Ecological performance of cultivated and feral populations of an invasive freshwater fish under current and near-future climatic conditions

Goldfish (Carassius auratus), a globally invasive species, is increasing in abundance in lakes and ponds within the Laurentian Great Lakes region. We hypothesized that current and future selection pressures imposed on feral goldfish populations adapted to urban and natural ponds would result in elevated ecological performance compared to cultivated pet-store goldfish. We tested the hypothesis experimentally by comparing the foraging efficiency (functional response metrics: maximum feeding rate and handling time) and temperature tolerance (critical thermal maximum, CTmax) of feral and cultivated goldfish under current (18°C) and projected future (26°C) surface water temperatures. After acclimation to projected temperatures, all goldfish populations displayed higher maximum feeding rates and temperature tolerances. Feral goldfish had higher maximum feeding rates than cultivated goldfish at both temperature treatments, indicating higher trophic impact potential. In contrast, CTmax did not vary significantly among populations. Collectively, these results indicate that temperature and intraspecific variation influence the foraging efficiency of goldfish and thus could mediate their ecological impacts. Population-level variation in foraging suggests that selection in urban and natural ponds may increase the ecological impact of introduced goldfish.