Reproductive dynamics and virtual population analysis (VPA) of Tilapia zilli (Perciformes: Cichlidae) in a tropical flood river basin

Abstract

A total number of 4044 specimens, comprising 2342 females and 1702 males of Tilapia zilli were caught from mid Cross river basin. There were significantly more females than males with the sex ratio of 1:1.4 for T. zilli (χ² = 101.29, df = 1, P < 0.05). Statistical analysis revealed that there were significant differences in the M: F sex ratio of T. zilli in all the four gonad maturation stages, immature stage, mature stage, ripe stage and spent stage. Gonadosomatic index ranged between 0.3 and 19.5 with a mean of 3.16 ± 0.46 for males and 7.44 ± 1.85 for females. Fecundity of T. zilli ranged from 628 to 3631 with a mean of 1583 ± 898 eggs. The fertility co-efficient for the fish was 0.30 ± 0.08. The smallest sexually mature female of T. zilli was 11.0cm TL. VPA results for T. zilli indicated one peak of fishing mortality (F) (0.99yr^-1). The peak of F occurred in the length class 13.0 to 14.0cm TL. Catch in number was 1210881 and population of survivors was 13019479.59. Recruitment to fishery was 77075319.18. Reproduction of T. zilli population is successful and the population structure indicates a species that is well adapted.