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 (<em>Oncorhynchus nerka</em>) 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 (<em>T</em>_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 (<em>M</em>O₂), cardiac output <i>V</i>·_b, heart rate (<em>f</em>_H), and cardiac stroke volume (<em>V</em>_s), as well as arterial and venous blood oxygen status in adult sockeye salmon swimming at temperatures that bracketed <em>T</em>_opt to elucidate possible limitations in oxygen uptake into the blood or internal delivery through the oxygen cascade. Above <em>T</em>_opt, the decline in <em>M</em>O_2max and aerobic scope was best explained by a cardiac limitation, triggered by reduced scope for <em>f</em>_H. The highest test temperatures were characterized by a negative scope for <em>f</em>_H, dramatic decreases in maximal <i>V</i>·_b and maximal <em>V</em>_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 <em>en route</em> 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 <em>f</em>_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.