Salinity and thermal tolerance of an invasive and native fish to inform management priorities

Aquatic invasive species are a major cause of global biodiversity loss. A leading hypothesis posits that invasive species tolerate wide ranges of abiotic conditions, thereby enabling them to displace native species in disturbed habitats. We investigated salinity and temperature tolerances of the invasive western mosquitofish ( Gambusia affinis ) and the native plains topminnow ( Fundulus sciadicus ) to inform management priorities. We conducted a 30-day laboratory experiment subjecting individually caged fish to six salinity concentrations (0-2400 mg/L), matching the range found in the South Platte River, Colorado, USA, where both species occur longitudinally with the native plains topminnow upstream and the invasive mosquitofish downstream. Survival was monitored daily, and critical thermal maxima were assessed on days 4, 10, and 30 to determine the effects of acute and chronic salinity exposure on thermal tolerance. Both species exhibited high 30-day survivability (> 95%) across all salinity concentrations, showing that the salinity range in their current habitat does not pose lethal effects. Western mosquitofish had a significantly higher critical thermal maxima (35 °C) than plains topminnow (32.5 °C). Critical thermal maxima decreased significantly over time, and it differed by salinity concentration but in only a single pair of concentration levels, showing limited sub-lethal effects of salinity. These results partially support the hypothesis of invasive species’ wider physiological tolerances. Our study suggests intensive water reuse elevating salinity may not directly threaten the native fish in this arid region, and the basin-scale longitudinal distribution pattern of the two species is more plausibly explained by their thermal tolerances.