Abstract
Freshwater ecosystems across the world are threatened due to human-mediated changes such as habitat conversion and species introductions. These changes can influence native ecosystems through the development of new direct and indirect interactions within food webs. Changes within the food web can impact threatened native species through diet shifts and habitat displacement. The successful introduction of European perch (Perca fluviatilis) to New Zealand rivers and lakes has resulted in rapid spread across the country. Juvenile perch have become highly abundant, with adult perch acting as strong top trophic competitors. However, there is a lack of knowledge about the direct impact of interspecific competition on the threatened longfin (Anguilla dieffenbachii) and shortfin (Anguilla australis) eels. This study’s overall aim is to investigate longfin and shortfin eels across a wetland complex (Sinclair Wetlands and the surrounding area), inferring the impact of perch as well as seasonal variation within the eel populations. The first aim is to understand the impact of interspecific interactions between invasive European Perch and two native eel species, through non-lethal sampling methods. Nine ponds in five pond clusters were sampled across the Waihola-Waipori wetland complex that varied in perch presence and relative abundance. Fyke nets were set overnight with longfins, shortfins and all other fish species measured and sampled. Eels were identified, weighed, and measured to total length to investigate catch per unit effort (CPUE) and condition index. A maximum of ten eels from each pond had mucus nonlethally sampled for stable isotope analysis. Results showed no significant impact of perch presence on the abundance and condition index of either eel species. The trophic position of eels showed a non-significant decreasing trend with increasing perch relative abundance. These findings suggest that perch may impact the trophic position of eels, potentially through interspecific competition. Although adult perch may compete with adult eels, abundant juvenile perch may act as prey, therefore contributing to the lack of relationships. The second aim is to understand the seasonal trends of both shortfin and longfin eel populations within a large pond in the Sinclair Wetlands. Eel populations were investigated over seasonal and lunar time periods. Data collected during sampling events in 2018, 2019 and 2023 were compiled and analysed. CPUE and condition index of both eel species were observed over changes in season and lunar periods. Results showed seasonal trends of ii increasing CPUE and condition index of both eel species towards the summer season. Greater CPUE and condition index during summer are predicted to be linked with increased water temperature and prey availability. Results also observed lower CPUE of both species associated with the lunar quarters closer to full moons. These lunar trends are predicted to be linked with prey avoidance and photophobic behaviours of eels during lunar periods with greater moonlight. This study contributes to the understanding of the complex impact introduced perch have on native eel populations while describing seasonal and lunar trends within wetland eel populations.