Impacts of elevated water temperature on the behaviour and thermal tolerance of the Bluntnose Minnow (<i>Pimephales notatus</i>)

Fish often show behavioural responses to elevated water temperature before they reach their upper thermal limit. We measured the activity, aggression, and thermal tolerance (critical thermal maximum, CT max ; agitation temperature, T ag ; thermal agitation window; and thermal safety margin) of the Bluntnose Minnow ( Pimephales notatus Rafinesque, 1820). Thirty fish underwent a 5-week acclimation period to two temperatures (ambient, 16 °C or hot, 25 °C). We took behavioural observations throughout the 5 weeks and conducted thermal tolerance trials at the end of the acclimation period. Aggression was marginally higher in fish acclimated to hot water, but there was no change over time (week 1 to 5). Activity was higher in the hot acclimation treatment, but again there was no change in activity over time. We found that CT max , T ag , and the agitation window (the difference between CT max and T ag ) were significantly greater in fish acclimated to 25 °C, but the thermal safety margin (the difference between acclimation temperature and CT max ) was smaller. This study highlights the significance of behavioural responses to elevated water temperature that may occur well before the upper critical thermal limit, which may result in negative physiological (metabolic costs) or behavioural (avoidance/refuge seeking behaviour) impacts over time.