Swimming kinematics of two scombrid fishes (Scomber japonicus, Euthynnus affinis)

The swimming movements (kinematics) of two active pelagic fishes from the family Scombridae were compared in order to test the hypothesis that the kawakawa tuna (Euthynnus affinis) is a more stiff-bodied swimmer than the chub mackerel (Scomber japonicus). These fishes are thought to represent two distinct locomotor styles of body-caudal fin propulsion: the kawakawa is categorized as a thunniform swimmer and the chub mackerel as a carangiform swimmer. This study, the first quantitative kinematics comparison of size-matched juvenile scombrids, confirmed different locomotor modes in the two species. At a given speed, kawakawa displayed a significantly greater tail-beat frequency but lower stride length, tail-beat amplitude, and propulsive wavelength than chub mackerel. Intervertebral lateral displacement and flexion were significantly lower in kawakawa than in chub mackerel, indicating that the kawakawa is indeed a more stiff-bodied swimmer. Despite these differences, the net cost of swimming was the same in the two species.