High unsteady (acceleration) performance of rainbow trout (L = 14.3 cm) and green sunfish (L = 8.0 cm) was studied in response to electric shock stimulus. Acceleration movements were divisible into a preparatory stage 1 and a main propulsive stage 2. Locomotory behaviour varied between faststarts and turning manoeuvres.

Taking the centre of mass for the stretched straight body as the reference point, distance covered with time was described by the equation;

distance covered = a. (time)b.

The mean value of b was 1.60 for trout and 1.71 for sunfish. The overall mean distance covered and time to the end of stage 2 was 5.36 cm in 0.078 sec for trout and 2.85 cm in 0.079 sec for sunfish. Velocity increased curvilinearly with time. Maximum values of 20 L/sec were observed, but overall mean values at the end of stage 2 were 8.5 L/sec for trout and 8.3 L/sec for sunfish. Acceleration rate was not uniform but decreased with time. Mean maximum values were calculated of 42 m/s2 for trout and 16 m/s2 for sunfish, but overall mean values for an acceleration movement were 13 m/s2 and 8 m/s2 for the two species respectively.

The observed acceleration behaviour is more advantageous than uniform acceleration because a greater distance is covered and greater velocities acquired in a shorter time, while the increased energetic cost is only 2–3 % of the total energy expended.

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