Effects of Lake Naivasha (Kenya) closure to fishing on Tilapia zillii (Gervais 1848) population characteristics

Abstract

Fishing activities on Lake Naivasha were banned for a year in 2001 following a rapid decline in fish catches attributed to overexploitation and
environmental degradation. Studies were done to estimate growth, mortality, and exploitation rate of Tilapia zillii from length frequency data
collected in January to December in 1995, 2002 and January to May 2003 from commercial catches in Lake Naivasha to evaluate the effect of the fishing ban. Experimental gillnetting studies were done during 2001 closure period. Data analysis was done using FISAT (FAO-ICLRAM Stock Assessment Tool) software. The asymptotic length (L„V) was estimated at
28.93, 29.00, 37.08 and 28.90 cm TL, growth curvature (K) of 0.56, 0.83, 0.36 and 0.60 yr-1, total mortality (Z) of 3.50, 5.14, 1.22 and 4.56 yr- 1 and exploitation rate (E) of 66, 71, 33 and 72 % in 1995, 2001, 2002 and 2003 respectively. The study showed remarkable changes in population characteristics immediately after the lifting of the ban in 2002. The annual catches also rose from 0.9 t in 2000 before fishing ban to 21 t in 2002
after lifting of the ban. The increase in L„V, catches and a reduction of K,
F and E immediately after lifting of the ban revealed that the closure of the fishery had a positive impact on the T. zillii fishery. However, the same parameters and catches in 2003 two years after lifting of the ban were at a level indicative of intense exploited again. Remedial measures to sustain the fishery are recommended. These include: limiting entry to the fishery, eradication of illegal fishing methods, reduced environmental  degradagation, encouraging alternative livelihood and involvement of the community in the fisheries management.

Keywords: environment degradation, co-management, population characteristics