The Role of the Pectoral Fins in Braking of Mackerel, Cod and Saithe

Abstract

Braking of mackerel, cod and saithe is analysed from high speed cine films. The body of the fish is curved, the pectoral fins are extended to a varying degree. In cod and saithe the dorsal and anal fins are curved. The role of the pelvic fins could not be studied in the available film sequences. The instantaneous velocities and decelerations were calculated. The maximum possible contribution to the braking force by the pectoral fins is discussed for the moments of highest deceleration. The pectorals, if fully extended perpendicular to the body and in a vertical position, cannot contribute more than about 30% of the braking force. The body and extended median fins, if held straight, deliver not more than about 15 %. So the curvature of the body and extended median fins is responsible for more than half of the braking force. The maximum power output of the abductor muscles of the pectoral fins during braking is discussed. From data in the literature it is assumed to be about 20 W kg -1 of muscle. The maximum velocity at which the fish can fully extend their pectoral fins in vertical position then will be about 0.5 L- 2/3 s -1 (L = body length). This is very low in comparison to maximum speeds (10 to 15 body lengths s -1 ). It could explain why a fully extended vertical position of the pectoral fins is not seen in the films. The surface drag coefficient of the braking fish at the instants of highest deceleration had values of 0.095 up to 0.428, about 8 to 36 times higher than for coasting cod, 1.5 to 7 times higher than for steady swimming cod and of about the same magnitude as for cod in the 'kick' phase of swimming. The morphology of the pectoral fin shows aspects limiting individual movements of the rays relative to each other: a firm connection between the proximal parts of the rays and the fibrocartilage pad, especially in mackerel, lamellae on the lateral hemitrichia pointing between the hemitrichia of the successive rays in cod and saithe, the tendency to a more collective insertion of the muscles on the rays and the situation that the muscles are hardly subdivided in bundles to individual rays. Also the movability of the proximal radials is restricted. In mackerel ligaments are present, blocking the abduction of the rays beyond a certain limit. No clear indications were found that the pectoral fins are adapted to deliver great braking forces. In mackerel the ratio of the weight of abductor to adductor muscles is about 2, in cod and saithe about 1, but the weight of the abductor muscles relative to the body weight is not significantly greater in mackerel than in cod and saithe. Backward movements of the pectoral fins, as e.g. in manoeuvring, will play a more important role in cod and saithe than in mackerel. From the kinematic, dynamic and anatomical data it can be concluded, that the role of the pectoral fins in braking is rectricted and confined to rather low speeds.