This
model has been used quite extensively, primarily because of the popularity of the computer
program GENPROD (Fox, 1975), which provides a method of estimating the parameters
using the equilibrium assumption.
Difference Models (Walters and Hilborn 1976)
Walters and Hilborn (1976) used a simple difference equation of the Schaefer's model:
B
Bt +1 =Bt + rB 1-
- Ct (15)
K
10
Report of the Workshop on Methods for the Estimation of MSY, Accra, Ghana 22nd 26th August 2005
where Bt is the biomass at time t, r and K have the same meaning as in the differential
equation version of the Schaefer's model, and Ct, the catch during time t is defined as:
Ct = qBtEt (16)
The model is therefore straightforward as it is derived from the differential equation, but
estimation of its parameters can be very difficult. A computer program CEDA (Catch Effort
Data Analysis), developed at the Renewable Resources Assessment Group (RRAG) by
Holden and Bravington (1992), under the Fish Management Program of the Overseas
Development Agency (ODA,) is a useful method for solving the problem of parameter
estimations of this model.
Summary on Biomass Dynamic Models
There are two alternative methods for assessing the dynamic response of fish populations to
exploitation; Biomass Dynamic Models and Age-structured Models. Historically, Biomass
Dynamic Models have been the second choice, used only when age-structured data were
unavailable. The work of Ludwig and Walters (1985, 1989), indicate that the choice between
the two methods may not be so clear-cut and in many circumstances, the Biomass Dynamic
Approach may provide a better answer, even when age-structured data are available. It is
better to think of the two methods as simply different; if Biomass Dynamic Models provide a
different answer from Age-structured Models, scientists should try to understand why they
are different and analyze the management implications of the different predictions, rather
than disputing which method is the best.
Exercise 2
Using catch and effort statistics from Ghana, Cameroon, Sierra Leone Angola and Benin (see
data below), derive the Maximum Sustainable Yield from these countries based on the
Schaefer's (1954) model.
11
Report of the Workshop on Methods for the Estimation of MSY, Accra, Ghana 22nd 26th August 2005
Ghana: Data for Distant Fleet Benin: National Fisheries Data
Year Catch F
C/f Year Catch Effort C/f
1994 634 132* 5* 1992 175 13* 13.4*
1995 1144 153* 8* 1993 337 20* 23.6*
1996 1981 147 3 1994 650 17 37.9
1997 1931 173 8 1995 670 17 39.5
1998 1983 180 11 1996 698 16 44.7
1999 1611 186 5 1997 624 13 46.9
2000 1542 154 11 1998 699 13 52.7
2001 1635 132 13 1999 666 13 52.5
2002 1633 103 48 2000 670 13 51.5
2003 1449 92 47 2001 488 7* 43.5
Cameroon: National Fisheries Data Angola: Nation Fisheries Data
Year Catch Effort C/f Year Catch Effort C/f
1995 9829 15* 229* 1995 6906 8344* 0.8
1996 9829 33* 213* 1996 12679
10920* 1.4
1997 7783 51 369 1997 18612 9210 2.1
1998 8471 61 196 1998 18033 9192 2.0
1999 6548 69 129 1999 17675 7942 2.2
2000 9522 71 123 2000 17449 7979 2.2
2001 7458 69 112 2001 16618 8743 2.0
2002 9799 68 129 2002 17205 9866 1.8
2003 7425 68 121 2003 16665 10608 1.7
2004 4302 64 107 2004 18277 11232 1.6
Sierra Leone: National Fisheries Data
Year Catch Effort C/f
1991 3263 2884* 0.4*
1992 9731 7373* 0.8*
1993 10045 5366 1.4
1994 5240 5290 1.7
1995 2935 3339 1.9
1996 4461 1860* 2.1
1997 2599 1258* 2.1*
1998 2028 992* 2.2*
1999 7182 2813* 2.2*
2000 5072 2293* 2.3*
2001 11165 3101 2.5
2002 5842 2895 2.5
2003 10117 3174 2.9
12
Report of the Workshop on Methods for the Estimation of MSY, Accra, Ghana 22nd 26th August 2005
5. ESTIMATION OF MSY USING ANALYTICAL MODELS
5.1. Introduction
A basic feature of Analytical Models or Dynamic Pool Models as developed by among
others, Baranov (1914), Thompson and Bell (1934), Beverton and Holt (1957) is that they
require the age composition of catches to be known. Gayanilo and Pauly (1997), justify that
requirement as:
1. If there are "too few old fish", the stock is overfished and the fishing pressure on the
stock should be reduced.
2. If there are" many old fish" the stock is underfished and more fish may be caught as
the yield is increased.
Analytical Models are therefore age-structured and require the estimates of growth and
mortality rates.
5.2. Concepts in Analytical Models
The concept to deal with this issue is that of a cohort. A cohort of a given fish species is made
up of all the fish in that species which hatched at the same period (say a month). The number
of fish in the cohort decreases with time due to natural mortality (predation, diseases, etc.).
However, while the number of survivors decrease with time, the average individual body
length and body weight increases.
The curve resulting from combining the number of survivors with their mean body weight is
the biomass curve, which displays a clear maximum. Thus, to realise the (hypothetical)
maximum yield in weight from that cohort, all fish should be caught exactly when the cohort
has reached the age corresponding to that maximum. This, of course, is not possible in
practice. However, the goal of Fish Stock Assessment is to manage fisheries in such a way
that catches come as close as possible to this theoretical maximum.
13
Report of the Workshop on Methods for the Estimation of MSY, Accra, Ghana 22nd 26th August 2005
The implication is that the fish should be caught neither too young nor too old. When the fish
is caught too young, there is growth overfishing of the stock; conversely growth underfishing
(recruitment overfishing) occurs when fish are caught too old. There are therefore two major
elements describing the dynamics of a cohort:
1. Increase of length and weight, due to growth;
2. Decline of number of survivors, due to fishing and/or natural processes.
When working with Analytical Models using length frequencies, another concept that needs
clear definition is that of age. As noted earlier with body length, we do not operate at the
level of individual fish, so age implies the average age of a cohort. To define age, one must
start with a definition of the day of birth. The obvious biological definition is the day larvae
hatch from the egg (if applicable). In the first part of their lives, the larvae or juveniles are not
affected by the fisheries, therefore we say that the fish are in the unexploited phase. At age tr
when fish reach the fishing grounds, they are called recruits. Recruitment here means the
number of fish that have attained age tr during a recruitment season. Recruitment intensity is
the number of recruits per unit time. The recruitment pattern of a typical tropical species is
continuous throughout the year with seasonal peaks. The occurrence of seasonal peaks in
tropical fisheries can be related to many factors, among these, the monsoons (Pauly and
Navaluna, 1983), the rainfall pattern (Djama, 1992) or the occurrence of seasonality in
upwelling periods.
At age tc (age at first capture), the cohort is assumed to be suddenly exposed to full fishing
mortality F. Because of this "knife-edge" selection, the catch from the cohort is assumed to
be zero before the cohort has attained the age tc.
5.3. Beverton and Holt's Yield-Per-Recruit Model
Beverton and Holt (1957) developed a Yield-Per-Recruit Model describing the state of the
stock and the expected yield in a situation where a given fishing pattern has been operating
for a long time, i.e. under steady-state conditions. They proposed the following equation:
-
-2
-3
Y
1 e kr1
3
e kr
3
1
e kr1
R = F.
2
Mr
e-
W -
-
-
Z
Z + K Z + 2K Z + 3K
14
Report of the Workshop on Methods for the Estimation of MSY, Accra, Ghana 22nd 26th August 2005
where: r = t t and r = t t
1
c
0
2
c
r
5.4. Estimations Procedures
The Food and Agricultural Organisation (FAO) of the United Nations and the International
Centre for Living Aquatic Resources Management, now known as World Fish Centre have
produced a joint software package for Fish Stock Assessment for use with microcomputers.
The software called FISAT (FAO-ICLARM Stock Assessment Tools), has all the methods
for parameter estimations (growth, mortalities etc.) and includes many analytical methods
such as the Beverton and Holt Yield Per Recruit (1957), the Beverton and Holt Relative Yield
Per Recruit Model etc.
Exercise 3
Draw the Y/R curve and compute F 0.1 for Nemipterus marginatus from the South China Sea
(Source FISAT, 1997) in table 2 below.
Table 2. Fishing Mortality and Yield Per Recruit Values for Nemipterus marginatus
F Y/R F
Y/R
0.00
0.000
1.0
1.215
0.01
0.030
1.1
1.247
0.1
0.270
1.2
1.272
0.2
0.485
1.3
1.293
0.3
0.656
1.4
1.310
0.4
0.794
1.5
1.323
0.5
0.905
1.6
1.334
0.6
0.995
1.7
1.342
0.7
1.068
1.8
1.348
0.8
1.127
1.9
1.352
0.9
1.175
2.0
1.355
15
Report of the Workshop on Methods for the Estimation of MSY, Accra, Ghana 22nd 26th August 2005
6. ESTIMATION OF THE STATUS OF THE FISHERIES USING THE LENGTH-
BASED METHOD
Froese and Binohlan (2000), provided a method for analysing length frequencies in order to
estimate the degree of growth over-fishing (over exploitation of immature fish species). This
model requires the determination or knowledge of biological parameters such as mean
asymptotic length of the fish (L), mean length at first sexual maturity (Lm), mean length at
maximum egg production (Lopt.) and the length-weight relationship. Table 3 summarises
these parameters.
Table 3. Summary of the Parameters used in the Study
Parameter Definition
L
Parameter of the VBGF expressing the mean asymptotic
length that a fish would reach if it is to grow indefinitely
Lm
Mean length at first maturity
Lopt.
Mean length at maximum egg production
WL = a*Lb
Relationship between the length of the fish and its weight
WLclass = Nclass*a*Lbclass
Weight of each length class
WT = WLclass
Total weight (landing)
Wposs. = a*Nopt*Lbopt.
Sustainable yield
NB: log Lopt = 1.053log(Lm) 0.0565
As mentioned earlier, methods for the estimation of these parameters are available. However,
where these estimations do not exist, many of these parameters can be retrieved from the
software, FishBase. FishBase can be obtained by directing requests to the World Fish Center,
Malaysia. Copies for participating countries have been obtained by the GCLME RCU.
Exercise 4
Length frequency were provided by participants from Cameroon.
16
Report of the Workshop on Methods for the Estimation of MSY, Accra, Ghana 22nd 26th August 2005
7. SUMMARY OF SOLUTIONS TO THE EXERCISES
Biomass Surplus
Models
Analytical
Length-based
Country
MSY F MSY F
Y/R F0.1
Lc F
Angola
15 656
17 125 9 250
Benin
684 15
Cameroon 17
000
19
000
39
35
Ghana 92
725
17
840
160
Nigeria 68
338
Sao-Tome e Principe
1 598
Sierra Leone
9 025
4 750
South China Sea
1.25
1.1
NB: MSY from Ghana using surplus production model represents distant fleet and should not
be compared with MSY using biomass estimates. The two methods give the same estimates in
Angola and Cameroon.
8. RECOMMENDATIONS
The following recommendations emanated from the various deliberations during the course
of the MSY Workshop. These recommendations express the need to:
· Organize a Regional Training Course on Stock Assessment Tools such as the FAO-
ICLARM Stock Assessment Tools (FISAT), the FishBase Software from the World
Fish Centre and the Ecopath with Ecosim software from the Fisheries Centre of the
University of British Columbia.
· Harmonize data collection and sampling procedures in order to facilitate data
exchange and comparison of results.
· Intensify data collection processes at national levels.
· Promote exchange of survey information and data among GCLME countries and
international organizations.
· Undertake at national levels, stomach content and benthos analysis for at least 10
species at each trophic level based on the manuals provided by the GCLME Project.
17
Report of the Workshop on Methods for the Estimation of MSY, Accra, Ghana 22nd 26th August 2005
9. ADOPTION OF THE REPORT
The report and recommendations of the Workshop were adopted by acclamation. The
participants requested the Secretariat to produce and put at their disposal the final version of
the report as soon as possible.
10. CLOSING CEREMONY
The Workshop on Methods for the Estimation of Maximum Sustainable Yield was brought
to an end at 6:00 pm on the 26th of August with a short ceremony chaired by Dr. Mohamed
Seisay, Fisheries Scientist, Ministry of Fisheries and Marine Resources, Sierra Leone, who,
in his closing remarks stated that the objective of the Workshop had been achieved.
The ceremony was honoured by the presence of the Hon. Deputy Minister for Environment
and Science, Dr. Gheysika Adombere Agambila, who saluted all participants who traveled
from far and near to attend the workshop, for their sacrifice and dedication and wished them
safe journey back to their respective countries.
18
Report of the Workshop on Methods for the Estimation of MSY, Accra, Ghana 22nd 26th August 2005
11. REFERENCE LIST
Baranov, F.I. (1914). The Capture of Fish by Gillnets. Mater. Poznaniyu Russ. Rybolov. 3
(6): 56-99.
Beverton, R.J.H and S.J. Holt (1957). On the Dynamics of Unexploited Fish Population.
Fish. Invest. Minist. Agric. Fish. Food.UK Series (2). No. 19: 533p
Djama, T. (1992). The State of Exploitation of the Commercial Demersal Fishery of
Cameroon. J. App. Ichthyol. Vol. 9: 12-17.
Fox, W.W. (1975). Fitting the Generalized Stock Production Model by Least-squares and
Equilibrium Approximation. U.S. Fish. Bull. 73: 23-37.
Froese, R. and C. Binohlan (2000). Empirical Relationships to Estimate Asymptotic Length,
Length at First Maturity and Length at Maximum Yield Per Recruit in Fishes with a Simple
Method to Evaluate Length Frequency Data. Journal of Fish Biology. 56: 759-773.
Gayanilo, F.C. Jr. and D. Pauly (eds.) (1997). FAO-ICLARM Stock Assessment Tools
(FISAT). Reference Manual. FAO Computerised Information Series (Fisheries). No. 8 Rome,
FAO. 262p
Graham, M. (1935). Modern Theory of Exploiting a Fishery and Application to North Sea
Trawling. J. Cons. Int. Explor. Mer. 10: 264-274.
Gulland, J.A. (1983). Fish Stock Assessment. A Manual for Basic Methods. Vol. 1
FAO/Wiley Series on Food and Agriculture. 223p
Hilborn, R. and C.J. Walters (1992). Quantitative Fisheries Stock Assessment. Choice,
Dynamics and Uncertainty. Routledge, Chapman and Hall, Inc. UK. 570p
Holden, S. and M.V. Bravington (1992). Catch Effort Data Analysis, the CEDA Package
User Manual. 90p
Ludwig, D. and C.J. Walters (1985). Are Age-structured Models Appropriate for Catch-
Effort Data? Can. J. Fish. Aquat. Sci. 42: 1066-1072.
Ludwig, D. and C.J. Walters (1989). A Robust Method for Parameter Estimation from Catch
and Effort Data. Can. J. Fish. Aquat. Sci. 46: 137-144.
Pauly, D. (1980). On the Interrelationships between Natural Mortality, Growth Parameters
and Mean Environmental Temperature in 175 Fish Stocks. J. Cons. CIEM 39 (3): 175-92
Pauly, D. and N.A. Navaluna (1983). Monsoon-induced Seasonality in the Recruitment of
Philippine Fishes. FAO Fish. Rep. 291 Vol. 3: 823-33.
Pella, J.J. and P.K. Tomlinson (1969). A Generalized Stock Production Model. Bull. Inter-
Am.Trop. Tuna. Comm. 13: 419-496.
19
Report of the Workshop on Methods for the Estimation of MSY, Accra, Ghana 22nd 26th August 2005
Quin, T.J. II, R.B Deriso and S.H. Hoag (1985). Methods of Population Assessment of
Pacific Halibut. IPHC Scientific Report No. 72.
Schaefer, M.B. (1954). Some Aspects of the Dynamics of Populations Important to the
Management of the Commercial Marine Fisheries. Inter-Am. Trop. Tuna Comm., Bull. 1 (2):
27-56
Thompson, W. F. and F.H. Bell (1934). Biological Statistics of the Pacific Halibut Fishery. 2.
Effect of Changes in Intensity upon Total Yield and Yield Per Unit of Gear. Rep. Int. Fish.
(Pacific Halibut) Comm. (8). 49p
20
Report of the Workshop on Methods for the Estimation of MSY, Accra, Ghana 22nd 26th August 2005
ANNEX 1: SPEECHES DELIVERED AT THE OPENING CEREMONY
A. WELCOME ADDRESS BY THE REGIONAL DIRECTOR OF THE GCLME
PROJECT, PROF. CHIDI IBE
Presented by
Dr Jacques Abe, Environment Expert of the GCLME Project
Mr. Chairman,
Hon. Deputy Minister for Fisheries,
Chief Director, Ministry of Environment and Science,
Dear Participants,
Distinguished Guests,
Members of the Press,
Ladies and Gentlemen,
It is an honor and great pleasure for me to be with you today at the opening of this important
Workshop on Methods for the Estimation of the Maximum Sustainable Yield (MSY). Let me
take this opportunity to welcome you all in this conference hall of the GCLME Project.
As you know, the total world fish production has increased steadily from 19 million tonnes in
1950 to more than 134 million tonnes in 2002, with marine capture fisheries being the largest
contributors (up to 70%). During the last decade, the overall total catches have started to level
off, and in many countries the catch per unit effort is persistently decreasing.
This situation is particularly in developing Africa, where the decline of the regional fisheries
adversely affects the social welfare of the populations. Fisheries stocks in the region are
today either fully exploited or overexploited. The future is even more threatened as human
populations continue to grow, putting fishery resources under continuous pressure of
exploitation. To bring back the socio-economic health in the sector, the following actions,
amongst others, should be undertaken:
1. Rehabilitate overexploited stocks and "Fine Tuning" of those fully exploited;
2. Reduce habitat degradation (pollution, coastal erosion, etc.);
3. Capitalize traditional knowledge (traditional management of the resources);
21
Report of the Workshop on Methods for the Estimation of MSY, Accra, Ghana 22nd 26th August 2005
4. Improve the institutional and legal framework for fisheries management at national,
sub-regional and regional levels.
Ladies and gentlemen,
With regards to the rehabilitation of overexploited fish stocks, the GCLME fishery resources
survey, which was launched in Tema on June 4th, 2005 and ended successfully on 16th July
2005, was the first step towards the rehabilitation of depleted fisheries. Although the results
of this survey (standing biomass estimates) seem interesting, they seldom interest the
fisheries manager and/or the public at large. What people want to know is the quantity of fish
that can be sustainably caught each year from their fishery. It is in answer to that important
concern, that the GCLME Project is organizing this Workshop on Methods for the Estimation
of Maximum Sustainable Yield (MSY). It is understood that MSY is the "Maximum harvest
that can be taken in perpetuity from a given fishery".
Mr. Chairman,
Given its scope and targets, the importance of this Workshop cannot be over emphasized. I
therefore foresee constructive discussions during the deliberations and important
recommendations at the end of the Workshop, for the benefit of the stakeholders of the
fisheries sector. The results of this meeting will without a doubt, constitute an important
challenge for the future of fisheries management in the GCLME region, the ultimate goal
being the sustainable increase of fish production and the consequent improvement of the
social-welfare of all the stakeholders of this important sector.
Thank you very much for your kind attention
22
Report of the Workshop on Methods for the Estimation of MSY, Accra, Ghana 22nd 26th August 2005
B. KEYNOTE ADDRESS DELIVERED BY MR. EDWARD OSEI NSENKYIRE,
CHIEF DIRECTOR, MINISTRY OF ENVIRONMENT AND SCIENCE
Mr. Chairman,
Hon. Deputy Minister of Fisheries,
Regional Director, GCLME,
Representatives of UN Agencies,
Distinguished Fisheries Experts,
Distinguished Ladies and Gentlemen,
It is an honour and a privilege for me to open this workshop on Maximum Sustainable Yield
(MSY).
About two months ago, precisely on the 1st of June 2005, the Hon. Minister for Fisheries,
Mrs. Gladys Asmah, opened a workshop on Fishery Resources Survey, Planning and
Methodologies.
In her speech, she said, and I quote "The Government of the Republic of Ghana attaches so
much importance to fish because it is a great source of protein for our people and fishery
activities provide livelihood for the coastal population" end of quote.
Mr. Chairman, these words are indicative of the fact that the Government of the Republic of
Ghana through the Ministry of Fisheries is putting in place a National Fisheries Policy for
sustained increase in fish production for the local and export markets.
The Food and Agriculture Organization (FAO) is assisting in setting up pilot projects to
strengthen fish farmers' organizations in various parts of the country. Farmers have been
trained to raise their own fingerlings for commercial-sized fishponds.
Mr. Chairman,
Distinguished ladies and Gentlemen,
The Guinea Current Large Marine Ecosystem Project is a multidisciplinary project. The
fisheries component of the project is about "Recovery and Sustainability of depleted fisheries
and marine living resources, including mariculture."
23
Report of the Workshop on Methods for the Estimation of MSY, Accra, Ghana 22nd 26th August 2005
The major concern under this component is the quantification of the extent to which fisheries
are overexploited. It is therefore imperative to determine the Maximum Sustainable Yield.
When this is done it will then be possible to translate the results into management tools for
the sustainability of the fisheries.
I am reliably informed that the fish trawl that was successfully launched on the 4th of June,
2005 in Tema, Ghana, has completed its journey and results obtained by scientists on board
the vessel will be analyzed at this workshop.
This workshop is a highly technical one and at the end of the week fisheries experts gathered
here are expected to come out with a publication of stock Maximum Sustainable Yield
estimates of the sixteen countries that are implementing the Project.
I wish our distinguished participants fruitful deliberations.
Thank you.
24
Report of the Workshop on Methods for the Estimation of MSY, Accra, Ghana 22nd 26th August 2005
C. STATEMENT DELIVERED BY THE HON. DEPUTY MINISTER FOR
FISHERIES, MR. DANIEL DUGAN
Mr. Chairman,
Chief Director, Ministry of Environment and Science,
Regional Director of the GCLME Project,
Distinguished Participants,
Honourable Ladies and Gentlemen of the Press,
Ladies and Gentlemen,
The fisheries sector plays an important role in the socio-economic development of our
country. It employs about 2 million people, representing 10% of our population. Fish
accounts for about 60% of animal protein intake and contributes 3% of Gross Domestic
Product. In spite of these contributions, the sector is faced with mirages of problems. It is to
revamp the fishing industry that the Government of Ghana under H.E. President John
Agyekum Kufour created the Ministry of Fisheries with a cabinet ministerial status.
One of the major problems facing the fishing industry, if not the greatest, is the over-
exploitation of our marine and fresh water resources. The seriousness of this is that, we do
not know the extent of over exploitation. My Ministry agrees with the concept of Maximum
Sustainable Yield (MSY) which Larkin in 1977 defined as "A species of fish each year
produces a harvestable surplus and if you take that much and no more, you can go on getting
it forever and ever". Mercer in 1982 defined MSY as the "Maximum harvest that can be
taken in perpetuity from a self-regenerating stock of animals".
My Ministry hopes that this workshop on the Maximum Sustainable Yield would give the
current status of our marine fisheries resources and also suggest management options. The
Ministry sees aquaculture as the way forward and options on marine culture is being
considered.
I pledge the Ministry's support in whatever way that will see sustainable management of the
marine fishery resources.
I wish you fruitful deliberations.
Thank you.
25
Report of the Workshop on Methods for the Estimation of MSY, Accra, Ghana 22nd 26th August 2005
ANNEX 2: LIST OF PARTICIPANTS
Countries
Angola
Benin
Maria Esperanca Pires dos Santos
Amélie A. Gbaguidi
Head of the Crustaceans Program
Chercheur
Instituto Naconal de Investigacao Pesqueira
Centre de Recherches Halieutiques et
(INIP)
Océanologiques
Tel/Fax: (244) 222 449519
BP 1665 Cotonou
Cell: (244) 912243214
Tel: (229) 32 12 63
E-mail: mariasantos35@hotmail.com
Cell: (229) 912691 / 229 300964
E-mail: aziable2002@yahoo.fr
Silvi Edith Nsiangango
Roger Djiman
Marine Research Institute Luanda
Chef de Centre
Tel: (244) 222 449519
Centre de Recherches Halieutiques et
Cell: (244) 923711371
Océanologiques
E-mail: sylpriscilla@hotmail.com
BP 1665 Cotonou (Bénin)
Kumbi Kilongo
Tel: (229) 326 214 / 339546
Head of the Regional Marine Research
Cell: (229) 997 488
Institute of Benguela, Angola
E-mail: rodjiman@yahoo.fr
Tel: (244) 309077
Cell: (244) 923 319481
E-mail: kkilongo@hotmail.com
Cameroon
Congo
Chiambeng George Yongbi
Atsango Claude Benoît
Senior Researcher
Chef de service de l'hydrobiologie
Fisheries Research Station, Limbe
Direction Générale de la Pêche et de
Postal Box: PMB 77 Limbe, Cameroon
l'Aquaculture
Tel. 237/7233321
BP 1650 Brazaville
Fax: 237/3333091 ext. 2
Tel : (242) 536 9793
E-mail: chiambeng@yahoo.fr
E-mail: atsangoclaude@yahoo.fr
Pierre Nolasque Meke
Itoua Niamba Frederic
Chef de Service Provincial des pêches du Littoral Chef de service de l'inspection et de la
Tel: (237) 3430642 / 9932553
surveillance des pêches ; Pointe-noire,
Fax: (237 342 1113
Tel: (242) 5461581
E-mail: pierremeke@yahoo.com
E-mail : itouaniamba@yahoo.fr
Sulem Steve Yong
Fisheries Research Officer
Institute of Agricultural Research for
Development
Tel: (237) 223 35 38/ Cell: (237) 753 6011
Fax: (237) 222 33 62
E-mail : yongsulem@yahoo.com
26
Report of the Workshop on Methods for the Estimation of MSY, Accra, Ghana 22nd 26th August 2005
Democratic Republic of Congo
Gabon
Mr Jean Claude Emene Elenga
Mr. Mba-Asseko Georges
Expert en Pollution Industrielle
Biologiste / Conseiller
Chef de Bureau Evaluation
Direction Générale des Pêches et de
Environnementale des Projets
l'Aquaculture
Ministère de l'Environnement et de la
BP: 9498 Libreville
Conservation de la Nature
Tel: (241) 72 14 12 / 748992
Cell: (243) 981 21166
Cell: (241) 06611140
E-mail: jcemene@yahoo.fr
Fax: (241) 764602
E-mail: gmbasseko@yahoo.com
Ghana
Ghana
Dr. Francis K. Nunoo
AnthonyYaw Karikari
Lecturer (Fisheries Scientist)
Research Scientist
University of Ghana
CSIR-Water Research Institute
P. O. Box LG 99
Box M 32, Accra
Legon, Accra. Ghana
Tel: (233) 21 779514
Tel: (233) 21 514656 (office)
Cell: (233) 20 8184215
Cell: (233) 20 2003075
Fax: (233) 21 777170
Fax: (233) 21 502701
E-mail: anthony_karikari@yahoo.co.uk
Email: fkenunoo@ug.edu.gh
Mr. Kwamena Quaison
Paul Bannerman
Biologist, Ministry of Environment and
Fisheries Biologist
Science
Marine Fisheries Research Division
Tel: (233) 21 660357
Ministry of Fisheries
Cell: (233) 244 755867
Tel: (233) 22 202346
Fax: (233) 21 666828
E-mail: paulbann@hotmail.com
E-mail: atobiggy@yahoo.co.uk
Quaatey Samuel N. K.
Theodore Quarcoopome
Marine Fisheries Research Division
Fisheries Research Scientist
Ministry of Fisheries
CSIR-Water Research Institute
Tel: (233) 22 202346
P.O. Box AH 38 Achimota
Email: samquaatey@yahoo.com
Tel: (233) 21 779514/ 775352
E-mail: qpome@yahoo.com
Francis K. Amerenku
Fisheries Research Scientist
CSIR-Water Research Institute
P.O. Box AH 38, Achimota, Ghana
Tel: (233) 244 573418
E-mail: fykamevenku@hotmail.com
27
Report of the Workshop on Methods for the Estimation of MSY, Accra, Ghana 22nd 26th August 2005
Guinea
Liberia
Momo Horace
Diallo Samba Tenin
Expert in Fisheries and Marine Biology
Biologiste des Pêches
Associate Vice President for Research
Centre National des Sciences
Cell: (231) 6 593 157
Halieutiques de Boussoura (CNSHB)
E-mail: momohorace@yahoo.com
B.P. 3738 / 3739, Conakry
République de Guinée (West Africa)
Yevewuo Z. Subah
Tel: (224) 22 96 98 / 46 44 91
Program Director
Cell: (224) 34 21 51
Fainmaba Fisheries Development Cooperative,
Fax: (224) 45 19 26 / 41 35 23 / 40 91 07
Inc.
Email: stdiallo@cnshb.org.gn
P.O. Box 10-02711
1000 Monrovia, 10 Liberia
Mr. Bangaly Kaba
Cell: (231) 6 517742
Océanographe Biologiste
E-mail yevewuozsubah@yahoo.com
CERESCOR,
BP 1615
Conakry, Guinea
Cell: (224) 33 15 25
E-mail: bkaba3@yahoo.fr
Guinea Bissau
M. Mario Biague
Expert en Conservation et Utilisation des
Zones Humides
Direction Générale de l'Environnement
Ministère de l'énergie et des ressources
Naturelles BP 399
Tel: (245) 7219726 / 6635494
E-mail: m_biague@yahoo.fr
Nigeria
Nigeria
Williams Akanbi B.
Isebor C. E.
Senior Research Officer
Chief Research Officer (Fisheries)
Nigerian Institute for Oceanography and Marine
Nigerian Institute for Oceanography and Marine
Research
Research
P. M. B. 12729, Marina, Lagos.
P. M. B. 12729, Marina, Lagos, Nigeria
Tel: (234) 80233441039
Tel: (234) 8033127885
Fax: (234) 1 2617530
Fax: (234) 1 2617530
E-mail: abwilliams2@yahoo.com
E-mail: ekaeteukut@yahoo.com
Parcy Abohweyere
Oyebanji Micheal O.
Chief Research Officer
Principal Research Officer (Plankton)
Nigerian Institute for Oceanography and Marine
Nigerian Institute for Oceanography and Marine
Research
P. M. B. 12729
Research
Marina, Lagos
P. M. B. 12729, Marina, Lagos
Cell: (234) 802 300 6855
Tel: (234) 8023064833
E-mail: parcyochuko@yahoo.com
Fax: (234) 1 2617530
E-mail: oyebanji20012002@yahoo.com
28
Report of the Workshop on Methods for the Estimation of MSY, Accra, Ghana 22nd 26th August 2005
Sao Tome e Principe
Sierra Leone
Virgina Godinho
Mohamed B. D. Seisay
Direccao, das Pesca
Fisheries Scientist
Ministerio de Economia;
Ministry of Fisheries and Marine Resources
C.P. 59, Largos das Alfandegas
Tel: (232) 235920 / 076 622609
Sao Tome e Principe
E-mail: mohamedseisay@yahoo.co.uk
Tel: (239) 22 20 91
Cell. (239) 907655
Reynold Johnson
E-mail: virginiacarvalho998@hotmail.com
Department of Geography/Institute of Marine
Biology and Oceanography
Idelecio Yoao
Fourah Bay College
Navigation, CPN no. 68 Marinha STP
Cell: (232) 076 629040
Tel: (239) 221520 / 905179
E-mail: treynold12001@yahoo.com
Tel: (239) 222936/22161
Cell. (239) 905953
E-mail: virginacarvalho998@hotmail.com
Togo
Togo
Mr Kossi Maxoe Sedzro
Mr. Ahoedo Kossi,
Ingénieur Agronome Statisticien des Pêches
Biologiste
Chef de Division des Pêches et de l'Aquaculture
Chef de la Section Ressources Halieutiques
BP 1095 Lomé
Division Pêche et Aquaculture
Tel: (228) 221 34 70
BP 1095 Lomé
Cell: (228) 907 03 33/ (228) 949 11 28
Tel: (228) 221 3470 / 906 9510
Fax (228) 221 71 20
Email: kahoedo10@yahoo.fr
E-mail: peche@laposte.tg /
ksedzro69@hotmail.com
GCLME RCU
GCLME RCU
Prof. Chidi Ibe
Dr Jacques Abe
Regional Director
Environment Expert
GCLME, RCU
E-mail:gclme@unido.org, jabel@hotmail.com
No. 1 Akosombo Road
PMB CT 324, Accra, Ghana
Mr. Napoleon Gbolonyo
Tel: 233-21 781225
Administrative Officer
Fax: 233-21 781226
E-mail: gbolonyo@yahoo.com
E-mail: gclme@unido.org, c.ibe@unido.org
Ms. Kelechi Ihemeje
Dr. Djama Theodore
Personal Assistant to Regional Director / Editor
Fisheries Expert
E-mail: kihemeje@yahoo.com
E-mail: gclme@unido.org,
theodoredjama@yahoo.co.uk
Ms. Selina Kuukuwa Yawson
Public Awareness and Participation Advisor
Mr. Joshua Okechuku Ndubuisi
E-mail: s.yawson@yahoo.com
ICT Specialist
E-mail: ndujosh@hotmail.com
Ms. Thelma Hodey
Secretary
Ms. Sylvia Osei Nsenkyire
E-mail: takrumah@yahoo.co.uk
Intern
E-mail: Feasy2k@yahoo.com
29
Report of the Workshop on Methods for the Estimation of MSY, Accra, Ghana 22nd 26th August 2005
ANNEX 3: WORK PROGRAMME
1. Opening Ceremony
2. Adoption of the Agenda and Organisation of the Session
3. Estimation of MSY from Fishery Survey Data
4. Estimation of MSY from Surplus Production Models
5. Estimation of MSY from Analytical Models
6. Estimation of the Status of the Fisheries from Length-Based Method
7. Working Group Presentations
8. Country Presentations
9. Recommendations
10. Adoption of the Report
11. Closing Ceremony
30
Report of the Workshop on Methods for the Estimation of MSY, Accra, Ghana 22nd 26th August 2005
ANNEX 4 : LIST OF DOCUMENTS
1. GCLME/02/2005/01 Programme
2. GCLME/02/2005/02 Agenda
3. GCLME/02/2005/03 Annotated Agenda
4. GCLME/02/2005/04 Aide Mémoire
5. GCLME/02/2005/05 Essay on MSY
6. GCLME/02/2005/06 Letter to the Editor
7. GCLME/02/2005/07 Structure and Dynamics of the Marine Fisheries of
Cameroon
8. GCLME/02/2005/08 Fish Stock Assessment: A Manual of Basic Methods
9. GCLME/02/2005/09 FAO-ICLARM Stock Assessment Tools
10. GCLME/02/2005/10 Quantitative Fisheries Stock Assessment: Choice, Dynamics and
Uncertainty
11. GCLME/02/2005/11 Status of the Population of Pseudotolithus senegalensis
(Valencienne, 1833) of Cameroon
12. GCLME/02/2005/12 The State of Exploitation of the Commercial Demersal Fisheries
of Cameroon
31
Report of the Workshop on Methods for the Estimation of MSY, Accra, Ghana 22nd 26th August 2005
ANNEX 5: SOLUTIONS TO THE EXERCISES
Solutions to Exercise 1
From the total production it was assumed that the proportions of species groups from the
survey should equally be reflected in the actual landings. Back calculations were then
performed using biomass proportions to estimate proportions of catches per group of species.
Cadima's formula was then applied to the data for the estimation of MSY.
Nigeria
MSY from total Biomass; MSYtotal = 0.3(Ytotal + M*Btotal). Replacing Ytotal by its value of 148
000 tonnes, natural mortality M by 0.5 and Btotal by 159589 we obtained, MSYtotal = 68338
tonnes/year. This value of MSY represents 43% of the carrying capacity. MSY from Biomass
of each group (Table below) is equal to MSY from the total biomass.
Group Biomass
Proportions
Proportions
Cadima's formula
Biomass
landings
MSY = 0.3 (Y+MB)
Sparidae
19518
0.12
18100
8358
Pomasyidae
18627
0.12
17274
7976
Sciaenidae
15057
0.09
13963
6448
Serranidae
2217
0.01
2056
949
Lutjanidae
141
0.00
131
60
Sharks
4893
0.03
4537
2095
Rays
1392
0.01
1291
596
Sphyraenidae
12558
0.08
11646
5377
Cephalopods
18457
0.12
17117
7904
Carangidae
66730
0.42
61884
28575
Total
159 589
1
148 000
68 338
Cameroon
MSY from total Biomass; MSYtotal = 0.3(Ytotal + M*Btotal). Replacing Ytotal by its value of 39
000 tonnes, natural mortality M by 0.5 and Btotal by 35370 we obtained;
MSYtotal = 17 000 tonnes/year. This value of MSY represents 48% of the carrying capacity.
MSY from Biomass per each group (Table below) is equal to MSY from the total biomass.
32
Report of the Workshop on Methods for the Estimation of MSY, Accra, Ghana 22nd 26th August 2005
Group Biomass
Proportions
Proportions
Cadima's formula
Biomass
landings
MSY = 0.3 (Y+MB)
Sparidae
2747
0.08
3029
1321
Pomasyidae
18411
0.52
20300
8852
Sciaenidae
4447
0.13
4904
2138
Serranidae
423
0.01
467
204
Lutjanidae
595
0.02
656
286
Sharks
306
0.01
337
147
Rays
238
0.01
262
114
Sphyraenidae
1572
0.04
1734
756
Cephalopods
1355
0.04
1494
651
Carangidae
5275
0.15
5816
2536
Total 35370
1
39000
17
000
Sao-Tome e Principe
MSY from total Biomass; MSYtotal = 0.3(Ytotal + M*Btotal). Replacing Ytotal by its value of 39
000 tonnes, natural mortality M by 0.5 and Btotal by 35370 we obtained, MSYtotal = 1 598
tonnes/year.
Group Biomass
Proportions
Proportions
Cadima's formula
Biomass
landings
MSY = 0.3 (Y+MB)
Sparidae
3172
0.79
2633
1266
Pomasyidae
31
0.01
25
12
Sciaenidae
0
0.00
0
0
Serranidae
202
0.05
168
81
Lutjanidae
233
0.06
193
93
Sharks
10
0.00
8
4
Rays
7
0.00
6
3
Sphyraenidae
0
0.00
0
0
Cephalopods
275
0.07
229
110
Carangidae
75
0.02
62
30
Total 4
005
1
3324
1
598
33
Report of the Workshop on Methods for the Estimation of MSY, Accra, Ghana 22nd 26th August 2005
Ghana
MSY from total Biomass; MSYtotal = 0.3(Ytotal + M*Btotal). Replacing Ytotal by its value of 248
800 tonnes, natural mortality M by 0.5 and Btotal by 120 571 tonnes we obtained, MSYtotal =
92 725 tonnes/year.
Group Biomass
Proportions
Proportions
Cadima's formula
Biomass
landings
MSY = 0.3 (Y+MB)
Sparidae
26673
0.22
55040
20513
Pomasyidae
3844
0.03
7932
2956
Sciaenidae
48819
0.40
100739
37544
Serranidae
400
0.00
824
307
Lutjanidae
702
0.01
1449
40
Sharks
0
0.00
0
0
Rays
0
0.00
0
0
Sphyraenidae
3742
0.03
7721
2878
Cephalopods
3708
0.03
7651
2851
Carangidae
32684
0.27
67444
25136
Total
120 571
1
248 800
92 725
Angola
MSY from total Biomass; MSYtotal = 0.3(Ytotal + M*Btotal). Replacing Ytotal by its value of
15376 tonnes, natural mortality M by 0.5 and Btotal by 73618 we obtained, MSYtotal = 15 656
tonnes/year. This value of MSY represents 21% of the carrying capacity.
Group Biomass
Proportions
Proportions
Cadima's formula
Biomass
landings
MSY = 0.3 (Y+MB)
Sparidae
30783
0.42
6394
6536
Pomasyidae
18858
0.26
3958
4016
Sciaenidae
22049
0.30
4567
4677
Serranidae
1928
0.03
457
426
Total
73 618
1
15 376
15 656
34
Report of the Workshop on Methods for the Estimation of MSY, Accra, Ghana 22nd 26th August 2005
Solutions to Exercise 2
Cameroon: Plot of Catch per Unit Effort against
Benin: Plot of Catch per Unit Effort against Effort
Fishing Effort
t 60
1000
f
for 50
y = -12.598x + 981.03
i
t
n
800
R2 = 0.8649
i
t
E 40
y = -2.8475x + 88.265
n
r
U
rt
600
R2 = 0.9092
30
r
U
f
o
400
Ef
20
h pe
200
a
t
c
h pe
10
C
a
tc
0
C
0
0
20
40
60
80
0
5
10
15
20
Fishing Effort
Fishing Effort
Catch per Unit Effort
Linear (Catch per Unit Effort)
Catch per unit Effort
Linear (Catch per unit Effort)
Angola: Plot of Catch per Unit Effort against
Ghana: Plot of Catch per Unit Effort against
Fishing Effort
Fishing Efforty = -0.6782x + 223.5
R2 = 0.6269
t
r
t
3.0
200
ffor
f
f
o
2.5
150
i
t
E 2.0
i
t
e
n
n
1.5
y = -0.0002x + 3.6588
100
r
U
r
u
1.0
R2 = 0.7103
50
h pe 0.5
h pe
a
tc 0.0
a
tc
0
C
C
0
5000
10000
15000
0
50
100
150
200
250
Fishing Effort
Fishing effort
Catch per Unit Effort
Linear (Catch per Unit Effort)
Fishing effort
Linear (Fishing effort)
Sierra Leone: Plot of Catch per Unit Effort against
Fishing Effort
3.5
3.0
2.5
2.0
y = -0.0004x + 3.7742
1.5
R2 = 0.7103
1.0
0.5
0.0
0
1000
2000
3000
4000
5000
6000
F i shi ng Ef f o r t
Catch per Unit Effort
Li
(C t h
U it Ef f
t )
Summary table for MSY and fishing effort estimates
Angola
Benin
Cameroon
Ghana Sierra
Leone
MSY 17 125
684
19 100
17 840
9 025
F
9 250 days-fishing
15?
39 Boats
160?
4 750?
35
Report of the Workshop on Methods for the Estimation of MSY, Accra, Ghana 22nd 26th August 2005
Solution to Exercise 3
The difference between successive Y/R values, put on a yearly basis, is used as
approximation to F0.1. In this case F0.1= 1.1
F Y/R
Diff./10
0.00
0.000
0.00
0.01
0.030
0.30
0.1
0.270
2.40
0.2
0.485
2.15
0.3
0.656
1.71
0.4
0.794
1.38
0.5
0.905
1.11
0.6
0.995
0.90
0.7
1.068
0.73
0.8
1.127
0.59
0.9
1.175
0.48
1.0
1.215
0.40
1.1
1.247
0.32
1.2
1.272
0.25
1.3
1.293
0.21
1.4
1.310
0.17
1.5
1.323
0.13
1.6
1.334
0.11
1.7
1.342
0.08
1.8
1.348
0.06
1.9
1.352
0.04
2.0
1.355
0.03
36
Report of the Workshop on Methods for the Estimation of MSY, Accra, Ghana 22nd 26th August 2005
Solutions to Exercise 4
Length frequency distributions analyzed were presented by the participants from Cameroon
and concerned the species Pseudotolithus typus, of the Sciaenidae family.
Input parameters;
· Linf = 80 cm; K = 0.2
· Lm = 23; Lopt = 47
· M = 0.5
Outputs of the length frequencies (A and B)
(A)
L e n g th Fr e q u e n c y Plo t
B io ma s s Plo t
L m
L o p t.
L in f.
3 0 0 0
2 5 0
2 5 0 0
2 0 0
i
e
s
2 0 0 0
nc
1 5 0
1 5 0 0
1 0 0
1 0 0 0
r
e
que
F
5 0 0
5 0
0
0
12
16
20
24
28
32
36
40
44
48
52
56
60
T o t a l le n g t h in c m
Fig . 2 : L e n g t h - Fr e q u e n c y Bio m a s s P lo t
(B)
Po te n tia l Y ie ld Plo t
A c tu a l c a tc h = 4 3 8 3 to n n e s
4 0 0
L m
L o p t
P o te n ti a l c a tc h = 5 6 5 2 to n n e s
e
l
d
3 0 0
2 0 0
t
i
a
l
Yi
t
e
n
1 0 0
Po
0
12
16
20
24
28
32
36
40
44
48
52
56
60
T o t a l L e n g t h in c m
F ig . 3 : L e n g t h - F r e q u e n c y P o t e n t ia l Y ie ld P lo t
37