COMMUNICATION OF VELOCITY CHANGES IN JACK MACKEREL (TRACHURUS-SYMMETRICUS) SCHOOLS

The latency and the velocity of the response of jack mackerel, T. symmetricus, to a velocity change by a schooling companion was studied in the laboratory. Fish were tested in groups of two and six fish each. One fish in each group was given an electric shock; the responses of the other fish in the group to the increase in speed by the electrically stimulated fish were filmed at 24 fps, and the films were analysed frame by frame. In both two-and six-fish tests latencies were shorter when the electrically stimulated fish was near the centre of the monocular field or in the binocular field of the responding fish, and they were longer when the stimulated fish was in the periphery of the monocular field of the responding fish. Latency was correlated with the angular velocity or the movement of the stimulated fish relative to the eye of the responding fish. These results suggested that latency depended upon visual perception of movement. In the six-fish experiments the velocity of the response was correlated with the velocity of the electrically stimulated fish and its angular position. Fish responded more strongly when the stimulus fish was to the side than when it was either in front or behind. In the two-fish experiments, on the other hand, response velocities were correlated only with stimulus velocities. These results suggested that fish used the speeds of all fish in visual range as a velocity reference but ones to the side played a greater role in speed regulation than did ones behind or in front. © 1969.