Warm northern river temperatures increase post-exercise fatigue in an Arctic migratory salmonid but not in a temperate relative

1. The Arctic is warming at twice the global average rate; how native and non-native anadromous fishes will respond remains largely unknown. Some native Arctic salmonids are already experiencing warm (>21 degrees C), physically challenging migratory river conditions and large diurnal temperature fluctuations (>10 degrees C). 2. We conducted field and laboratory experiments to determine how these extreme conditions may affect the capacity for migration in Arctic and temperate salmonids. 3.In adult migratory Arctic char, reflex impairment following a handling challenge increased with temperature, indicating more extensive fatigue. In Arctic char smolts, temperature did not affect initial critical swimming performance (U-max1), however, there was a threshold for repeat swimming performance (U-max2) near 20 degrees C, above which recovery was impaired. 4. Following a simulated diurnal warming scenario (11-21 degrees C), U-max1 increased in rainbow trout and remained constant in Arctic char as in the field while U-max2 remained constant in rainbow trout it was drastically reduced in Arctic char. Furthermore, at warm temperatures, Arctic char were unable to recover to routine levels of oxygen uptake after exercise, while rainbow trout were. Warming also had more pronounced effects on blood composition and plasma glucose and lactate concentration in Arctic char than in rainbow trout. 5. In general, rainbow trout, a temperate salmonid, had superior swimming performance, aerobic capacity, and warm tolerance than Arctic char, an Arctic salmonid. 6. The present flow and temperature regimes in some Arctic rivers may restrict migration of native salmonids by limiting their ability to recover from fatiguing exercise. Non-native, temperate salmonids are likely better suited to overcome these particular physical and thermal challenges.