Pectoral fin locomotion in the striped surfperch. I. Kinematic effects of swimming speed and body size

Swimming trials at increasing velocity were used to determine the effects of steady swimming speed on pectoral fin kinematics for an ontogenetic series of striped surfperch Embiotoca lateralis, ranging from 6 to 23 cm in standard length (SL). The fin stroke cycle consisted of a propulsive period, the duration of fin abduction and adduction, and a 'refractory' period, during which the fin remained adducted against the body. Pectoral fin-beat frequency (fp) measured as the inverse of the entire stride period, as in past studies, increased curvilinearly with speed. Frequency, calculated as the reciprocal of the propulsive period alone, increased linearly with speed, as shown previously for tail-beat frequency of fishes employing axial undulation. Fin-beat amplitude, measured as the vertical excursion of the pectoral fin tip during abduction, increased over a limited range of low speeds before reaching a plateau at 0.35­0.40 SL. Pectoral fin locomotion was supplemented by intermittent caudal fin undulation as swimming speed increased. At the pectoral­caudal gait transition speed (Up-c), frequency and amplitude attained maxima, suggesting that the fin musculature reached a physiological limit. The effects of body size on swimming kinematics differed according to the method used for expressing speed. At a given absolute speed, small fish used higher stride frequencies and increased frequency at a faster rate than large fish. In contrast, the relationship between fp and length-specific speed (SL s-1) had a greater slope for large fish and crossed that for small fish at high speeds. We recommend that comparisons across size be made using speeds expressed as a percentage of Up-c, at which kinematic variables influencing thrust are size-independent.