The life history characteristics of introduced Nile tilapia (Oreochromis niloticus L.) in Lake Victoria, including, sex ratio, fecundity, reproduction, weight‐length relationship and body condition were studied and compared with those of other populations. Samples were collected by trawling and seining in the Kenyan sector of Lake Victoria between June 1998 and December 2000. Males predominated over females (sex ratio 1.42 : 1 : 00). O. niloticus spawned throughout the year but with a peak between December and June. Length at first maturity was (mean ± SD) 30.81 ± 0.09 for females and 34.5 ± 6 0.48 for males. There was little seasonal variation in relative condition, which ranged from 0.92 to 1.05 in males and 0.94 to 1.07 in females. Gonadosomatic index (GSI) was low during the postspawning period (July to October) and high during the protracted breeding period (December and June). Fecundity ranged from 905 to 7619 oocytes for fish of 28 to 51 cm total length (TL) respectively. The relationships between fecundity (F) and total length (L), weight (W) and ovary weight (OW) were: F = 8.159L1.53, F = 96.269W0.4504, F = 1806 + 39.4OW. The slope b of the weight‐length relationship was 3.08–3.32 for males and 3.07–3.22 for females. Growth was allometric in both cases and was significantly different from the expected value of 3. The life history strategy of O. niloticus is discussed in context of environmental changes occurring in the lake.

Unlike other lakes in tropical regions, Lake Naivasha exhibits low fish species diversity. The fishery is based on introduced fish species that are not native to the lake. The lake ecosystem is very fragile, as reflected in its fluctuating fish catches. The fishery almost completely collapsed in 2001, due partly to mismanagement and ignorance about needed conservation measures. The Kenyan government, like many governments around the world, recognized the need to involve fishers and other stakeholders, in order to ensure better management of the lake fisheries resources. Also referred to as collaborative management (co-management), this management approach provides the opportunity to share responsibility for managing the fishery resources between the government and the community. A total ban on fishing was imposed in February 2001, lasting for 1 year, to allow lake fish stocks to recover. This study analyses the co-management process for Lake Naivasha, and discusses the role played by the community during the fishing ban, as well as the successes and challenges encountered, and the tribulations and perceptions of the fishers regarding the new management strategy. It also comprises the first documentation of community participation in fisheries management begun for Lake Naivasha in the year 2001.

No abstract

In their adaptation to the rapidly shifting environment of the African Great Lakes, small pelagic fish appear to be the most successful fish species. This paper examines the fecundity, size at maturation and the growth and population parameters of Rastrineobola argentea in Lake Victoria. R. argentea may have a low absolute fecundity of a few thousand eggs, but its relative fecundity by weight is enormous: 70 times higher than for L. niloticus and almost 4,000 times that of the tilapia species O. niloticus and O. esculentus. Its food consists mainly of zooplankton, which is superabundant in the environment and for which there is not much competition. In response to increased predation and fishing, the species has reduced its size at maturity as well as its maximum size. Population parameters show a typical r-selected life strategy. The reproductive potential and diet alone do not seem to have enabled R. argentea to thrive as larval densities of R. argentea can be significantly related to a combination of abiotic factors like water conductivity, temperature, Secchi depth and dissolved oxygen concentration.

Studies were conducted on reproductive characteristics of Oreochromis niloticus from 1998 to 2000. The results were combined with published work on growth parameters of O. niloticus from 1985 to 1999 in order to establish the current survival strategies exhibited by O. niloticus in the Nyanza Gulf of Lake Victoria. The study revealed that size at maturity had decreased concurrently with increasing fishing mortality. Observations on reproductive effort point to a fish species under stress. It is observed that the behavioural change in O. niloticus is not due to size selective predation but due to size selective exploitation. Indications that O. niloticus in the Nyanza Gulf of Lake Victoria allocates more energy for reproduction than for somatic growth (i.e. increased turnover rate) are multiple. It is concluded that O. niloticus in the Nyanza Gulf exhibits an ‘r’-selected life history strategy in order to survive stressful conditions.