Spatiotemporal variation in fishing patterns and fishing pressure in Lake Victoria (East Africa) in relation to balanced harvest

Balanced harvest (BH) refers to applying moderate fishing pressure across a broad range of species, trophic levels (TL), stocks, or sizes in an ecosystem in proportion to productivity (gross production per biomass unit) or production (total cumulated biomass over a given period) instead of exerting pressure on particular taxa or sizes. Both modelling and empirical studies have shown that BH can lead to higher fish yield than selective fishing, with minimal changes to ecosystem structure and function. This concept has stimulated considerable debate, and one question that needs to be addressed is how close or far fisheries are from being balanced. Here, we investigated whether fishing on Lake Victoria (East Africa), the world’s second largest inland fishery, is consistent with BH, and whether there is any link between past and present fishing patterns and life history traits of major commercially-exploited species: Nile perch (Lates niloticus) and Nile tilapia (Oreochromis niloticus). We found exploitation rates to be relatively higher in high TL groups than low TL groups; however, the overall fishing pattern has been consistent with BH until recently (from 2000 onwards). Exploitation rates (E) above 50% of annual production (“overexploitation”) were observed for large Nile perch, Nile tilapia, and other less productive demersal groups (for example, catfishes, squeakers, and lungfish). Unexpectedly, there was no evidence of significant reduction in body size of Nile perch associated with this fishing pattern, which may be attributed to limited compliance of minimum size regulations. On the other hand, we found a significant reduction in body size of Nile tilapia, which tended to be associated with high fishing pressure. We conclude that the fishery is currently inefficiently utilized in terms of food energy value, whereby groups with highest production (including small Nile perch) are underexploited (E < 10%). However, moving towards BH in the case of Lake Victoria, and generally in many inland fisheries, will require a paradigm shift both in mindset and the law.