Trophic niche variation and overlap between invasive mrigal carp and native mud carp in southern China

The ecological impacts of freshwater invasive fishes can develop through interspecific competition where they share similar resources to native species, but with this potentially mitigated by high ecological opportunity in prey resources. The trophic ecology of invasive mrigal carp Cirrhinus mrigala and native mud carp Cirrhinus molitorella were assessed in the Pearl River, southern China, through applying DNA metabarcoding on samples collected in both the wet and dry seasons between 2019 and 2021. Both fishes had diets comprising of items including algae, ciliates, arthropods, plants, rotifers, protists, and fungi. The interaction of year and season explained the most variation in the dietary composition, individual niche width and population niche width (PNW) for each species, and the interspecific niche overlap, with their niche widths being lowest but interspecific niche overlap being highest during the 2021 dry season. The ecological opportunity, influenced by river discharge and water temperature was in negative relationships with the interspecific niche overlap. The interspecific niche overlap was significantly negatively affecting the between individual component (BIC) of PNW for each species, but with the negative effects on PNW and its within individual component (WIC) being non-significant. The significant positive linear relationships between PNW and its components of WIC and BIC for each species provided a dual expansion case for the niche variation hypothesis. The dynamic nature of the trophic interactions of these fishes indicates the importance of considering temporal and seasonal variability within invasion risk screening processes and impact assessments in sub-tropical rivers.