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2 Acta Ichthyologica et Piscatoria

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Gillnet selectivity for the north African catfish, Clarias gariepinus (Actinopterygii: Siluriformes: Clariidae), from the Upper Okavango Delta, Botswana

100%

Bokhutlo T. , Mosepele K.

nr 2

Background. Successful fisheries management requires estimation of gillnet selectivity for optimum exploitation of the resource. In the Okavango Delta, no study has assessed the selectivity of gillnets for Clarias gariepinus (Burchell, 1822) which is an important component of both the subsistence and the commercial gillnet fishery catch. The aim of this study was to simulate the harvesting pattern of the commercial gillnet fishery and provide gillnet selectivity parameters for C. gariepinus. This will help fishery managers with information on the appropriate mesh sizes needed for sustainable utilisation of the catfish resource. Materials and Methods. Monthly gillnet sampling was conducted over a period of 8 years from 2001 to 2009. The SELECT method was used to estimate gillnet selectivity for C. gariepinus using catch data from four mesh sizes (73 mm, 93 mm, 118 mm, and 150 mm). Results. The 93 mm and 118 mm mesh sizes were the most efficient when capturing C. gariepinus accounting for 44.6% and 21.9% of the total catch, respectively. Mean fish length increased with increasing mesh size and was significantly different between mesh sizes (P < 0.001). The modal fish lengths for the four mesh sizes were estimated at 41.63 cm, 53.23 cm, 66.35 cm, and 85.54 cm in order of increasing mesh size. Conclusion. The modal fish lengths for meshes 93 mm and 118 mm are greater than the size at maturity and therefore the current commercial gillnet fishery which uses 100 mm mesh size may not be a threat to the C. gariepinus population in the Upper Okavango Delta. Future studies should aim to conduct fishery dependent selectivity studies in the lower Okavango Delta to determine if selectivity changes with location and gear. Moreover, a comprehensive age-based stock assessment is required to establish the mesh size that optimizes yield without adversely depleting the spawning stock biomass.

Dynamics of the feeding ecology of selected fish species from the Okavango River delta, Botswana

80%

Mosepele K. , Mosepele B. , Wolski P. , Kolding J.

nr 4

Background. This study contributes to the understanding of the potential impact of changes in flooding patterns and (potential) fish production as a consequence of upstream developments in floodplain systems. Therefore, stomachs of eight fish species from the Okavango Delta were analysed to evaluate the feeding ecology of floodplain fish (and the effect of seasonal flooding), using the delta as a case study. Materials and Methods. In total, 2101 fish stomachs of eight species, representing six families, were collected (in all seasons) from the delta using experimental fishing nets, from 2004 to 2009. Frequency of occurrence, Levin’s diet breadth index, Pianka’s overlap index, trophic levels, and Bray–Curtis similarities were used to evaluate feeding preferences. Detrended correspondence analysis was used to study temporal variations in diet. Multiple linear regressions were used to determine the influence of flooding on diet. ANOVA and MANOVA were used to determine the level of significance among variables, while LSD post hoc analysis revealed the source of significance. Results. Cluster analysis and Pianka’s index highlighted inter- and intra-specific competition for food among different species and age classes, ANOVA highlighted dynamic changes in inter- and intra-specific trophic level partitioning, while detrended analysis showed that the terrestrial environment is subsidizing the aquatic environment. Regression analysis showed that Schilbe intermedius diet was driven by discharge (F = 7.03; P = 0.045; R2 = 0.58) while that of Marcusenius macrolepidotus was driven by water depth (F = 25.88; P = 0.04; R2 = 0.93). Conclusion. The terrestrial–aquatic ecotone is important in fish growth of seasonal floodplains. Energy uptake is optimised through cannibalism to ensure species survival. Furthermore, species inter-relations are dynamic due to variations in food availability driven by seasonal flooding, which shortens and lengthens the food chain periodically. This dynamic relation is pronounced at low floods when predation and competition increases within the fish community. The evidence from this study has shown that predation, cannibalism, inter- and intra-specific competition are regulating factors in floodplain fish communities, driven by seasonal flooding.