The recovery of locomotory activity following exhaustive exercise in juvenile rainbow trout (Oncorhynchus mykiss)

This study investigated the recovery of locomotory activity in exhausted juvenile rainbow trout (Oncorhynchus mykiss, approximately 6-10-cm fork length) in response to two conditions: (1) direct transfer to a range of velocities (0-15 cm s(-1)) in a swim flume (forced swimming) and (2) direct transfer to a pool downstream of a swim channel where a choice of velocities was presented: 2-3 cm s(-1) in the lower half of the pool, a range of velocities from 7 to 40 cm s(-1) in the upper half the pool near the channel entrance, and a velocity of 57 cm s(-1) in a swim channel emptying into the pool (volitional swimming). Exhausted trout showed a pronounced delay in the recovery of normal locomotory activity. With forced swimming, the time required to resume swimming was inversely proportional to water velocity. At 15 cm s(-1), almost all exhausted fish recovered immediately, whereas it took about 1 h for recovery at a current of 5 cm s(-1). In contrast, nonexhausted fish responded to imposed velocity with immediate rheotactic responses (orientation and station holding) at all test velocities. In voluntary swim trials, exhausted trout showed a marked preference for holding station in current in the downstream pool (approximately 11 cm s(-1)) but took, on average, 2 h longer than nonexhausted fish to make transits in the swim channel. Moreover, their ground speed in the swim channel was significantly slower. We conclude that swimming performance is impaired for at least 6 h by exhaustive exercise. Maladaptive behaviors during this time include a preference for current near the surface over cover and a reduced capacity for burst activity, both of which would translate into greater predation risk and reduced ability to forage.