Increased density of Bluehead Sucker larvae decreases critical thermal maximum

Streams used by Bluehead Suckers Catostomus discobolus experience variable water temperatures of high and low extremes and extreme rates of change. Acute and chronic laboratory temperature tests have been used to develop protective thermal criteria, but these methods do not allow for testing the lethality of temperature interactions with other stressors. For instance, fish density and temperature interactions are likely to occur during low‐flow periods with high densities of fish stranded in a stagnant pool; but standard thermal tests only included one fish.We examined the effect of two acclimation temperatures that represent field temperatures (8°C and 18°C) on hatch success and larval survival, and the critical thermal maximum (CTMax) and minimum temperatures (CTMin). We then investigated how the interaction of density (n = 1, n = 10, n = 25 fish per 2 L) and temperature influence CTMax or CTMin.We found maximum egg hatch success and larval survival at 18°C and the temperature range for Bluehead Sucker larvae at 18°C was 7.2 ± 1.6°C to 32.1 ± 2.4°C. However, when the density of the larvae was high (n = 25 per 2 L) the upper thermal limit significantly decreased by 5.9°C.Our results indicate higher fish densities negatively affect the tolerance of the fish to increased temperature. Thus, density and temperature interactions should be considered when developing protective temperature criteria.