Effect of rate of change during diel temperature fluctuations on growth, acute and chronic stress, and pathology of multiple warm water fish species

Rapid temperature change can lead to thermal shock and death whereas gradual temperature change can have a stimulating effect.  Unknown is when temperature change becomes harmful.  To answer this question we examined the acute and chronic stress responses in warmwater fish during 4-6 weeks of diel temperature fluctuations of ±3.9°C at 0.8°, 1.1°, 2.2°, or 4.4°C•hr^-1 or a constant temperature control under cool (mean 7.8°C), warm (mean 20.0°C) and hot (mean 27.8°C) conditions.  We examined ~250 individuals from each of 4-6 species from three feeding guilds at each condition, and 8 species in total.  We measured cortisol, glucose and triglyceride levels after one day of temperature cycling and at the end, measured growth and performed histopathological analysis on a portion of individuals.  No species exhibited an acute or chronic stress response to temperature cycling during the cool or warm conditions.  Temperature cycling stimulated growth versus the control in 3/4 of species under cool conditions. Under hot conditions there was an increase in the acute and chronic stress response in 3/4 of species.  Temperature change as little as 1.1°C•hr^-1 can be stressful near a species upper tolerance, but within a species comfort zone temperature change of 4.4°C•hr^-1 may not be stressful.