From behaviour to genes: anti-predator responses of brown trout (Salmo trutta) under winter conditions

Temperature has major effects on the performance of poikilotherms. In encounters with predators, low winter temperatures constrain predator detection and escape capabilities in prey fishes. Most studies of the anti-predator responses of fish under winter conditions focus on endothermic terrestrial predators, whereas effects of piscivorous fish are generally overlooked. The studies presented in this thesis explore behavioural and physiological responses of juvenile brown trout (Salmo trutta) at winter temperatures of 3 and 8 °C in the presence and absence of a winter-active piscivorous fish (burbot, Lota lota). In Paper I, I report behavioural responses of trout in relation to the time of day. At the lower temperature and in the presence of burbot, trout reduced their activity. Trout used overhead shelter the most during the day and in the presence of burbot. Trout also spatially avoided burbot at night and at dawn but not during the day. In Paper II, I examined plasma cortisol and mRNA expression of stress-related genes. A redundancy analysis showed that both temperature and the presence of burbot explained a significant amount of the observed variation. Trout had higher cortisol levels when exposed to the burbot. Analyses of individual gene expressions revealed that trout had higher mRNA expression at 3 than at 8 °C for 11 of the 16 examined genes. Only one gene, RBP1, was expressed to a higher degree in the presence of burbot, but there were also interaction effects between temperature and burbot presence for two genes coding for serotonin and glucocorticoid receptors. My studies show that piscivorous fish shape anti-predator responses of juvenile brown trout, both behaviourally and at the gene level, under winter conditions. The observed thermal effects on mRNA levels underscore the importance of temperature in fish stress responses, with implications for stream salmonids in a warmer climate. Low winter temperatures constrain physiological performance in stream fishes, with possible consequences for encounters with predators. This thesis explores behavioural and physiological responses of juvenile brown trout (Salmo trutta) at 3 and 8 °C in the presence and absence of a winter-active piscivorous fish (burbot, Lota lota). In Paper I, I report behavioural responses of trout in relation to time of day. Lower temperature and burbot presence reduced trout activity. Use of shelter by trout was greatest during the day and in the presence of burbot. Trout spatially avoided burbot at night and at dawn, but not during the day. In Paper II, I examined plasma cortisol and mRNA levels of stress-related genes. Trout had highest cortisol levels in the presence of burbot. For 11 of the 16 examined genes, trout had higher mRNA expression at 3 than at 8 °C. For one gene, RBP1, trout had higher expression in the presence of burbot, and there were interaction effects between temperature and burbot treatments for genes coding for serotonin and glucocorticoid receptors. Piscivorous fish shape anti-predator responses of juvenile brown trout under winter conditions, and thermal effects underscore the temperature dependence of fish stress responses, with possible effects in a warmer climate