Swimming kinematics of juvenile kawakawa tuna (Euthynnus affinis) and chubmackerel (Scomber japonicus)

The swimming kinematics of two active pelagic fishes from the family Scombr idae were compared to test the hypothesis that the kawakawa tuna (Euthynnus affinis) uses the thunniform mode of locomotion, in which the body is held more rigid and undergoes less lateral movement in comparison with the chub mackerel (Scomber japonicus), which uses the carangiform swimming mode, Th is study, the first quantitative kinematic comparison of size-matched scomb rids, confirmed significantly different swimming kinematics in the two spec ies. Ten kawakawa (15.1-25.5 cm fork length, FL) and eight chub mackerel (1 4.0-23.4 cm FL), all juveniles, were videotaped at 120 Hz while swimming at several speeds up to their maximum sustained speed at 24 degreesC, Compute rized motion analysis was used to digitize specific points on the body in s equential video frames, and kinematic variables were quantified from the pr ogression of the points over time. At a given speed, kawakawa displayed a s ignificantly greater tailbeat frequency, but lower stride length, tailbeat amplitude and propulsive wavelength, than chub mackerel when size effects w ere accounted for. Midline curvatures subdivided on the basis of X-rays int o individual vertebral elements were used to quantify axial bending in a su bset of the fish studied. Maximum intervertebral lateral displacement and i ntervertebral flexion angles were significantly lower along most of the bod y in kawakawa than in chub mackerel, indicating that the kawakawa undergoes less axial flexion than does the chub mackerel, resulting in lower tailbea t amplitudes. However, lateral movement at the tip of the snout, or yaw, di d not differ significantly interspecifically. Despite these differences, th e net cost of transport was the same in the two species, and the total cost was higher in the kawakawa, indicating that the tuna juveniles are not mor e efficient swimmers.