Abstract:

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.