The biology and distribution of Haplochromis spp in the Nyanza Gulf prior to the total invasion of the Gulf of Nile perch, Lates niloticus (L)
The work reported was conducted during the watershed period of 1976 when Nile perch (Lates niloticus) started to replace Haplochromis spp. in dominance in the Nyanza Gulf of Lake Victoria. Seventy four "groups" of Haplochromis species flock obtained from a stock assessment survey of that year were used in the study. The length of fish examined varied between 57 and 237 mm total length, and between 1 and 182 g wet weight, with means of 101.05 mm and 18.53 g respectively. Frequency distribution curves for both sexes were unimodal with a maximum between 70 and 130 mm. The t-test showed that the two sexes came from the same population. Clutch size per fish (mean weight 25.3 g and mean total length 106.7 mm) was 78 eggs. The minimum size at maturity was 89 mm for males and 93 mm for females. Living condition coefficient was highest at developing stages. Sex ratio calculations per "group" were found illogical as most "groups" were exclusively monosexual. Most of the Haplochromine "groups" fed on phytoplankton (41%), others on molluscs (21%), fish material (12%), insect larvae (9%), adult insects (8%), macrophytic detritus including sand grains (7%) and zooplankton (4%). Feeding competition was lowest among the grazers on the abundant phytoplankton and highest among the adult aquatic insect eaters. Nematode parasitic infestation was common among female fish. Haplochromis spp. were collected in all the hauls and usually in greatest concentrations from a depth of 4 m through to 49 m. Over 80% of the 74 "groups" were represented in the 4-9 m depth interval, 59% in the 10-19 m depth interval, 68% in the 20-29 m depth interval, 30% in the 30-49 m depth interval and only about 10% of the groups were represented in the deepest 50-69 m depth interval.
- Conference Papers