The effects of acclimation to temperature and chloride salinity on the thermal tolerance of goldfish (Carassius auratus)

Understanding how nonnative aquatic species respond to changing temperatures and salinities is crucial for risk assessment in an era of rapid global change. Popular ornamental species such as goldfish (Carassius auratus) have become widespread and locally abundant, owing to frequent release events and broad environmental tolerance. The critical thermal maximum (CT_max) of cultivated goldfish obtained through the pet trade was measured to assess how their thermal tolerance is affected by acclimation to realistic levels of a co-occurring abiotic stressor. Goldfish were exposed to a combination of chloride concentrations (0 ppt, 1 ppt, and 6 ppt; selected based on those measured in urban ponds subjected to road salt pollution) and current and elevated temperatures (18 °C, 21 °C, and 25 °C; selected based on climate projections for the Great Lakes basin). Results revealed a positive response to acclimation temperatures, with those fish exposed to temperatures near the species’ growth optimum having the highest CT_max, irrespective of chloride treatment. Thermal tolerance further characterized using metrics of agitation and acclimation potential revealed that high chloride levels (6 ppt) cause sub-optimal performance during heat stress, but acclimation to intermediate temperatures buffers these negative effects. Therefore, the effects of multiple potential stressors on thermal tolerance could mediate the invasive success and impact of ornamental goldfish released in urban waterbodies. Goldfish populations presently acclimating to increased warming and salt pollution in urban ponds would likely have a competitive advantage when subsequently introduced to wild ponds that will be altered by these stressors under expanding urbanization in future years.