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Intraspecific variation and plasticity in rainbow trout responses to climate change stressors Strowbridge, Nicholas Robert


Global climate change threatens to reduce available habitat for cold-water fish such as rainbow trout (Oncorhynchus mykiss). Phenotypic plasticity might help individuals to cope, but may not be sufficient and survival could require adaptation in situ. Here, I assessed levels of phenotypic variation in thermal and hypoxia tolerance that could act as the substrate for adaptation. Furthermore, I also characterized the extent of thermal plasticity in these traits in multiple strains of rainbow trout. To characterize phenotypic variation, I used a common garden breeding approach with 25 family crosses in each of several strains of British Columbia rainbow trout, assessing critical thermal maximum (CTmax) and incipient lethal oxygen saturation (ILOS). Using California strains, I investigated the extent of thermal plasticity in CTmax, ILOS, and (in collaboration) standard and maximum metabolic rate, absolute aerobic scope (AAS), critical oxygen tension (Pcrit), and measures of cardiac performance such as maximum heart rate (ƒHmax) and cardiac gene expression. I found little among but large within-strain variation in CTmax and ILOS, whereas post-trial mortality clearly differentiated the strains. There was little correlation between upper thermal and hypoxia tolerance at the individual level. I observed significant plasticity in CTmax, with associated declines in AAS and ƒHmax and increases in the expression of stress-related genes. However, plasticity in all these measures reached a limit at a high but ecologically relevant temperature. Taken together, these findings suggest that thermal plasticity will not be sufficient to allow rainbow trout to cope with climate change, but that trout populations possess substantial phenotypic variation in climate-change relevant traits that may allow adaptation in situ. However, differences between strains were not evident for all traits, and managers will need to take a multifaceted approach when examining the effects of climate change on natural and stocked strains of rainbow trout.

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