Fisheries Resources Research Institute (FIRRI), P.O. Box 343, Jinja, Uganda. E-mail: firi@infocom.co.ug

The distribution, abundance and population structure of the fish stocks in the Uganda portion of Lake Victoria with options for their management

During the last 30 years, physical, chemical and biological changes 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. The only fish stock assessment exercise in Lake Victoria was from 1969 to 1971, before the introduction and establishment of the Nile perch.

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 Nile perch, Lates niloticus (L.), 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 Nile tilapia, O. niloticus (L.), 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 185.9 kg hr-1, of which 89.4% was L. niloticus. The relative abundance of the fish caught decreased with increasing water depth from 229.0±38.1 kg hr-1 for all fish species in the 4-10 m depth zone to 5.0 kg hr-1 at 50-60 m depth zone during the survey period 1997-2000. From the swept area method, the abundance index for all fish species living within 3-m column of water from the bottom 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. Only 29.3% of 121,000 tonnes estimated for the bottom dwelling 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 90,000 tonnes is Nile perch suggesting cropping of immature fish.

During 1997-2000, size at first maturity for L. niloticus was estimated at 21.5 and 22.5 cm total length for males and females respectively, while 50% maturity was 27.1 and 26.8 cm total length. Length at infinity (Loo) for L. niloticus was 256 cm total length and the growth rate (k) was 0.292 year-1. Caridina nilotica was generally the most important food item for L. niloticus in all habitats, constituting 49.7% and 45.6% occurrence during 1999 and 2000, respectively.
During the lakewide bottom trawl survey of 1969-1971, haplochromine cichlids were the most abundant (83% by weight) while Lates niloticus catches were insignificant (<0.1%). A mean catch rate of 797 kg hr-1 was estimated for waters between 4 m and 30 m deep during 1969-1971. However, haplochromine abundance in the trawl catches declined from 91.4% in 1981 to almost zero in 1985, while the contribution of Lates niloticus increased from 5% to 96% during the same period. The mean catch rates for all fish species combined declined from 595 kg hr-1 in 1981 to 355 kg hr-1 in 1983 and to 155 kg hr-1 in 1985. During the survey of May 1993 to October 1997, Lates niloticus contributed 96.5% of the total catch by weight.

The paper recommends that measures be put in place to reduce or stop capture of immature fish, human activities that lead to reduction in suitable water habitat for fish should be minimised, and regular surveys should be continued to monitor possible changes of the fish stocks in the lake.