Impacts of Nile tilapia cage culture on water and bottom sediment quality: The ability of an eutrophic lake to absorb and dilute perturbations

Abstract

Environmentally sustainable aquaculture depends on accurate understanding of the impacts of aquaculture-derived organic matter (AOM) and the ability of aquaculture systems to absorb and dilute perturbations. To this end, the present study assessed the impacts of AOM from cage culture of Nile tilapia on the ecology of Lake Victoria, Kenya, using fish cages near Anyanga Beach in Siaya County from December 2018 to October 2019. Four locations were surveyed for organic loadings from cage cultures, located 0, 50, 150 and 500 m (as a control site) away from the cages. The cage aquaculture produced increased P and N concentrations near the cages and a decreased N:P molar ratio. These changes stimulated algal growth which, in turn, affected the water quality. The organic material accumulated on the bottom under the cages, increasing the benthic BOD (BOD, >10 mg/g), a sensitive indicator of the ecological footprint of the cage aquaculture. Further, the negative ORP observed in the benthic layer suggested anoxic bacterial metabolism, possibly causing build-up of sulphides and methane. These changes altered the abundance and composition of both limnetic and benthic communities. At the beginning of the study, 22 zoobenthic taxa existed around the cages and 18 at the reference sites. Only 3 saprophilous taxa, chiefly gastropods (Physella spp.), bivalves (Sphaerium spp.) and oligochaetes (Tubifex spp.) were present at the cage site and 17 at the reference site at the end of the culture period. The Shannon diversity index exhibited a declining tendency with the length of culture period at the cage site, signifying a negative impact of aquaculture on biodiversity. The water quality recovery after cage disturbance is rapid (<4 months), noting there was no significant difference in the water quality recorded at the cage site and the other sampling sites after a fallow period of 4 months. However, the recovery of the sediment and meiofauna was far from complete at the end of this period. Moving the cages slightly (50–100 m) away from the former location may allow the benthic communities to recover and alleviate the problem. Further, the fallowing period, particularly for the Anyanga Beach site, should be extended from four to at least 5 months to allow for the environment to recover. With the rapid increase of cage fish farming in the African Great Lakes Region and with the potential for its occurrence in other lakes, there is a need to develop regulations to guide the industry, as well as the need for continuous monitoring of the environment, in order to provide information to guide investments and ensure sustainable cage farming.