Seasonal and environmental effects on upper thermal limits of eastern sand darter ().

Anthropogenic stressors are predicted to increase water temperature, which can influence physiological, individual, and population processes in fishes. We assessed the critical thermal maximum (CT) of eastern sand darter (), a small benthic fish listed as threatened under the in Canada. Field trials were conducted stream side June-November 2019 in the Grand River, Ontario, to encompass a range of ambient water temperatures (7-25°C) for which agitation temperature (T) and CT were determined. Additional measures were taken in the comparatively more turbid Thames River to test the effect of turbidity on both measures. In the Grand, T and CT ranged from 23°C to 33°C and 27°C to 37°C, respectively, and both significantly increased with ambient water temperature, with a high acclimation response ratio (0.49). The thermal safety margin (difference between ambient temperatures and CT) was smallest in July and August (~11°C) indicating that eastern sand darter lives closer to its physiological limit in summer. The between-river comparison indicated that turbidity had no significant influence on T and CT. Comparison of CT with in-river temperatures suggested that mean stream temperature 24 hours before the trial was most important for determining CT. Fish mass, temperature variance and maximum temperature in the 24-hour period prior to the CT trial were also shown to have some effect on determining CT. Overall, study results better define the sensitivity of eastern sand darter to temperature changes across the growing season and provide information to assess the availability of suitable thermal habitat for conservation purposes.