Climate change-induced increases in summer water temperature have been associated with elevated mortality of adult sockeye salmon (Oncorhynchus nerka) during river migration. We show that cardiorespiratory physiology varies at the population level among Fraser River sockeye salmon and relates to historical environmental conditions encountered while migrating. Fish from populations with more challenging migratory environments have greater aerobic scope, larger hearts, and better coronary supply. Furthermore, thermal optima for aerobic, cardiac, and heart rate scopes are consistent with the historic river temperature ranges for each population. This study suggests that physiological adaptation occurs at a very local scale, with population-specific thermal limits being set by physiological limitations in aerobic performance, possibly due to cardiac collapse at high temperatures.

Pink salmon, Oncorhynchus gorbuscha, are the most abundant wild salmon species and are thought of as an indicator of ecosystem health. The salmon louse, Lepeophtheirus salmonis, is endemic to pink salmon habitat but these ectoparasites have been implicated in reducing local pink salmon populations in the Broughton Archipelago, British Columbia. This allegation arose largely because juvenile pink salmon migrate past commercial open net salmon farms, which are known to incubate the salmon louse. Juvenile pink salmon are thought to be especially sensitive to this ectoparasite because they enter the sea at such a small size (approx. 0.2 g). Here, we describe how 'no effect' thresholds for salmon louse sublethal impacts on juvenile pink salmon were determined using physiological principles. These data were accepted by environmental managers and are being used to minimize the impact of salmon aquaculture on wild pink salmon populations.

Pink salmon (Oncorhynchus gorbuscha) enter seawater earlier and smaller (0.2 g) than any other salmonid following a two-stage salinity tolerance process initiated around yolk-sac absorption and completed with seawater entry. For the first time, this two-stage ontogeny of salinity tolerance was characterized by either holding posthatch pink salmon in fresh water or transferring them to seawater every 2 weeks. A window of salinity tolerance around yolk-sac absorption was evidenced by a period of zero morbidity in seawater compared with 100% morbidity for newly hatched alevins and 25% morbidity for fry (∼0.2–0.3 g). Increased hypo-osmoregulatory ability at the time of yolk-sac absorption was indicated in fish held in fresh water under constant photoperiod (12 h light : 12 h dark) and temperature (5 °C) by a switch from catabolic to anabolic growth, increased gill Na+K+-ATPase activity and α-1b/α-1a isoform expression, and a plateau in whole-body water content, implying that pink salmon go through a form of smoltification. A large increase in whole-body [Na+] observed in fresh water at yolk-sac absorption may represent a unique strategy for maintaining water balance once fish enter seawater.

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