Swimming performance of delta smelt: maximum performance, and behavioral and kinematic limitations on swimming at submaximal velocities.

Swimming performance, measured as critical swimming velocity (Ucrit) and endurance, and swimming behavior and kinematics were measured in delta smelt Hypomesus transpacificus, a threatened estuarine planktivore. Most fish (58 % of the Ucrit test group) were capable of achieving and sustaining moderately high velocities: mean Ucrit was 27.6±5.1 cm s-1 (s.d.). Ucrit was not affected by either acclimation temperature (12­21 °C) or fish size (3.2­6.8 cm standard length) and was generally comparable with values measured for other similarly sized fishes. The remaining 42 % of the fish failed to swim at velocities above 10­15 cm s-1. Interestingly, of the fish that provided a Ucrit measurement, 62 % experienced at least one temporary swimming failure between 10 and 20 cm s-1. Endurance was highly variable and, for all velocities, not normally distributed; the only significant decrease, from 6 h to 64 min, occurred between 10 and 15 cm s-1. Kinematic analyses of stroke frequency, stroke amplitude, stride length, glide frequency, glide duration, proportion of time spent stroking and the number of strokes between successive glides showed that delta smelt employed three velocity-dependent swimming gaits: a discontinuous 'stroke-and-glide' swimming behavior below 10 cm s-1; a continuous swimming behavior above 15 cm s-1 and up to Ucrit; and a discontinuous 'burst-and-glide' swimming behavior at velocities above Ucrit. Swimming failure at velocities between 10 and 20 cm s-1 coincided with the transition from 'stroke-and-glide' swimming to continuous swimming; delta smelt were unable or unwilling to swim steadily in the flume within this transition velocity range. These results underscore the importance of monitoring and quantifying behavior in experiments intended as physiological performance tests of whole animals.