Prediction of regulation effects on natural biological rhythms in south‐central African freshwater fish

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AbstractFish in south‐central African freshwaters are governed by biological rhythms. First, these are associated with the markedly seasonal summer annual rainfall when breeding takes place with an initially high mortality among newspawned juveniles. Second, a longer term rhythm of annual rainfall variability occurs in cycles up to or more than a decade. These cause water levels to vary from exceptionally high levels to near desiccation in endorheic systems.Studies of the shallow endorheic lakes Mweru Wa'Ntipa (Zambia), Ngami (Botswana), and Chilwa (Malawi) are cited showing that species diversity rapidly increases by colonization from adjacent ‘inocula’ (relict communities surviving in more permanent nearby water bodies, e.g. deep pools in perennial streams) when filling starts. During the drydown phase of the cycle, diversity decreases through lack of annual breeding environment and diminution of water quantity and quality. A few, usually three, species remain longer which, being physiologically hardy or showing phenotypic plasticity, are tolerant of higher salinity, temperature, etc., conditions associated with dry‐out. Despite the lakes' wide geographical separation these taxa in every case are a single species of the tilapia, genus Oreochromis, the catfish Clarias gariepinus, and the minnow Barbus paludinosus, which build up large populations valuable to local fisheries.Evidence from the Pongolo River, Zululand, regulated by a large dam, shows that fish of downstream endorheic lakes react similarly to artificial water deprivation, with mainly the same taxa persisting. Managed water releases timed to coincide with breeding rhythms largely restore the conditions which previously prevailed. Data from Zambezi impoundments indicate conversely that reaction of previously riverine fish to large man‐made lakes is for previously sparse lentic taxa like Oreochromis to proliferate greatly while lotic forms like Labeo are reduced in number.Since fish stocks are a valuable natural resource, such basic scientific results may be applied with great advantage. The collaboration between engineer, biologist, and management authority can minimize adverse environmental effects and predict better opportunity cost assessments of a projected river manipulation.