Can non-native perch (Perca fluviatilis) support native eel populations in a wetland complex?

It is well established that non-native fish can become invasive and outcompete and displace native fish populations. However, little research has explored the potential benefits that non-native fish may provide to native fish populations. To address this information gap, we examined how the availability of non-native perch (Perca fluviatilis) as prey could benefit populations of the endemic longfin eel (Anguilla dieffenbachii) and the native shortfin eel (Anguilla australis) in two neighbouring open-water wetlands in the Rakatu-Redcliff wetland complex in the Southland region of Aotearoa New Zealand. The Redcliff wetland fish community comprised native fish only, while the Rakatu wetland comprised native fish and non-native perch. We compared the size, condition, population density, population size structure and diet of eels in these wetlands. While eels were not necessarily larger or better conditioned in Rakatu wetland, their population density was three times higher than the Redcliff wetland, with young-of-the-year perch comprising ca. 40% of their diet. Furthermore, juvenile eel density was four times lower in Redcliff wetland, suggesting that cannibalism may occur at this site to support the existing eel population. Based on our findings, we suggest that freshwater managers consider the predator–prey dynamics of both native and non-native fish before removing non-native species, to avoid unintended negative consequences for native predatory fish species.