Nile tilapia(Oreochromis niloticus)invasion impacts trophic position and resource use of commercially harvested piscivorous fishes in a large subtropical river

Background Although freshwater ecosystems cover less than 1%of the earth’s surface,they support extremely high levels of biodiversity and provide vital ecosystem services.However,due to the introduction of non-native fishes,aquatic ecosystem functioning has been altered,and in some cases,declined sharply.Quantifying the impacts of invasive species has proven problematic.In this study,we examined the relative trophic position of native piscivorous fishes to estimate the effects of invasive Nile tilapia on food webs in the downstream sections of an invaded large subtropical river,the Pearl River,China.Furthermore,we quantified how native piscivorous fish diets changed as the Nile tilapia invasion progressed.Results The trophic position of the widely distributed and locally important economically harvested piscivorous culter fish(Culter recurviceps),mandarinfish(Siniperca kneri),and catfish(Pelteobagrus fulvidraco)lowered significantly in the invaded Dongjiang River compared to an uninvaded reference Beijiang River.The lower trophic position of these piscivorous fishes was reflected by a major reduction in the proportion of prey fish biomass in their diets following the Nile tilapia invasion.Small fishes in the diet of culter fish from the reference river(33%small fishes,17%zooplankton)shifted to lower trophic level zooplankton prey in the invaded river(36%zooplankton,25%small fish),possibly due to the presence of Nile tilapia.Additionally,small fishes in the diet of mandarinfish in the reference river(46%small fishes,11%aquatic insects)declined in the invaded river(20%aquatic insects,30%small fishes).Similarly,the diet of catfish from the reference river shifted from fish eggs(25%fish eggs,25%aquatic insects)to aquatic insects in the invaded river(44%aquatic insects,5%fish eggs).Conclusions The results of this study contributed to a growing body of evidence,suggesting that Nile tilapia can modify trophic interactions in invaded ecosystems.It is crucial to understand the processes outlined in this study in order to better assess non-native aquatic species,conserve the stability of freshwater ecosystems,and improve current conservation strategies in reaches of the Pearl River and other similar rivers that have experienced invasions of non-native species.