Current distribution, abundance and population structure of the fish stocks in Lake Victoria and options for their management

Abstract

During the last 30 years, changes (physical, chemical and biological) have occurred in Lake Victoria and, consequently, the fish stocks of the lake have been modified. The extent of the modification of these fish stocks is not fully known. A total of 793 hauls (of 30 minutes duration) were taken during experimental bottom trawl surveys in the Uganda sector of Lake Victoria during the period November 1997 to December 2000 to estimate composition, distribution and abundance of the major fish species in waters 4 to 60 m deep. Seventeen fish groups were caught with Lutes niloticus (Nile perch) constituting the largest biomass (87.8%) followed by Nile tilapia, Oreochromis niloticus (L.) (8.6%). Haplochromines and L. niloticus occurred in all areas sampled while O. niloticus and other tilapiines were restricted to shallower waters (<20 m). In waters deeper than 40 m, fish was rarely recorded during bottom trawling: the echosounder indicated no fish at the bottom: the fish signals were concentrated in the depth zone 10-20 m from the surface. The mean trawl catch rate in the zone where artisanal fishermen operate (i.e. in waters <30 m depth) was 195.2 kg hr^-1, of which 87.9% was L. niloticus. Species diversity and relative abundance of the fish caught decreased with
increasing water depth from 232.2±38.7 kg hr^-1 for all fish species in the 4-10 m depth zone to 7.7 kg hr^-1 at 40-50 m depth zone during the survey period 1997-2000. From the swept area method, the abundance index lor all fish species in the 4-40 m depth zone in the Uganda portion of the lake was estimated at around 142,000 tonnes during the period 1999-2000. The abundance index for L. niloticus and O. niloticus were estimated around 121,000 and 15,000 tonnes, respectively, during the period 1999-2000; 307,000 tonnes was estimated for L. niloticus in the Uganda sector of the lake. Only 29.3% of 121,000 tonnes estimated for Nile perch during 1999-2000 was for mature fish (>50 cm total length) and the rest were juveniles. Estimated yield from the Uganda sector is around 107,000 tonnes annually, of which about 72,000 tonnes is Nile perch suggesting cropping ot at least 40,000 tonnes of immature fish. This calls for urgent management measures.