Influence of seasonal temperature on the repeat swimming performance of rainbow trout Oncorhynchus mykiss

While the temperature dependence of exercise performance in fishes is reasonably well documented, information on the temperature dependence of metabolic recovery and reperformance is scant. This study examined the recovery of swimming performance after exhaustive exercise in rainbow trout Oncorhynchus mykiss at seasonal temperatures ranging from 5 to 17degreesC and explored the relationship between performance and preceding metabolic state. The primary objective of the study was to test the hypothesis that increased temperature increases the capability of rainbow trout to repeat a critical swimming speed (U-crit), as assessed by two consecutive critical swimming speed tests separated by a 40 min rest interval. An additional expectation was that certain plasma ionic, metabolic and humoral parameters would be correlated with how well fish reperformed and so plasma levels of lactate, potassium, ammonia, osmolality, sodium and cortisol, as well as hematocrit, were monitored before, during and after the swim challenges via an indwelling cannula in the dorsal aorta. As expected, performance in the first U-crit test (U-crit1) was positively related to temperature. However, the relationship between U-crit1 and reperformance (U-crit2) was not dependent on acclimation temperature in a simple manner. Contrary to our expectations, U-crit2 was less than U-crit1 for warm-acclimated fish (14.9 +/- 1.0degreesC), whereas U-crit2 equaled U-crit1 for cold-acclimated fish (8.4 +/- 0.9 degreesC). Cold-acclimated fish also exhibited a lower U-criti1 and less metabolic disruption compared with warm-acclimated fish. Thus, while warm acclimation conferred a faster U-crit1, a similar swimming speed could not be attained on subsequent swim after a 40 min recovery period. This finding does not support the hypothesis that the ability of rainbow trout to reperform on U-crit test is improved with temperature. Both plasma lactate and plasma potassium levels were strongly correlated with U-crit1 performance. Therefore, the higher U-crit1 of warm-acclimated fish may have been due in part to a greater anaerobic swimming effort compared with cold-acclimated fish. In fact, a significant correlation existed between the plasma lactate concentration prior to the start of the second test and the subsequent U-crit2 performance, such that U-crit2 decreased when a threshold plasma lactate level of around 12.2 mmol l(-1) was surpassed for the initial swim. No other measured plasma variable showed a significant relationship with the U-crit2 performance. We conclude that warm-acclimated fish, by apparently swimming harder and possibly more anaerobically compared with cold-acclimated fish, were unable to recovery sufficiently well during the fixed recovery period to repeat this initial level of performance, and this poorer repeat performance was correlated with elevations in plasma lactate levels.