A report of the gillnet survey in the Ugandan waters of Lake Victoria under IFMP (January to March 2006)

Under the Implementation of the Fisheries Management Plan (IFMP) for Lake Victoria Result area 4, quarterly gillnet surveys are undertaken to monitor changes in fish stocks and environmental parameters in the shallow nontrawlable areas of the lake For purposes of monitoring surveys, the Ugandan sector of Lake Victoria is divided into 3 zones as shown in Figure 1. During the second quarter of APE2, two gillnet surveys were undertaken in zones 1 and 3 in February and March 2006 respectively. The purpose of the surveys was to monitor changes in the fish stocks and their biological characteristics, water quality, algal dynamics and invertebrate communities; as detailed in the various sections of the report. The surveys followed those conducted in November and December 2006 in the same zones. Results of the surveys showed that the number of fish taxa was higher in the near-shore fleets (0-100m) decreasing towards offshore. The near-shore areas were also associated with high primary productivity and hence secondary production to which Caridina and other invertebrates are part. These organisms are an important source of food for the fish and this may partly account for the high number of fish species recorded in this area of the lake. It was also observed that although Nile perch was the most dominant fish species recorded in all the stations during the surveys, haplochromines, Brycinus sadleri, Brycinus jacksonii Oreochromis niloticus and various mormyrid species contributed significantly to the fish biomass. The presence of many fish species and their coexistence with the predator, Nile perch is attributed to the presence of macrophyte cover and rocky habitats which serve as refugia in the shallow inshore habitats of Lake Victoria. In addition, the vegetated habitats are an important source of food for the fishes. As reported in macro-invertebrate studies, big populations of Caridina and other invertebrates were recorded among macrophyte beds. Caridina is an important source of food for juvenile Nile perch and other fish species so are the other invertebrates especially chironomid larvae, odonata nymphs and molluscs. Resurgence in haplochromine cichlids was observed during the surveys. The presence of haplochromines cichlids in all the sites especially Thruston Bay where it ranked the second by percentage contribution in number, is evidence of the recovery of this group of fishes which had declined largely due to predation by L. niloticus. Caridina nilotica has also increased in biomass and is a major component of the Nile perch diet. This could have reduced predation pressure on the haplochromines by Nile perch and has possibly contributed to recent resurgence in haplochromines cichlids in the lake in the shallow nontrawlable areas of the lake Rastrineobola argentea was found to be an important prey item for Nile perch and other fish species such as Clarias gariepinus. Measures should therefore be taken to ensure sustainable harvesting of Dagaa so that there is enough left to sustain the fishery of Nile perch and other species.

Possible fish stock size assessment and available production survey as developed on Lake Kariba

This paper is an outline of methods practically useful for the evaluation of ichthyomass, fish abundance, available production and yield in lakes and rivers. Terms and concepts are reviewed, and difficulties stemming from the use of "predetermined" mathematical models are discussed. Sampling with toxicants in blocked-off areas was found to be the most practical method and is described in detail. For the total estimation of ichthyomass the spatial ranges of fish distribution must be determined; the results of echo-sounding surveys for horizontal, vertical, topographical, seasonal and diel fish distribution are given. Some of the most important methods for computing available production are listed and applied to Lake Kariba as an example. In particular, a method based on the balance between the main predator and prey species is reviewed. The ecological production survey concept is finally stressed as applied to multispecies fish stocks.

The role of man on the changing fishery potentials of Lakes Victoria and Kyoga

The traditional lucrative fisheries of Lakes Victoria and Kyoga were based on similar multispecies itchthyofaunas. Man's activities on and around these lakes have both directly and indirectly assisted to modify the natural trends and components of the commercial fisheries. The exotic L.niloticus and O.niloticus together with the native R. argentea form the major component of current commercial landings. The total catches are higher but it is not yet clear whether the increase would endure given the fragility of predator/prey systems. The trophodynamics are still modifying and it is not certain how ecosystem function would be influenced. It is, therefore, prudent and desirable to undertake appropriate research investigations in order to guide the multiobjective activities of man on these lakes.