Management of the Nile tilapia (Oreochromis niloticus (L.)) fishery in the Kenyan portion of Lake Victoria, in light of changes in its life history and ecology

Abstract

This study reports on the population parameters, catch distribution and feeding ecology of Nile tilapia (Oreochromis niloticus) from bottom trawls and commercial catches obtained in the Kenyan portion of Lake Victoria during 1997-2006. The population parameters were analysed using the FAO-ICLARM stock assessment tool (FISAT). The fish biomass and the food ingested by the fish were estimated using the swept area and point methods, respectively. Immature fish comprised ഠ70% of the total fish population. The asymptotic length, maximum weight, maximum age, exploitation rate (E) and length at 50% maturity of Nile tilapia have decreased, whereas the growth curvature and fishing mortality have increased. The commercial catches increased from 13.93 t in 1997, to 23.70 t in 1999, decreasing thereafter to 18.73 t in 2005. The bottom trawl catches increased from 46.90 kg ha -1 in 1997, to 401.48 kg ha -1 in 2000, decreasing thereafter to 15.57 kg ha -1 in 2006. The major food items ingested by the fish were algae, insects and other fish. Population parameters, and the catch and diet of O. niloticus, have changed over the years in Lake Victoria. The population characteristics suggest a population under stress, attributable to intense catch exploitation. Even under intense exploitation (E = 0.68), however, the mature fish constituted ഠ30% of the population. The commercial catches are still high, indicating a very resilient fishery. Nevertheless, despite this resilience, the future of Oreochromis fishery is threatened by increased fishing capacity in the lake, and there is need to re-evaluate the effectiveness of current fishery management measures, with the goal of possibly adopting new measures. Enactment of new fishery policies also should provide for co-management to enhance the management process. Furthermore, there is a need to reduce fishing capacity and illegal fishing methods, and to seek alternative livelihoods for lake fishers and other stakeholders.

Recurrent changes in cichlid dentition linked to climate‐driven lake‐level fluctuations

“…This observed change to unicuspid FR teeth and, eventually, an (almost) entirely unicuspid oral dentition is known to occur in large individuals of several other Oreochromis species, mainly in sexually mature males (Trewavas 1983). Most cichlids display dietary shifts as they grow (Njiru et al 2008), and these are often accompanied by changes in different aspects of their trophic morphology (Streelman et al 2007). This study is, to our knowledge, the first to document this ontogenetic change in cichlid oral dentition with geometric morphometrics, and it reveals that the changes occur gradually, at least in O. hunteri.…”

“…Most cichlids display dietary shifts as they grow (Njiru et al. ), and these are often accompanied by changes in different aspects of their trophic morphology (Streelman et al. ).…”

Molecular genetic diversity and differentiation of Nile tilapia (Oreochromis niloticus, L. 1758) in East African natural and stocked populations

“…This might be the case for the L. Kyoga population. For example, although O. niloticus boosted the capture fisheries in the Lake Victoria basin (Lakes Victoria and Kyoga) following introductions, the species was subsequently overexploited between the 1970s and 80s [53,54]. This was reflected in the dramatic decline of the stock sizes and increased fecundity, which are clear indicators of overfishing [54].…”

“…For example, although O. niloticus boosted the capture fisheries in the Lake Victoria basin (Lakes Victoria and Kyoga) following introductions, the species was subsequently overexploited between the 1970s and 80s [53,54]. This was reflected in the dramatic decline of the stock sizes and increased fecundity, which are clear indicators of overfishing [54]. The low diversity in River Nile could be linked to low gene-flow connectivity with other water bodies due to hydroelectric power dams that have been constructed along the river (the upper Nile of the Ugandan side), which increases the effect of genetic drift.…”

The responses of Nile tilapia Oreochromis niloticus (Linnaeus, 1758) in Lake Wamala (Uganda) to changing climatic conditions

“…; Njiru et al . , ). The result is a knowledge gap on how Nile tilapia, a commercially important species in both capture fisheries and aquaculture, responds to the conditions created by climate variability and change.…”