No abstract

This paper discusses the status of tilapiine species both native and those introduced into Lakes Victoria and Naivasha. In Lake Victoria, Oreochromis niloticus, O. leucosticus, Coptodon zillii and Coptodon rendalii were introduced. Establishment of non-native species was associated with declines in the populations of the native O. esculentus and O. variabilis. Oreochromis niloticus dominates the commercial fishery, but populations of the other introduced species persist. Predation by Nile perch (Lates niloticus), overfishing, lake pollution and eutrophication have impacted negatively on the fish stocks. In Lake Naivasha, O. niloticus, C. zillii and O. esculentus were introduced. These species have also declined in catches due to the deteriorating lake conditions and effects of overfishing. Oreochromis leucostictus and O. niloticus dominate fishery catches, whereas C. zillii is rarely caught. The biological and population parameters of the tilapiines have shown great variations over time. O. niloticus and O. leucosticus attain early maturation. These changes are discussed as adaptive characteristics in response to intense fishing and changing ecological conditions. This therefore, calls for a need to improve on the management measures applicable in the two lake ecosystems, such as fishing effort control, pollution control and protecting the surrounding wetland from degradation.

Samples (6404) of Nile perch were collected monthly from commercial catches between June 2014 and June 2015. The total length (cm), weight (g) and sex of fish were all determined in the field, with 3771 (59%) being male and 2059 (32%) being female fish, for an overall sex ratio of 1.83:1.00 (male: female). The average length and weight for all fish were 55.38 ± 0.14 cm TL and 2355.10 ± 22.30 g, respectively. The size of male fish ranged from 15.5 to 128.5 cm TL and weighed between 190 and 25 000 g, whereas that of female ranged from 21.0 to 130.0 cm TL and weighed between 124 and 25 800 g. The value of the regression slope b of the length–weight relationship was equal to 3.04. The mean (±SD) condition factor for all fish was 1.23 ± 0.13. The condition factor was different in the length groups, with the highest (1.32 ± 0.13) and lowest (1.12 ± 0.20) values recorded in length groups 100–110 and <30 cm TL, respectively. The highest and lowest condition factors were recorded between January and March, respectively. The length–weight relationship and condition of Nile perch in Lake Victoria have deteriorated greatly, this finding being linked to the reduction of its prey species in the lake.

This study investigated the growth, mortality and recruitment of Lates niloticus in Lake Victoria basis on length–frequency data collected during the period 2014‐2015. The asymptotic length (L∞) had a value of 124 cm TL, growth curvature (K) of 0.22 year−1, total mortality (Z) of 0.96 year−1, a natural mortality (M) of 0.42 year−1, a fishing mortality (F) of 0.54 year−1, an exploitation rate (E) of 0.57 and a growth performance index (normal∅) of 3.53. Logistic selection model showed that 50% of fish of 46.09 cm TL encountering the gear are retained. There were two peak recruitment periods, a minor one in March and a major one in July, accounting for 12.04% and 22.04%, respectively, of the total fish catch. The Beverton and Holt's relative yield‐per‐recruit model indicated the indices for sustainable yields are 0.32 for optimum sustainable yield (E0.5), 0.60 for maximum sustainable yield (Emax) and 0.51 for economic yield (E0.1). Compared to previous findings, there is a great decline in the sizes of Nile perch stocks in Lake Victoria. Thus, managing the fishery requires strict adherence to the slot size of 50–85 cm TL, and restrictions on illegal gear and methods, by the devolved governments through monitoring, control and surveillance in liaison with the Beach Management Units (BMUs).

The Nile tilapia (Oreochromis niloticus) was introduced into Lake Victoria in the early 1950s and 1960s and has since become the dominant tilapiine in the lake. This study investigated the growth and population parameters of O. niloticus in Lake Victoria on the basis of length–frequency data collected during the period June 2014 and June 2015. The asymptotic length (L∞) had a mean (±SE) value of 46.24 ± 0.04 cm TL, growth curvature (K) of 0.69 ± 0.25 year−1, total mortality (Z) of 2.18 ± 0.80 year−1, a natural mortality (M) of 1.14 ± 0.28 year−1, a fishing mortality (F) of 1.05 ± 0.53 year−1, an exploitation rate (E) of 0.46 ± 0.08, a growth performance index (∅) of 3.14 ± 0.17 and a length at first capture (LC50) of 20.31 ± 0.40 cm TL. Comparing the results of this study with previous studies indicates the parameters K, Z and M have increased, whereas ∅, F, E and LC50 have decreased. Changes in these parameters could be attributed to the existing high fishing capacity, and changing lake conditions. Thus, management measures should include continued restriction on illegal fishing methods and gears, such as the use of undersized gillnets (<5 in. mesh size) and beach seines. More attention also should be directed to the implementation of measures to control pollution of the lake from its various sources.