Impact of anthropogenic pollution and artisanal fishing on the population of Tilapia spp. Oreochromis niloticus and Oreochromis amphimelas in Lake Manyara, northern Tanzania

Lakes are among the main sources of protein and livelihood to huge communities of rural people, and some of them house endemic fish species. The livelihood of about 200,000 rural people depends on Lake Manyara resources which also houses a population of the endemic and endangered fish, the Manyara Tilapia (Oreochromis amphimelas). Despite this importance, fishery in the lake is nearly under open access and it is not known how this has affected fish stocks. The lake is also under strong influence from overgrazing and poor farming practices in the highlands and adjacent areas which are polluting the lake. However, it is also not known how this has affected the growth and survival of fish in the lake. We assessed the impact of artisanal fishing and anthropogenic pollution on the stock of Manyara Tilapia and Nile Tilapia in Lake Manyara. We found that fish stocks in the lake are under heavy fishing pressure. Nearly all harvested Nile Tilapia were immature, and the majority of Manyara Tilapia were first time spawners caught at the length of their first maturity. This prevented the fishes from spawning at least once in their lifetime. Anthropogenic pollution has also hypereutrophied the lake and degraded the ecological quality for growth of Tilapia fish. Therefore, urgent site-specific mitigation measures and conservation actions are required to safeguard community livelihood and continued existence of the endangered Manyara Tilapia.