How body size and salinity affects thermal tolerance of a range-expanding fish

Objective We investigated the influence of body size and salinity on the thermal tolerance of Common Snook Centropomus undecimalis (hereafter, “snook”). Methods Juvenile snook (small = 59–156 mm standard length, large = 188–341 mm standard length) were collected from Tampa Bay, Florida. Snook were acclimated in a recirculating aquaculture system, where they underwent a quarantine period at 25°C with gradual salinity adjustments to either 3, 15, or 30‰ for large juveniles and 3‰ for small juveniles. Snook were then randomly selected for chronic lethal minimum trials during which temperature was decreased by 1°C per day. Temperatures at which the fish ceased feeding, lost equilibrium, and died were recorded. Results In the chronic lethal minimum trials, small juvenile snook exhibited greater cold tolerance for two end points—cessation of feeding and death—compared with larger juveniles at 3‰. For large juveniles, death occurred at lower temperatures in both the low- and high-salinity trials (9.2°C) than in the midsalinity trials (10.1°C) (i.e., fish were less hardy at midsalinity). In the low-salinity trial for large juveniles, cessation of feeding (16.1°C) occurred before the other salinity trials for large juveniles as water temperatures were lowered, but loss of equilibrium occurred after (10.1°C). Conclusions Juvenile snook undergo complex ontogenetic changes resulting in reduced cold tolerance in larger individuals. Salinity differences further modify cold tolerance. At the onset of a cold event, juvenile snook occupying waters of low salinity (i.e., rivers) are less likely to lose equilibrium, while those occupying waters of midsalinity are more likely to die if they cannot find adequate refuge from cold water. These observations likely apply to adult snook; thus, information on cold tolerance at various salinities, combined with knowledge of fish behavior, provides insights into factors affecting overwintering and snook resilience to climate variability, particularly as the species continues to expand its range north.