Use of a Ramp Velocity Test to Measure Critical Swimming Speed in Rainbow Trout (Onchorhynchus mykiss)

Abstract The objective of this study was to estimate critical swimming speed (U crit ) in rainbow trout ( Oncorhynchus mykiss ) using a protocol considerably shorter than the traditional step-test protocol. A ramped U crit protocol was developed in which water velocity was initially accelerated to approximately 75% of the fatigue velocity over a 5- to 6-min period. Subsequently there were at least two stepped increases (30 min duration) before the fish fatigued. For one test group, U crit was measured first with the step-test protocol (velocity steps of 0.2 body lengths s −1 (bl s −1 ) lasting 30 min) and then again using the ramped U crit protocol after a 24-hr recovery period. The interpolated velocity at fatigue for the step-test (1.72 ± 0.08 bl s −1 , N = 5) was not significantly different compared with that for the ramped U crit protocol (1.64 ± 0.07 bl s −1 ; N = 5). However, the test duration was over 2 hr shorter with the ramped U crit protocol. In a second test group, the velocity at fatigue was 10% lower when the ramped U crit protocol preceded the step-test protocol. A third test group were given a practice high speed swim shortly after being introduced to the swim tunnel. In this group the fatigue velocity with the ramped U crit protocol (2.10 ± 0.06 bl s −1 ; N = 4) was identical to that obtained with the step-test (2.10 ± 0.06 bl s −1 ; N = 4). We conclude that the ramped test protocol can accurately estimate U crit with considerable time saving provided a brief practice swim is performed during the period when the fish is habituating to the swim tunnel.