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

Abstract

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; R^sup 2^ = 0.58) while that of Marcusenius macrolepidotus was driven by water depth (F = 25.88; P = 0.04; R^sup 2^ = 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.