Life history traits and growth of Nile Perch, Lates Niloticus (l.), in Lake Victoria, Uganda: Implications for management of the fishery

Abstract

A study of life history traits and growth of Nile perch, Lates niloticus L. in northern Lake Victoria (Uganda) was undertaken between 2006 and 2008 with a major aim of providing information that could facilitate the development of predictive sustainable management strategies for Nile perch. Comparisons were drawn from historical records collected from earlier studies. From experimental gill-netting conducted in the shallow nearshore waters of Hannington Bay, Thruston Bay, and Napoleon Gulf, considerable changes in fish taxa richness and abundance of Lake Victoria were discerned and appeared to be associated with the increase in fishing effort targeting Nile perch, coupled with the continuous decline in the Nile perch catches in the commercial fishery. Age information was derived using marginal-increment and edge analyses to validate the periodicity and timing of opaque zone formation in otoliths of the Nile perch. Deposition of annuli was bimodal corresponding to the two seasonal peaks of rainfall characteristic of this equatorial region. Male Nile perch attained maximum size faster than their female counterparts, while the females reached a much larger size at older ages. Age data combined with maturity data showed that males reached 50% maturity at 1 year and females at 1.5 years. The difference found between the age distribution of the Nile perch in this study and those in some earlier studies with respect to the maximum age of the fish sampled (7 years versus 13-14 years) suggested heavy harvest of larger, older Nile perch in Lake Victoria. In comparison to historical patterns, it was found that the Nile perch had experienced a reduction in size at first maturity (L50) with a shift towards maturation at earlier ages and smaller sizes correlated with strong size selective mortality. The shift to lower L50 is likely to decrease the overall reproductive potential because smaller females produce fewer eggs. In comparison to those collected in 1970, the female Nile perch sampled in this study exhibited lower total fecundity (mean = 1,853,438 ± 196,805 eggs), suggesting reduced allocation to gonads. An increase in the proportion of haplochromines in Nile perch diet was found to be coincident with an increase in fishing pressure on the Nile perch, a decline in Nile perch catches, and haplochromine cichlid recovery; with the Nile perch exhibiting a much smaller size (15 cm total length) at shift to piscivory, and a better condition (K = 1.24) for Nile perch that had a high proportion of haplochromines (versus invertebrates) in their stomachs. The study illustrates that the stability and sustainability of the Nile perch fishery depends on defining a proper balance between harvesting and biodiversity recovery. Overall, size selective mortality and its effects on yield-determining traits such as L50 and fecundity, seem largely responsible for the changes observed over time in the Nile perch population. The degree to which these responses are phenotypically plastic and/or genetic is not known; however, the high percentage reductions in these traits may contribute major losses in yield to the fishery and could negatively affect population growth and rate of recovery.