Pelagic food web as the basis of fisheries in Lake Tanganyika: A bioenergetic modeling analysis

Abstract

Fisheries in Lake Tanganyika, East Africa, are mainly based on two predominantly planktivorous clupeids (Stolothrissa tanganicae and Limnothrissa miodon) and a centropomid predator (Lates stappersi), caught with lift nets, purse seines, and beach seines by traditional, artisanal, and industrial fishers. The biological basis and sustainability of the present fisheries were assessed in a comprehensive project “Research for the Management of the Fisheries on Lake Tanganyika” in 1992–1998. Production in the whole lake was estimated for the entire pelagic food chain leading to the commercially important fish species. Preliminary calculations based on a constant production efficiency suggested that while the crustacean zooplankton production was sufficient to maintain the estimated planktivorous fish production, the food requirements of piscivorous fish exceeded the production of potential prey. Here, the food consumption by pelagic fish was estimated with a bioenergetic model using actual diet data for different size classes. In order to reveal potential differences in the food web sustaining the fisheries, separate calculations were made for different parts of the lake, which harbor different fish community compositions and size structures. According to the bioenergetic calculations, the food requirements of the planktivorous fish were a reasonable fraction (25–38%) of the zooplankton production. In contrast, very high predation pressure was indicated on shrimps (73–104%), and especially on prey fish (>100%), suggesting that the total biomass of the prey planktivores had been underestimated, or that the predatory fish biomass was overestimated. Annual catch of Stolothrissa tanganicae was 18–35% of estimated production in individual countries and 25% in the whole lake. For Limnothrissa miodon , the corresponding ratio was moderately low (19–22%) in Tanzania and Zambia, but high (55–61%) in Democratic Republic of Congo and Burundi; in the whole lake, the exploitation rate was 30%. For Lates stappersi , the catch/production ratios were very high (76–112%) in all parts of the lake, and even the lakewide average exploitation rate was as high as 94%. These figures suggest that the present clupeid fishery is on a sustainable basis, while the Lates populations are clearly overexploited.