Size, diversity, dominance and trophic structures of the fish community in a tropical lake: a case study for the Sinazongwe area, Lake Kariba, Zambia

This study investigated the size, diversity, dominance and trophic structures of the exploited inshore fish community of the Sinazongwe area, Lake Kariba, Zambia, for the period 1980-1995. It was based on fish records of scientific gillnet surveys and on data available in the literature. The size and diversity spectra exhibited, like those of exploited demersal marine fish communities, regular decreasing patterns, indicative of high fish numbers and diversity in smaller sizes and vice versa among larger sizes. The size spectra patterns were recurrent from year to year and well fitted by quadratic functions. The diversity (Shannon index) and dominance (J' index) decreased and increased linearly, respectively, with fish length. Diversity for larger lengths has decreased as evidenced by the annual variations of diversity spectra's slopes. The abundance-biomass comparison plots indicated years of undisturbed, strongly and moderately stressed fish community. The proportions of predators, especially of Hydrocynus vittatus, and the mean trophic level decreased linearly over years. The average diversity trophic spectra remained unchanged. The mean abundance and biomass trophic spectra indicated a top-predator controlled community over 1980-1984, and vice versa thereafter. The trophic structure descriptors better captured the fish community changes under increasing fishing pressure and revealed for the first time the “fishing down marine food webs” effects for Lake Kariba. The community attributes studied are potentially promising alternatives for the assessment and management of tropical exploited freshwater fish communities