EFFECTS OF COLD TEMPERATURE ON SEAWATER ACCLIMATION OF RAINBOW TROUT (Oncorhynchus mykiss)
In considering the effect of environmental challenges on physiological processes, like the challenges faced by migratory fishes, most studies focus on these factors individually and few studies have considered them in combination. Rainbow trout (Oncorhynchus mykiss) is a species of migratory salmonid that encounters a variety of environmental challenges within their lifetime and must face sustained changes in salinity and temperature during the transition from freshwater to seawater. This study examined the interaction of cold temperature on osmoregulation during salinity acclimation over time. Freshwater reared rainbow trout were transferred to either freshwater or seawater at either 15°C or 5°C and blood, muscle, and gill tissue were sampled over 4 days. Following exposure to salinity and temperature treatments, fish transferred to 15°C seawater were able to acclimate to seawater within 2-4 days whereas fish transferred to 5°C seawater did not successfully salinity acclimate within 4 days. Rainbow trout in 5°C seawater exhibited significantly higher blood plasma ion and osmolality levels and significantly lower white muscle water content in comparison to control and 15°C transferred fish. In this 5°C group, gill Na+/K+-ATPase activity levels did not increase, while those acclimated to 15°C showed a characteristic rise in activity. This inability to up-regulate gill Na+/K+-ATPase in fish exposed to 5°C seawater suggests the limiting mechanism for successful seawater acclimation may be timely up-regulation of gill Na+/K+-ATPase. Taken together, these data suggest that cold temperatures mitigate normal salinity acclimation processes in rainbow trout.