Introduction

Two recent events in British Columbia, Canada demonstrate that relatively small temperature shifts can have rapid and pronounced effects on fish populations. The first event involved a warm mass of marine water moving to a higher than normal latitude (an El Niño effect) and bringing with it a large stock of mackerel. These more active mackerel preyed extensively upon juvenile salmon and herring that normally grow in these coastal waters (B. Hargreaves, personal communication).

The second event involved the upstream spawning migration of sockeye salmon in the Fraser River and their normally spectacular but successful negotiation through Hells Gate and other ferocious rapids, the most formidable natural barriers on the river. In 1994 water temperature was several degrees higher than the yearly average for the Fraser River, reaching 20 °C during one week in the migration window. Sockeye salmon, on reaching Hells Gate, were observed to lose their upstream orientation and retreat downstream (S. Hinch, personal communication). Estimates suggest that an unusually large number of sockeye salmon failed to negotiate these warmer than normal rapids, and of those that did reach the spawning grounds, many laid eggs that did not hatch (C. Walters, personal communication).

These two examples underscore the importance of swimming performance (or the lack thereof) for fish survival and in reproductive success. Cardiovascular performance plays a central support role in aerobic swimming and is often regarded as the limiting factor for maximum exercise.