The three-dimensional structure of fish schools

Abstract

1. Methods for producing and analyzing long-term three-dimensional records of the positions of individuals within fish schools are described. Detailed analysis of internal structure and overall shapes of schools of three species which school to varying degrees are reported: Data from cod (Gadus morhua), a weak facultative schooler, saithe (Pollachius virens), a strong facultative schooler, and herring (Clupea harengus), an obligate schooler, are presented. 2. Fish do not position themselves at random within schools (Figs. 1 and 2). This is shown to result from the maintenance of minimum approach distances between fish (Fig. 3). Comparison of the frequency of neighbors at different distances with that expected at random (Fig. 3), however, demonstrates that fish space themselves more regularly than one would expect if the structure resulted wholly from minimum approach distances. 3. Herring and saithe are shown to swim at nearly but not exactly the same depth as their neighbors (Fig. 5). Neighbors are more-or-less equally common in all directions (bearings) around fish, although herring show some tendency toward taking up positions at 45° and 135°, the positions expected if school structure were a cubic lattice. School structure is present in a statistical sense only. 4. Herring are shown to maintain proportionately larger interfish distances than do saithe or cod (Fig. 7). These results are discussed in terms of the body structure of the three species and the antipredator function of schooling. Shape of cod schools and, to a lesser degree, saithe schools, is shown to be highly variable. Herring schools, however, appear fairly constant in their external shape (Fig. 10). 5. Factors affecting the structure of schools of saithe are studied. Increasing the number of fish in the school or the speed at which it is swimming results in smaller interfish distances (Figs. 8 and 9).