Effects of temperature on fish aggression and the combined impact of temperature and turbidity on thermal tolerance

Deforestation can increase light penetration and runoff entering adjacent freshwaters leading to increased average water temperature, stronger diel temperature fluctuations, and increased water turbidity. Changes in temperature extremes (particularly upper peaks) are important for fishes as their body temperature and rate of oxygen consumption varies with environmental temperature. Here, we compare effects of diel-fluctuating versus stable water temperature regimes on the behaviour and upper thermal tolerance (measured as Critical Thermal Maximum, CT_max) of the Bluntnose Minnow, Pimephales notatus. Fish were acclimated to either a static 18°C, static 24°C or a diel-fluctuating treatment of low to high (18-24°C) for a total of 10 weeks. Activity level and aggression were measured for 6 consecutive weeks during the acclimation period. Activity level remained high across treatments and over time. However, fish from the diel-fluctuating treatment exhibited a significant increase in aggression over the day as temperatures increased from 18°C to 24°C. Following acclimation, upper thermal limits of fish from each treatment were measured under two conditions: clear water (<2 NTU) and turbid water (25 NTU). This was to evaluate effects of acute turbidity exposure that might arise with heavy rain on deforested streams. CT_max was lowest in fish acclimated to static 18°C and highest in fish acclimated to static 24°C; fish acclimated to diel 18-24°C showed an intermediate CT_max. Exposure to acute turbidity during CT_max trials significantly lowered CT_max across all treatments, highlighting the importance of multiple-stressor studies in evaluating upper thermal tolerance of fishes.