Studies of the ecology of the blackbass. Micropterus salmoides (lacepede) (pisces:centrarchidae) in Lake Naivasha, Kenya

Abstract

The relative abundance, distribution, feeding ecology and the reproductive biology of the blackbass, Micropterus salmoides (Lacepede) in Lake Naivasha are described. Blackbass population is low as compared to that of the tilapiines in the lake. Depth is the major parameter affecting the distribution of the species, the numbers decreasing with increase in depth ( r= -0.89. P > 0.05). The food habits of the species changes with the size of the fish. In order of importance, juvenile blackbass feeds on corixids, chironomids, young crayfish, fish fingerlings and aquatic weeds while the adults feed on crayfish, fish fingerlings, corixids, frogs, chironomids. aquatic weeds and beetles. The frequency of occurrence of crayfish and fish fingerlings in the stomach of the adult was almost equal. There was a positive correlation between the size of the blackbass and the size of the prey eaten (r = 0.900, P < 0.001). M. salmoides is a diurnal feeder but shows increased feeding activity at dawn. around noon and towards dusk. Monthly variation in fullness index showed active feeding from August to-February. salmoides in Lake Naivasha has a single breeding period which extends from August to January. This period is characterized by the presence of highest numbers of mature male and female fish in monthly catches as well as high gonadosomatic index. The minimum size at sexual maturity for females occurred at a size-class of 30 - 34.9 em while males mature at an early size-class of 25 - 29.9 em. The numbers of males to females in the population did not differ significantly from the expected 1:1 ratio (X2 = 18.876, P < 0.100). Fecundity ranged from I, 300 eggs to 3.524 eggs with a mean of 2.203 eggs. There was a positive corrv.ation between fecundity and : (L) body weight (r = 0.92) and (Li ) ovarian weight (r = 0.89). A negative correlation was observed between fecundi ty and: (i) body length (r ~ 0.06) and (ii) egg diameter (r = -0.98). The equation for length - weight relationship revealed that growth in M. salmoides i s allometric. Males showed better condition than females. Condit ion and feeding activity were highly correlated (r = 0.84). There was low incidence of nematode {Contracaecum sp~ infestation in M. salmoides.