Cardiorespiratory collapse at high temperature in swimming adult sockeye salmon

Elevated summer river temperatures are associated with high in-river mortality in adult sockeye salmon (Oncorhynchus nerka) during their once-in-a-lifetime spawning migration up the Fraser River (British Columbia, Canada). However, the mechanisms underlying the decrease in whole-animal performance and cardiorespiratory collapse above optimal temperatures for aerobic scope (T_opt) remain elusive for aquatic ectotherms. This is in part because all the relevant cardiorespiratory variables have rarely been measured directly and simultaneously during exercise at supra-optimal temperatures. Using the oxygen- and capacity-limited thermal tolerance hypothesis as a framework, this study simultaneously and directly measured oxygen consumption rate (MO₂), cardiac output V·_b, heart rate (f_H), and cardiac stroke volume (V_s), as well as arterial and venous blood oxygen status in adult sockeye salmon swimming at temperatures that bracketed T_opt to elucidate possible limitations in oxygen uptake into the blood or internal delivery through the oxygen cascade. Above T_opt, the decline in MO_2max and aerobic scope was best explained by a cardiac limitation, triggered by reduced scope for f_H. The highest test temperatures were characterized by a negative scope for f_H, dramatic decreases in maximal V·_b and maximal V_s, and cardiac dysrhythmias. In contrast, arterial blood oxygen content and partial pressure were almost insensitive to supra-optimal temperature, suggesting that oxygen delivery to and uptake by the gill were not a limiting factor. We propose that the high-temperature-induced en route mortality in migrating sockeye salmon may be at least partly attributed to physiological limitations in aerobic performance due to cardiac collapse via insufficient scope for f_H. Furthermore, this improved mechanistic understanding of cardiorespiratory collapse at high temperature is likely to have broader application to other salmonids and perhaps other aquatic ectotherms.