Repeat swimming performance of mature sockeye salmon following a brief recovery period: a proposed measure of fish health and water quality

Measurements of swimming ability, such as critical swimming speed (U-crit), have commonly been used as indicators of the effects of environmental challenges on the general health of fish. In this study, we introduce repeat swimming performance as a particularly sensitive means to assess fish health and the effects of environmental stressors. Adult sockeye salmon (Oncorhynchus nerka) performed two U-crit,tests separated by a 40-min recovery period. When recovery ability was expressed as a ratio of Ucrit values in the first and second swim challenges (U-crit,U-2/U-crit,U-1) control fish exhibited recovery ratios of unity (0.98 +/- 0.01 (mean +/- SEM)). In contrast, the recovery of fish pre-exposed to between 0.12 and 0.77 mg.L-1 dehydroabietic acid (DHA) for 8-14 h, and swimming in either hypoxia or normoxia, was impaired. These fish had recovery ratios significantly lower than unity (0.92 +/- 0.02) despite swimming to a similar initial U-crit as control fish. The effect of pre-exposure to DHA was also evident in measurements of oxygen consumption and plasma lactate concentration. Unhealthy fish exhibited significantly lower initial and second U-crit values than control fish. To account for the low initial swimming performance of these fish, a normalized recovery ratio was introduced ((U-crit,U-1/U-crit,U-1(control) + U-crit,U-2/U-crit,U-1)/2). This index of recovery (0.65 +/- 0.08) identified the poor physical status of these fish.