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PAGES Africa
Pan Africa Workshop, July 2004
Environmental changes and their influence to the fish stocks in Lake Victoria- Tanzanian waters.
Dr Oliva C. Mkumbo1, Mr. Charles Ezekiel1 and Dr Ian G. Cowx2
1Tanzania Fisheries Research Institute, Mwanza Tanzania
2University of Hull, Hull U.K.
Marked changes have occurred in Lake Victoria ecosystem, both in the abiotic and biotic parameters. Changes in the physical-chemical parameters have also resulted to profound changes in the fish stocks – their diversity and abundances and consequently to the stability of the ecosystem. Patterns of variation on Abundance and distribution of stocks in Lake Victoria, Tanzania waters presents an example of the influence of environmental factors on fish stocks.
Mean biomass indices (Figure) from bottom trawl surveys varied in different quarters. A general pattern of low biomass indices in the first quarter with increase to a peak in the third quarter followed by a decline to the fourth quarter was observed. The pattern was repeated over two years sampled (1999&2000). The first quarter had the lowest biomass estimates in all sampled areas of the Tanzanian waters over the sampled period.
Figure: Mean Biomass estimates from bottom trawl surveys per square and quarter sampled in the Tanzanian waters (95% C.L. attached)
The third quarter in a year is generally the period when Lake Victoria is well mixed, while the first quarter is the period of thermal stratification (Talling, 1966; Akiyama et al., 1977, Wanink & Kashindye, 1998, Mkumbo, 2002). Mean oxygen concentrations were above 5 mg L-1 as deep as 60 m with minimum temperature variations from surface to bottom waters in August/September 2000. Conductivity decreases during the third quarter especially in shallow waters influenced by the lack of run-off during the dry season. This in turn limits algae blooms and increases water transparency, consequently favouring a wide distribution for the Nile perch stock and its availability to the bottom trawl. In the first quarter, oxygen concentration dropped to 0.28 mg L-1 with the thermocline forming at around 40 m deep, so it was unlikely for Nile perch to be distributed in the bottom waters.
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