Njiru1, M., I.G. Cowx2, J.B. Okeyo-Owour3 & M. Muchiri4

1Kenya Marine and Fisheries Research Institute. (KMFRI). P.O. Box 1881,
E-mail: rmnjiru@yahoo.ac.ukor kmfkisu@net2000ke.com
2 University of Hull International Fisheries, Institute, Hull HU6 7EX, UK,
E-mail: i.g.Cowx@biosci.hull.ac.uk
3 School of Environmental Studies. Moi University, P.O. Box 3900 Eldoret, Kenya.
4 Moi University. Fisheries Department , P.O. Box 3900, Eldoret, Kenya.
E-mail: Mu-fish @net2000ke.com

Changes in population characteristics of Nile tilapia, Oreochromis niloticus (L.) in Nyanza Gulf of Lake Victoria, Kenya: What are the management options?

Length frequency data collected from commercial landings were used to estimate growth parameters (K, L¥), mortality (Z, F, M), growth performance index (f'), exploitation (E) and recruitment pattern in O. niloticus from the Nyanza Gulf of Lake Victoria, Kenya. The asymptotic length (L¥) had a mean (±S.D) of 58.78 ± 2.42 cm TL, K of 0.59 ± 0.05 yr-1, Z of 1.92 ± 0.64 yr-1, M of 1.00 ± 0.06 yr-1, F of 0.91 ± 0.12 yr-1, E of 0.49 ± 0.04 % and f' of 3.32 ± 0.02. Fifty percent (L50) entry into the fishery ranged from 26.18 ± 12.50 cm TL. Length at first maturity had a mean (±S.D) of 31.73 ± 1.86 for females and 34.56 ± 0.97 for males. Males were more abundant than females. Recruitment occurred throughout the year, with two peaks corresponding to the rainy season.

A comparison with previous studies in the gulf indicates that O. niloticus is caught at a lower mean size, K, Z, M, F, have increased and the fish is maturing earlier. These changes suggest a population that is stressed, probably by exploitation, but still the stock exhibits high growth performance and grows to a large size. Remedial measures to sustain the fishery include putting a ban on illegal fishing methods, limiting entry to the fishery, updating fisheries laws and regulations, encouraging farming to reduce pressure on the fishery and involvement of the community in fisheries management.