Study on fish poisoning in Lake Victoria

This document reports on a fish poisoning study conducted in Lake Victoria waters of Tanzania, Kenya and Uganda. The study was mainly by aquarium experiments and, to a smaller extent, by experimental fishing. An inventory of poisons used at the time of catching fish in the three countries was established through interviews and surveys along the lake shores. The findings of the study show that, among the riparian countries, Tanzania was the most affected in terms of the variety of poisons used and extent of the illegal fishing. The most commonly used poisons in Tanzania were the pesticides Thiodan and Diazinon. Poisonous plants were also used such as Euphorbia tirucalli and Tephrosia vogelli. Kenya followed Tanzania in fish poisoning but it was found from the surveys that pesticides were virtually not used in catching fish. Poisonous plants used in Kenya were Cassia fallacina, Cassia didymobotra and Albizia gummifera. There was no information from Uganda on fish poisoning. The concentrations used for pesticides were not clearly defined but about 2 litres of the pesticide concentrate were applied on an area of 500 metres long and 20-50 metres wide from the shores with varying depths. The concentrations of the plant poisons used were almost uniform for all plants used whereby 5 kg were applied on an area 90, metres long and 20-50 metres wide from the shores with depths of about 2 metres. From the aquarium experiments MLC and LC sub(50) values of the studied poisons were estimated when applied on Alestes, Oreochromis nilotica and Haplochromis sp. Susceptibility of different sizes of Oreochromis nilotica was established and it was found that smaller sizes were more susceptible. Susceptibility of different species varies with the applied poisons. In general Cassia fallacina is more effective than Albizia gummifera and among the pesticides Thiodan is more effective than Diazinon. Noticeable differences were observed between poisoned and non-poisoned fish in biological and organoleptic characteristics. Each poison had a generally different pathological manifestation on different fish species. Among the processing techniques, smoking and salting had the greatest buffering effect against deterioration due to poisoning.