| Summary: | Herring (Clupea harengus L.), swimming freely, were recorded on high-speed video and the positions of their heads found at 5 ms intervals. The displacements, velocities and accelerations of various positions along the head were estimated. An analysis of this data showed that, with continuous swimming, the lateral motions of the head could be described reasonably well as the sum of side-slip movements (with equal lateral displacements for all points along the length of the head) and yawing movements around a pivoting position, P, on the mid-line of the head, about 16%-18% of the body length of the fish from the snout. The phases of the lateral displacements due to yaw were close to those of the lateral velocities at P; the lateral displacements and velocities, both being measured in a direction perpendicular to that in which the fish was moving. The lateral velocities were in good agreement with the products of the steady forward velocity of the fish U and the angle alpha (in radians) between the direction in which the head was pointing and the direction in which the fish was swimming. The angular velocity, OMEGA, of the turning of the head was close to being equal to 0.87A/U, where A is the acceleration of the head at P in the direction perpendicular to that in which the head was pointing. OMEGA and A were in phase. These facts give support to a theory described by Lighthill in the preceding paper on how clupeids might 'turn their heads' during swimming so as to swim more economically and diminish the large stimuli that a fish's own movements would otherwise give to the receptor organs of the lateral line system. An analysis of data taken from earlier work on cod (Gadus morhua) by J. J. Videler and C. S. Wardle, and on bream (Abramis brama) by R. Bainbridge, showed that these fish probably make head movements with the same properties.
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