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UBC Theses and Dissertations

Thermal tolerance of Pacific salmon (Oncorhynchus spp.) : methodological and inherent variability in upper thermal tolerance limits and their use in assessing vulnerability to climate change Mayer, Noa Eliza Berman


Aquatic systems are warming and exceeding upper thermal limits (UTL) of Pacific salmon (Oncorhynchus spp), yet quantifying UTL and understanding how they inform resilience to climate change is challenging. This thesis focuses on quantifying upper thermal tolerance limits in Pacific salmon with specific reference to Kokanee salmon (O. nerka). I conducted a systematic review involving 168 papers investigating UTL in five Pacific salmon species and found considerable variation among species, within species, and across life stages -largely due to different methodological approaches (e.g. CTmax/UILT, Aerobic/Cardiac Scope, Thermal Migration Barriers, Rearing Mortality, Thermal Preference/Avoidance). Each method displayed strengths and weaknesses owing to logistics, time scale, and ecological realism, I recommend reporting an ‘UTL range’ instead of single UTL values, to reflect inherent and methodology-based variation. Comparing studies with similar experimental design showed that within species, UTL was higher for populations that historically encountered higher temperatures suggesting local thermal adaptation. Within populations, UTL differed across the lifecycle -highest in fry for some populations and in migrating adults for others. UTL has not been assessed for spawning fish and few studies examined estuarine and marine stages, limiting life history perspectives. My analysis suggests pink salmon are most resilient to warming due to their exceptional aerobic capacity, high straying rates, short generation time and limited exposure to high temperatures. In an additional experiment, I conducted thermal tolerance trials using two commonly employed methodologies: The Critical Thermal Maximum (CTmax) and Thermal Performance Curves for Aerobic Scope (AS). Juvenile and spawning adult fish were held overnight at a range of experimental temperatures (12˚, 17˚, 20˚, 22˚ and 24˚C).. In CTmax trials water temperatures were increased at a constant temperature (0.3˚Cmin⁻¹) until fish lost equilibrium. CTmax values ranged on average from 28.7˚C – 29.7˚C and differed significantly between life stages. Absolute AS – the difference between maximal and standard metabolic rates – were found to differ significantly between male and female adults, however Optimal (Topt) and functional temperature limits (Tpejus) occurred at similar temperatures (Topt: ~15˚C, Tpejus: ~20˚C). The large degree of variation in upper thermal tolerance metrics highlights the need for careful selection of values when conducting vulnerability assessments.

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