Age, growth and mortality of Clarias gariepinus (Siluriformes: Clariidae) in the Mid-Cross River-Floodplain ecosystem, Nigeria

Okechukwu Idumah Okogwu

doi:10.15517/rbt.v59i4.3433

Abstract

Clarias gariepinus is a threatened highly prized species used for some elite ceremonies by the local communities. Artisanal fishers take advantage of this species annual breeding migration from the lower Cross River to the floodplain lakes in Mid-Cross River during the rainy season, and some migrant stocks are not able to spawn. Since there is a lack of information on this species population dynamics in the Mid-Cross area, this study aimed to evaluate the age, growth and mortality to support the development of effective management plans. For this, monthly overnight gill net catches (from 6 to 72mm mesh sizes) were developed between March 2005 and February 2007. Growth parameters were determined using the FiSAT II length-frequency distribution. A total of 1 421 fish were collected during the survey. The asymptotic growth (L(infinity)) was 80.24cm, growth rate (K) was 0.49/year while the longevity was 6.12 years. The annual instantaneous rate of total mortality (Z) was 2.54/year and the natural mortality (M) was 0.88. Fishing mortality (1.66/year) was higher than the biological reference points (F(opt) = 0.83 and F(limit) = 1.11) and the exploitation rate (0.66) was higher than the predicted value (E(max) = 0.64) indicating that C. gariepinus was over exploited in the Mid-Cross River-Floodplain ecosystem. Some recommended immediate management actions are to strengthen the ban of ichthyocide fishing, closure of the floodplain lakes for most of the year, restricted access to the migratory path of the fish during the flood period and vocational training to the fishers. In order to recover and maintain a sustainable harvest, I suggest that a multi-sector stakeholder group should be formed with governmental agents, community leaders, fishers, fisheries scientists and non-governmental organizations. These short and long term measures, if carefully applied, will facilitate recovery of the fishery.

Highlights

Clarias gariepinus is highly prized by indigenous consumers in Afikpo and nearby communities due to the size, tasty flesh and the absence of intramuscular bones
The annual instantaneous fishing mortality rate was higher than the biological reference points derived from the Patterson (1992) equation (Fopt = 0.83 and Flimit = 1.11), suggesting that C. gariepinus was over exploited in the Cross River
The estimated b value by the non-linear regression analysis of the length-weight relationship was significantly below three, which shows that the fish exhibits allometric growth pattern

Summary

Introduction

Clarias gariepinus is highly prized by indigenous consumers in Afikpo and nearby communities due to the size, tasty flesh and the absence of intramuscular bones. C. gariepinus like Chrysichthyes nigrodigitatus and other catfishes of the Cross River migrate seasonally to the floodplain lakes in the Mid-Cross to breed between May and October (Idodo-Umeh 2003) These migrant stocks are highly vulnerable to the gillnets of artisanal fishers and many of them do not make it to the lakes or back to the lower Cross. Artisanal fishers were believed to have minimal impact on fish abundance and diversity, recent studies have shown that they may be contributing significantly to the reduction in fish stock and diversity (King 2007) This is because over 90% of world fishers are artisanal and some of them employ unorthodox methods to catch fish. The use of ichthyocides (generic killers that kill both target and non-target species of all sizes) in the Cross River basin is

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