Acoustic estimates of commercial fish species in Lake Victoria: Moving towards ecosystem-based fisheries management

Data from the first series of lakewide fisheries acoustic surveys of Lake Victoria, East Africa, have been re-analysed according to current protocols. Surveys took place in February and August each year between 1999 and 2002. The primary aim has been to estimate the standing stock of Nile perch and dagaa, the main species taken in commercial fisheries on the lake. The results show that over the period of the surveys from 1999 to 2002 there was no significant trend in the standing stock of either species with time although there was a significant seasonal effect higher values in February as compared to August. Information from bottom trawls during the surveys supports these conclusions. The results have been considered in the context of a food web from which it is concluded that in order for sufficient food to be present for the Nile perch there must be a significant proportion of the decapod crustacean Caridina present. It is noted that this species can be estimated acoustically using multifrequency echosounders but is very difficult with the single frequency system used on the 1999–2002 surveys. Refinements in the methodology will permit the simultaneous assessment of several key components in the food web and open the way to effective ecosystem-based fisheries management.