Some aspects of the biology and life-history strategies of Oreochromis variabilis (Boulenger 1906) in the Lake Victoria Basin

Abstract

Oreochromis variabilis (Boulenger), a fish species endemic to Lake Victoria, was abundant, forming an important component of the indigenous fisheries stocks before and up to the late-1950s. Catches declined drastically thereafter, and only sporadic catches are currently found in Lake Victoria. Remnants population of the species, however, are found in several small waterbodies (SWBs) within the lake basin. The life-history characteristics of O. variabilis in Lake Victoria, including, sex ratio, reproduction and length–weight relationship, were compared to those in selected three SWBs in the lake basin. Fish samples were collected by monofilament gillnets of 30–255 mm between 2001 and 2005. Males predominated over females from all the sampled sites (sex ratio 1.00:0.33). Length at first maturity (Lm50) had mean (±SE) of 18.48 ± 1.50 cm TL for males, and 16.87 ± 0.95 cm TL for females, and did not exhibit any significant differences between habitats. Fecundity ranged between 73 and 14 800 eggs for fish of 13.5–18.6 cm TL, respectively. Absolute fecundity of O. variabilis was proportional to the body weight, but nearly proportional to the cube of the fish length. Egg diameter varied from 0.3 to 5.19 mm, with a mean (±SE) of 3.44 ± 0.08 mm. Growth was allometric in both male and female, being significantly different from the expected value of 3 (P < 0.05). The life-history strategy of O. variabilis is discussed within the context of changes in the lake and the SWBs.