Functional traits and stable isotopes reveal trophic interactions in a highly invaded reservoir: implications for fisheries

Freshwater reservoirs are increasingly threatened by human activities and biological invasions, which disrupt their structure and functioning. Although artificial, reservoirs act as species refuges and provide important ecosystem services. This study explores the functional diversity and trophic ecology of the fish community in Zimapán Dam, a highly disturbed reservoir in central Mexico. We focused on species coexistence, ecological niches and impacts on local tilapia fisheries. The community is dominated by nine invasive species and includes only one native species. We identified seven functional groups, most of which are composed of a single species, indicating niche differentiation. However, tilapia exhibits a complex trophic position, with significant dietary overlap with other non-natives, such as bluegill and juvenile largemouth bass, while also serving as potential prey for larger predators, including adult largemouth bass, lowland cichlid and channel catfish. These interactions may be contributing to the observed decline in tilapia, a species of key economic importance for local fisheries. Our study highlights trophic dynamics in a highly invaded ecosystem, potentially undermining local fisheries and the ecosystem services they support. We recommend a collaborative management approach that includes adaptive monitoring and targeted control of invasive predators to support both ecological balance and community livelihoods.