| Summary: | Many populations of Pacific salmon (Oncorhynchus spp.) now encounter warmer rivers during their once-in-a-lifetime spawning migrations, which can result in premature mortality and may have contributed to some of the population declines observed in recent decades. One objective of this thesis was to determine the effects of high water temperatures on wild caught adult sockeye (O. nerka) and pink (O. gorbuscha) salmon at the level of the individual, tissue and cell through a series of temperature exposure studies. Fish held at 19°C consistently had higher mortality compared with fish held at a cooler temperature (13°C or 14°C) for both species with sockeye salmon demonstrating sex-specific mortality patterns. Sockeye salmon had higher plasma chloride levels when held at 19°C and both sockeye and pink salmon had reduced levels of plasma sex steroids at 19°C compared with fish at 13°C, which suggests an osmoregulatory disturbance and a potential reproductive consequence, respectively. Sockeye and pink salmon held at 19°C showed evidence of a common cellular stress response in their gill transcriptome characterized by the upregulation of genes associated with heat shock, immune and oxidative stress responses, variable regulation of genes involved in protein biosynthesis and the downregulation of genes involved in ion transport.
The second objective of this thesis was to characterize some of the physiological effects of senescence and mortality on Pacific salmon. I found dramatic declines in plasma osmolality, chloride and sodium levels that occur days in advance of a fish's death and determined that plasma chloride levels are strong predictors of longevity in maturing sockeye salmon. I also examined the effects of mortality on the transcriptome of sockeye salmon for the first time. Dying fish were characterized by an upregulation of several transcription factors associated with apoptosis and the downregulation of genes involved in immune function and antioxidant activity, consistent with immunosuppression.
Many of these results, especially the gene expression results, are novel for Pacific salmon. This thesis greatly improves our knowledge of the physiological effects of high river water temperatures and senescence on adult Pacific salmon. Management implications of these results and future research directions are discussed.
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