Methods for measuring the three-dimensional structure of fish schools

Abstract

Two methods are described for measuring the three-dimensional co-ordinates of the individual fish in a school photographed from above. The Stereo Method uses a double-prism device which produces two images on the film taken from positions slightly apart, and the distance of the fish from the camera can be determined by its parallax against the background. The Shadow Method depends on the fact that the shadow of a an object in the sun is farther to the side the higher the object above the background. The coordinates in the horizontal plane are easily determined once the distance of the fish from the background is known. The advantages and disadvantages of the two methods are considered, and a sample set of photographs of a school of 10 pilchards, made with the Shadow Method, is analysed. These results are then used to calculate the distance and bearing in the horizontal and vertical planes between nearest neighbours in the school, and to give information about the regularities in geometrical spacing of the fish. The photographs were also measured to show to what extent the fish were swimming parallel.