Fisheries Management and Ecology, 2001, 8, 37–45
Growth and mortality of the catfish,
Hemisynodontis membranaceus (Geoffroy St.
Hilaire), in the northern arm of Lake Volta,
Ghana
P. K. OFORI -DANSON
Water Research Institute, Council for Scientific and Industrial Research, Achimota, Ghana
C. J . VANDERPUYE
Department of Oceanography and Fisheries, Uni6ersity of Ghana, Legon, Ghana
G. J . de GRAAF
NEFISCO, Amsterdam, and EUROCONSULT, Arnhem, the Netherlands
Abstract Estimates of growth and mortality of the catfish, Hemisynodontis mem-
branaceus (Geoffroy St. Hilaire), in Lake Volta were obtained from length composition
data compiled in 1995 and 1996. The von Bertalanffy growth function (VBGF) estimates
were: L=44.5 cm standard length; K=0.62 year−1; and t0= −0.23 years. Natural
mortality rate, M, was 1.20 year−1. Total mortality rate, Z, was computed as 4.39
year−1 and the exploitation ratio (E=F/Z) was 0.72. Although the fish is estimated to
have longevity of about 5 years, those exploited are normally less than 2 years of age,
which is indicative of growth over-fishing. In order to arrest over-exploitation of the
species, there is a need to establish ‘lake reserves’. In addition, the fisheries management
should be devolved from the state to the local level to compel fishermen to take greater
responsibility for the sustainability and conservation of the fisheries.
KEYWORDS : growth, Hemisynodontis membranaceus, Lake Volta, mortality, over-fishing.
Introduction
The River Volta is the second largest river in West Africa after the Niger. It has four
major tributaries: the White, Black and Red Volta and the Oti (Fig. 1), the catchment
areas of which cover roughly two-thirds of Ghana. A dam at Akosombo led to the
creation of Lake Volta in 1964.
During the last three decades, the lake has undergone great changes in its ecology,
limno-chemistry and socio-economy. For instance, increased pressure on land along the
Correspondence: P. K. Ofori-Danson, Water Research Institute, Council for Scientific and Industrial Research,
P.O. Box 38, Achimota, Ghana. Fax: +233 21 777665/761030 (e-mail: wri@ghana.com)
© 2001 Blackwell Science Ltd 37
38 P. K. OFORI-DANSON ET AL.
banks has led to high rates of deforestation. This has resulted in increased soil erosion
leading to the transportation of high loads of silt and nutrients through rivers into the
lake, thereby contributing to its eutrophication. Furthermore, wetlands bordering the
lake are being converted into agricultural land or land for grazing cattle, and therefore
may not be able to act as natural filters for nutrients and silt, and now do not provide
breeding grounds for many fish species.
In anticipation of these problems, multi-disciplinary studies under the Volta Lake
Research and Development Project (VLR&DP) were undertaken mainly under the aegis
of the Food and Agricultural Organization/United Nations Development Programme
(FAO/UNDP) during the first decade of the lake’s existence (FAO/UNDP 1971, 1979).
These studies came to an end when the VLR&DP was phased out in 1978. Since then,
systematic data collection from the lake’s natural resources has been lacking. There have
been calls for renewed studies to facilitate their management due to declining catches
with possible stabilization of the lake’s ecology after 30 years of impoundment.
This paper focuses on catfish, Hemisynodontis membranaceus (Geoffroy St. Hilaire),
which is one of the five major commercially important species encountered in the
northern arm of the lake. This zone currently constitutes the heaviest fish landing sectors
Figure 1. Map of Lake Volta, Ghana.
© 2001 Blackwell Science Ltd, Fisheries Management and Ecology 2001, 8, 37–45
GROWTH AND MORTALITY PARAMETERS OF CATFISH 39
of the lake, and boasts the largest fish market on the lake at Yeji (Agyenim-Boateng
1989).
Management of the fisheries sector in the country has so far been inadequate, with
issues of equity and sustainability only recently being addressed. The sector is facing
major problems such as over-exploitation of some fishery resources, inadequate infor-
mation for planning and management, destructive fishing practices, habitat damage,
conflicts between fisheries and post-harvest losses. The primary objective of the study
is to provide information that is relevant to the management of synodontid catfish in
Lake Volta.
Materials and methods
Sampling and data source
Two-stage stratified sampling method was implemented. In the first stage, the water
areas in stratum VII, the Yeji region, were geographically divided into three minor
strata, namely: northern sub-stratum; central sub-stratum; and southern sub-stratum
(Fig. 1). Further stratification was based on the size of the fishing site or village as
follows: small site (having 0–10 canoes); medium site (having 11–50 canoes); and
large site (having more than 50 canoes). The canoe catch and fishing effort were
recorded in a specific catch assessment survey (CAS). For this, three fish landing sites
were selected in the northern and southern sub-strata, respectively, and four sites were
selected in the central sub-stratum.
Length composition of the catch
Monthly length frequency (standard length, SL, to the nearest centimetre) data were
compiled from sample length measurements during experimental fishing with a purse
seine (locally called winch-net), and the distributions determined at 1.0-cm length
intervals. Appropriate routines of FISAT (Gayanilo, Sparre & Pauly 1995) were em-
ployed to correct the length frequency data for possible selectivity of the purse seine.
Growth parameters
Fish growth was assumed to follow the von Bertalanffy growth function (VBGF),
which has the basic form: Lt=L[1−exp(−K(t− t0))]
Estimates of the von Bertalanffy growth parameters, the asymptotic length (L) and
the growth coefficient (K), were derived using the ELEFAN routine in the FISAT
suite of programs (Gayanilo et al. 1995). The theoretical age at length zero (t0) was
obtained from Pauly’s (1979) equation:
log10(− t0)= −0.392−0.275 log10 L−1.038 log10 K
© 2001 Blackwell Science Ltd, Fisheries Management and Ecology 2001, 8, 37–45
40 P. K. OFORI-DANSON ET AL.
Longevity and growth performance index (8 %)
Longevity was calculated from Pauly’s (1984) equation: tmax=3/K. Pauly and Munro’s
(1984) growth performance index (or phi-prime index) 8 % was computed from the
equation: 8 %= log10 K+2 log10 L.
Mortality parameters
Total annual instantaneous mortality rates, Z, were estimated for each year separately by
constructing linearized length–converted catch curves (Sparre & Venema 1992). Natural
mortality rates, M, were computed by the empirical equation of Pauly (1980), using a
mean annual temperature of 29.8 °C. The fishing mortality rates, F, were calculated as
Z−M. The exploitation ratio, E, was calculated as equal to the fraction of death caused
by fishing (Ricker 1975): E=F/Z.
Results
Length–weight relationship
The length–weight relationships for H. membranaceus in Lake Volta are as follows:
W=0.0223SL2.9368 (r=0.9596) for males and W=0.0067SL3.2205 (r=0.9740) for fe-
males. The exponent b is closer to 3 in males indicating that weight growth is isometric.
Growth and mortality parameters
The length frequency distribution for the two successive years from FISAT analyses is
shown in Figure 2. A summary of the parameters that describe growth in length (K, L
and t0) and derived growth performance index (8 %) is provided in Table 1.
Using the estimated value of the average growth coefficient (K=0.62 year−1), the
longevity, tmax=3/K, was calculated as about 4.84 years and the von Bertalanffy growth
model for H. membranaceus is described as Lt=44.1[1−exp(−0.62(t+0.23))]. From
the length–weight relationships and the estimated L, the asymptotic weights (W) were
calculated as 1.55 and 1.36 kg, respectively, for males and females. These values are
significantly different (PB0.05) and suggest that there are differences between the
growth in weight of males and females.
The average instantaneous total mortality rate, Z, was estimated in 1995/96 (Fig. 3) for
fish ranging between 14.8 and 30.0 cm SL as 4.39 year−1 and the average instantaneous
natural mortality coefficient, M, was calculated as 1.20 year−1 (Table 1). The reliability
of the estimated M was ascertained using the M/K ratio because this ratio has been
reported to be within the range of 1.12–2.5 for most fishes (Beverton & Holt 1966). The
values of M/K were 1.88 and 2.02 in 1995 and 1996, respectively (Table 1). Thus, the
average instantaneous fishing mortality coefficient (F=Z−M) was calculated as 3.19
year−1 and the exploitation ratio (E=F/Z) as 0.72.
© 2001 Blackwell Science Ltd, Fisheries Management and Ecology 2001, 8, 37–45
GROWTH AND MORTALITY PARAMETERS OF CATFISH 41
Figure 2. Length frequency distribution output from FISAT for H. membranaceus caught in 1995 and 1996
with a superimposed growth curve.
Discussion
The maximum observed size (SL=33.0 cm) for H. membranaceus is among the relatively
large mochokid species found in the Sahelian waters (Entsua-Mensah, Osei-Abunyewa &
Palomares 1995; Machena 1995). Machena (1995) estimated the asymptotic length (L)
for Synodontis zambezensis (David & Poll) and Synodontis nebulosus (Peters) in Lake
Kariba as 32.6 cm TL and 14.7 cm SL, respectively, which further supports the assertion
that H. membranaceus is among the relatively large mochokid species found in the
sub-region.
Baijot & Moreau (1997) estimated that the range of 8 % mean values for some
important fishes in Africa ranged between 2.65 and 3.32, and considered these as low
growth rates. In this study, the 8 % mean estimate for H. membranaceus was 3.09 which
falls within this range; thus, the fish might also be considered to exhibit a slow growth
rate in Lake Volta. This slow growth rate in Lake Volta and in some similar water bodies
© 2001 Blackwell Science Ltd, Fisheries Management and Ecology 2001, 8, 37–45
42 P. K. OFORI-DANSON ET AL.
in Africa might be induced by important changes in the physical and chemical character-
istics of the waters, accentuated by persistent drought and poor rains over recent years.
The exploitation ratio of the purse seine for H. membranaceus, ranging between 14.8
and 30 cm SL, was beyond the expected optimal exploitation level, Eopt=0.5, indicating
over-exploitation by the purse seine. The estimated longevity (tmax) for H. membranaceus
is about 5 years indicating that it is short-lived. However, the fish caught were mostly less
than 2 years of age, implying that they are normally caught before they grow large
enough to contribute substantially to the stock biomass; this is indicative of growth
over-fishing. This was expected because the number of canoes has increased by about
300% between 1975 and 1995 (Coppola & Agadzi 1977; Ofori-Danson 1999). This is in
Figure 3. FISAT output of linearized length–converted catch curves for H. membranaceus caught from the
Yeji area of Lake Volta during 1995 and 1996.
© 2001 Blackwell Science Ltd, Fisheries Management and Ecology 2001, 8, 37–45
GROWTH AND MORTALITY PARAMETERS OF CATFISH 43
Table 1. Estimated population parameters of H. membranaceus caught in the Yeji area of Lake Volta
Parameters 1995 1996 Mean
L (SL, cm) 44.0 45.0 44.5
K (year−1) 0.68 0.55 0.62
t0 (years) −0.21 −0.26 −0.23
tmax (years) 4.42 5.45 4.84
8 % 3.12 3.05 3.09
M (year−1) 1.28 1.11 1.20
M/K 1.88 2.02 1.95
F (year−1) 2.40 3.98 3.19
Z (year−1) 3.68 5.09 4.39
E=F/Z 0.65 0.78 0.72
contrast with findings from the fish stock assessment programme carried out between
1969 and 1977 by the VLR&DP (FAO/UNDP 1979), which reported no sign of
over-fishing of the stocks and encouraged increases in fishing effort measured by the
number of canoes. It is apparent from the current results that this situation can no longer
be recommended for the fishery and that urgent management intervention is required.
The management implication from the results is that the fishery of H. membranaceus
in Lake Volta is faced with problems of growth over-fishing and economic over-capital-
ization because effort is increasing without a corresponding increase in catch. It is
possible that this situation may apply to the other commercially important fish stocks in
the lake (e.g. the Nile tilapia, Oreochromis niloticus L., Chrysichthys spp., Labeo spp. and
Schilbe spp.). There are simply too many canoes to support the production available.
Currently, fisheries management is solely in the domain of the state working through
the Directorate of Fisheries. It is apparent that this system of fisheries management,
where access and other development measures are regulated by the state, is now
inappropriate. There is therefore a need to devolve fisheries management to the local level
to compel fishermen to take greater responsibility for the sustainability and conservation
of the fisheries resource. This can be implemented initially through the adoption of a
community-based fisheries management (CBFM) approach (Yamamoto 1995; Pomeroy,
Pollnac, Predo & Katon 1996). The initial stage would involve the establishment of
community-based fisheries management committees (CBFMC), largely composed of
members from the fishing villages. The CBFMC would be designed specifically to
encourage fishing communities to discuss problems and propose solutions relating to
fisheries and the aquatic environment. In addition, the CBFMC must have some control
over their adjacent fishing areas, and have the ability to make and enforce their own
regulations. The main benefit of the CBFMC to the government is that the conservation
actions necessary to exploit the fishery resources on a sustainable basis become the
responsibility of the community. The Department of Fisheries would then work to
support the undertakings and needs of all fishermen. It is anticipated that ownership of
© 2001 Blackwell Science Ltd, Fisheries Management and Ecology 2001, 8, 37–45
44 P. K. OFORI-DANSON ET AL.
fish resources, access rights to them and adherence to the fisheries management process
would gradually be addressed by the CBFMC. In this regard, studies that will elucidate
the possibilities for adopting aspects of property rights in fisheries should be commis-
sioned for Lake Volta.
Meanwhile, a more practical means to arrest over-exploitation of the species and
contribute to the protection of the fishery as a whole would be the periodic prohibition
of fishing in certain areas. This may be achieved through the establishment of ‘lake
reserves’ to protect the spawning stock biomass and through the monitoring of their
effects as a management strategy. Unfortunately, preventing fishermen from fishing in
certain areas often has intangible benefits to them. Thus, it is necessary to address some
of the many social problems in the lake area (e.g. poverty, education, growing population
and employment) in order to win the support of fishermen and make changes in
behaviour possible. The areas to be delineated as ‘lake reserves’ should be chosen on the
basis of sound research and fishermen collaboration.
Acknowledgements
This work is part of a Ph.D. thesis supervised by Professor C.J. Vanderpuye, Head of the
Department of Oceanography and Fisheries, University of Ghana, Legon, Ghana. Our
sincere thanks extend to project coordinator, Mr L.I. Braimah and the technical staff of
the project ‘FAO/UNDP Integrated Development of Artisanal Fisheries (IDAF)’ based
at Yeji in the Brong Ahafo Region of Ghana for their field assistance.
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