Upper thermal limits of growth in brook trout and their relationship to stress physiology

Despite the threat of climate change, the physiological mechanisms responsible for reduced performance at high temperatures remain unclear for most species. Elevated but sublethal temperatures may act via endocrine and cellular stress responses to limit performance in important life-history traits such as growth. Here, brook trout (Salvelinus fontinalis) subjected to chronically elevated or daily oscillating temperatures were monitored for growth and physiological stress responses. Growth rate decreased at temperatures above 16 degrees C and was negative at 24 degrees C, with an estimated upper limit for positive growth of 23.4 degrees C. Plasma cortisol increased with temperature and was 12- and 18-fold higher at 22 and 24 degrees C, respectively, than at 16 degrees C, whereas plasma glucose was unaffected by temperature. Abundance of heat shock protein 70 (HSP70) in the gill increased with temperature and was 11- and 56-fold higher at 22 degrees C and 24 degrees C, respectively, than at 16 degrees C. There was no relationship between temperature and plasma Cl-, but there was a 53% and 80% decrease in gill Na+/K+-ATPase activity and abundance at 24 degrees C in comparison with 16 degrees C. Daily temperature oscillations of 4 degrees C or 8 degrees C (19-23 degrees C or 17-25 degrees C) were compared with 21 degrees C controls. Growth rate decreased with temperature and was 43% and 35% lower by length and mass, respectively, in the 8 degrees C daily oscillation treatment than in the controls. There was no effect of temperature oscillation on plasma cortisol or glucose levels. In contrast, gill HSP70 abundance increased with increasing daily oscillation and was 40- and 700-fold greater at 4 degrees C and 8 degrees C daily oscillation, respectively, than in the constant temperature controls. In individuals exposed to 17-25 degrees C diel oscillations for 4 days and then allowed to recover at 21 degrees C, gill HSP70 abundance was still elevated after 4 days recovery, but not after 10 days. Our results demonstrate that elevated temperatures induce cellular and endocrine stress responses and provide a possible mechanism by which growth is limited at elevated temperatures. Temperature limitations on growth may play a role in driving brook trout distributions in the wild.