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.

Factors influencing mercury accumulation in three species of forage fish from Caddo Lake, Texas, USA

Most studies that have examined mercury （Hg） contamination of fish have focused on game species feeding near the top of the food web, while studies that examine forage fish that feed near the base of the food web are rare. We conducted a survey of Hg contamination in three species of forage fish, brook silverside （Labidesthes sicculus）, threadfin shad （Dorosoma petenense） and gizzard shad （Dorosoma cepedianum）, from Caddo Lake, Texas, USA and found species-specific differences in Hg concentrations. We examined total length, age, trophic position （determined using δ15N）, and growth rate of forage fish as factors that could have influenced within-and between-species differences in Hg concentration. Total length and age were the best predictors of within-species differences in Hg concentration. Between-species differences in Hg concentrations were most strongly influenced by trophic position.

Isotopic niche provides an insight into the ecology of a symbiont during its geographic expansion

The study of the recent colonization of a symbiont and its interaction with host communities in new locations is an opportunity to understand how they interact.The use of isotopic ratios in trophic ecology can provide measurements of a species'isotopic niche,as well as knowledge about how the isotopic niches between symbiont and host species overlap.Stable isotope measurements were used to assess the sources of carbon assimilated by the host species(the bivalves Mytilus galloprovincialis and Scrobicularia plana)and their associated symbiont pea crab Afropinnotheres monodi,which occurs within these bivalves'mantle cavities.The mixing model estimates suggest that all of them assimilate carbon from similar sources,particularly from pseudofaeces and particulate organic matter in this symbiotic system based on filter feeding.The symbiotic species occupy comparable trophic levels and its association seems to be commensal or parasitic depending on the duration of such association.The pea crab A.monodi reflects a sex-specific diet,where males are more generalist than the soft females because the latter's habitat is restricted to the host bivalve.The high isotopic overlap between soft females and M.galloprovincialis may reflect a good commensal relationship with the host.