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.

During winter, juvenile salmonids in boreal streams are vulnerable to predation, mainly from mammals and birds. Encounters with terrestrial predators can be reduced or avoided if fish limit activity to the darker periods of the day or to periods with surface ice. As piscivorous fish also are active in winter, they may be a threat under low light conditions when juvenile salmonids do not avoid terrestrial predators. Abiotic conditions, especially temperature, have major effects on fish in winter. High temperatures alter ice conditions in winter and lead to increased metabolism and physiological performance of fish. Water temperature also influences embryogenesis, with repercussions for fish throughout their life. Considering the rapid warming of winters in boreal regions, insights into how salmonids are adapted to winter conditions can aid in efforts to predict and mitigate anthropogenic effects that alter the winter environment.

In this doctoral thesis, I explore anti-predator responses of brown trout (Salmo trutta) during its early life stages. I have examined the effects of predators, temperature, light and ice on the behaviour and physiology of juvenile trout during winter. In addition, I have studied how temperature and predators affect embryogenesis and the behaviour of fry after hatching. Anti-predator responses were evident in both the behaviour and physiology of juveniles and during embryogenesis. Trout exhibited diel behavioural changes when piscivorous fish were present, and were more vigilant towards piscivorous fish in darkness. Furthermore, temperature affected trout behaviour and physiology, with higher activity levels and lower mRNA expression of stress-related genes at higher temperatures. Trout also behaved differently depending on the temperature they experienced as eggs, as increased egg-incubation temperatures resulted in trout being more active and prone to risk taking.

This thesis explores how winter conditions affect anti-predator responses of brown trout (Salmo trutta) during its early life stages. Salmonids in streams are vulnerable to predation from mammals, birds and piscivorous fish in winter. Abiotic conditions such as water temperature also affect the fish, and winters in boreal regions are getting warmer as a result of global change. Warmer winters are expected to increase the metabolism of fish, but also change the physical environment by reducing ice cover and altering light conditions. Eggs of salmonids are affected by warmer winters as well, with direct effects on embryos and indirect effects on the fish after hatching.  

I have examined how predators, temperature, light and ice affect juvenile brown trout during winter, and how temperature and predators affect embryos and the behaviour of fry after hatching. Anti-predator responses were evident in juveniles and during embryogenesis. Juvenile trout exhibited diel behavioural changes when piscivorous fish were present, and increased their vigilance towards piscivorous fish in darkness. Trout exhibited higher activity and lower expression of stress-related genes at higher temperatures. Increased egg-incubation temperatures resulted in trout being more active and risk taking.