Assessment and management of Nile perch (Lates niloticus L.) stocks in the Tanzanian waters of Lake Victoria
Lake Victoria contributes more than 60% of the total fish yield in each of the respective riparian countries, Kenya, Uganda and Tanzania. Nile perch Lates niloticus contributed more than 60% to the total yield from the lake. Although accused of causing ecological instability of the ecosystem due to its predatory effects, the fishing industry became socially and economically dependable on the Nile perch. In the mid 1990s signs of overfishing were observed and concerns for the sustainability of the fishery were raised. To properly manage a fishery, knowledge of the factors that regulate the dynamics of the stock(s), their abundance and distribution is essential. Unfortunately data on the status of the Nile perch stocks are inadequate. This study was conducted on the Tanzanian part of Lake Victoria from 1997 to 2001 to address this problem. Reference is also made to the Kenyan and Ugandan national waters for comparison. Bottom trawl and catch assessment surveys were conducted to assess the status of the stocks. Abundance estimates and distribution patterns were determined. Current exploitation levels and practises were analysed and linked to growth and mortality, feeding and reproductive characteristics of the stock to determine status. Key environmental parameters were investigated and linked to the variations in the biological aspects and distribution patterns observed. There was an indication of reduced anoxic problems in the offshore deep waters and signs of improvement in the eutrophic state of the lake. Mean oxygen concentrations in the waters sampled varied from 8.02±0.73 mg L⁻¹ in the surface waters to 3.2±4.36 mg L⁻¹ in the bottom waters of 68 m deep, while Secchi disk readings at stations of 5-10 m depth ranged from 0.84±0.3 m in November to 1.9±1.02 m in August/September and in offshore waters of 50-6Om depth the readings were 3.08±0.62 m in February to 5.52±1.7 m in August/September. Distribution patterns of fish were highly aggregated but variable and were greatly influenced by seasonal patterns of oxygen and temperature, while reproduction and recruitment were related to rainfall patterns. Using the swept area method, biomass was estimated at 306,000 t for the Tanzanian waters and around 620,000 t for the whole lake, with a mean density of 9.87 t km⁻² and 10.56 t km⁻² respectively. Very high fishing mortality (1.55 yr⁻¹) and exploitation rates (0.84) were estimated using an L∞ of 218 em TL and a growth constant (K) of 0.16 estimated during the study. Excessively high fishing effort was observed in the 2000 frame survey while catch compositions reveal high dependence on juveniles for the Nile perch fishery. The size at first maturity was at 54.3 cm TL (1.6-yr.) and 76.7 em TL (2.5 yr.) for males and females respectively. About 83% of the catch survey data were below size at first maturity for males and 99% for females. Bottom trawl data (88% juveniles) suggested high recruitment in the stock. However the models indicated unsustainable exploitation of the fishery. A reduction of exploitation rate by 50% and increase of size at capture for optimum yield is recommended. The dominance of juveniles in the catch with the current yields (estimated at 138 323.85±6 229.14 t) higher than the sustainable yield (calculated at 108941.9 t yr⁻¹, using Cadima's formula) demands immediate management initiatives. Co-management is singled out as the most effective option for a functional system to implement control, monitoring and surveillance strategies within management process. With dynamic systems within the stocks, the environment as well as socioeconomic influences, and with continuous monitoring, adaptive and precautionary management strategies are recommended. Without reliable catch trend data it is difficult to confidently make predictions. The need to have a well-structured catch assessment survey system for reliable catch statistics is recommended. Priority areas to further research are also identified.