Population variability in thermal performance of pre-spawning adult Chinook salmon

Climate change is causing large declines in many Pacific salmon
populations. In particular, warm rivers are associated with high levels of
premature mortality in migrating adults. The Fraser River watershed in
British Columbia, Canada, supports some of the largest Chinook salmon
(Oncorhynchus tshawytscha) runs in the world. However, the Fraser River is
warming at a rate that threatens these populations at critical freshwater
life stages. A growing body of literature suggests salmonids are locally
adapted to their thermal migratory experience, and thus,
population-specific thermal performance information can aid in management
decisions. We compared the thermal performance of pre-spawning adult
Chinook salmon from two populations, a coastal fall-run from the
Chilliwack River (125 km cooler migration) and an interior summer-run from
the Shuswap River (565 km warmer migration). We acutely exposed fish to
temperatures reflecting current (12, 18°C) and future projected
temperatures (21, 24°C) in the Fraser River and assessed survival, aerobic
capacity (resting and maximum metabolic rates, absolute aerobic scope
(AAS), muscle and ventricle citrate synthase), anaerobic capacity (muscle
and ventricle lactate dehydrogenase), and recovery capacity (post-exercise
metabolism, blood physiology, tissue lactate). Chilliwack Chinook salmon
performed worse at high temperatures, indicated by elevated mortality,
reduced breadth in AAS, enhanced plasma lactate and potassium levels, and
elevated tissue lactate concentrations compared to Shuswap Chinook salmon.
At water temperatures exceeding the upper pejus temperatures (Tpejus,
defined here as 80% of maximum AAS) of Chilliwack (18.7°C) and Shuswap
(20.2°C) Chinook salmon populations, physiological performance will
decline and affect migration and survival to spawn. Our results reveal
population differences in pre-spawning Chinook salmon performance across
scales of biological organization at ecologically relevant temperatures.
Given the rapid warming of rivers, we show that it is critical to consider
the intra-specific variation in thermal physiology to assist in the
conservation and management of Pacific salmon.