There are other advantages to aggregating into shoals, defined as any group of fish, or eventually schools, when fish travel and turn together as if one organism. First off, if there are more fish for a predator to choose from, each individual is less likely to get eaten. If the fish look enough alike in both size and coloring, the predators can become confused, unable to pick one fish out of the bunch and thus are less efficient hunters. Larger groups also are better at watching for predators because there are more eyes on the lookout (Megurran 1990).
When the fish are all riled up in the presence of a predator, they do form these dense bait balls as each fish scrambles to get to the center to hide. This centripetal motion literally forms a sphere: the shape that has the most interior volume and keeps the fewest number of fish on the surface and thus more vulnerable to predators. Of course, this is not the shape that a shoal traveling casually forms. Instead, we have “circles, discs, ellipses, triangles, wedges, crescents, and lines” (McFarland and Moss 1967). Thus predation alone cannot be the only factor shaping shoals; is there another central factor and what is it?
via Guest Blog: Now in 3-D: The shape of krill and fish schools.