Straight, forward, unrestrained swimming behaviour, with periodic lateral oscillations of body and tailfin, was described and compared for saithe and mackerel. A method was developed for kinematic analysis of forward motion, lateral displacements and body curvature, based on a Fourier-series approach.

The dimensionless kinematic quantities showed relatively small variations over large ranges of swimming speeds. The speed range of mackerel was twice as fast as that of saithe. The main difference in swimming style between the species was a somewhat greater tail amplitude and a correspondingly stronger curvature near the caudal peduncle in mackerel than in saithe.

The swimming style of both species yields a Froude efficiency close to the maximum value possible, given the observed amplitude increase in the posterior part.

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