Why do fish school?

By Jamie Gove

The swirling black mass from above was hard to miss, particularly given the flurry of activity of squawking birds dive bombing the ocean surface, only to emerge seconds later with a flopping small fish in their beaks. As an amazing of a spectacle as it was from above the surface, I wasn’t prepared for what was happening below. Jumping in off the side of the boat, it took my eyes a few seconds to focus and my mind even longer to comprehend the scene unfolding before me. A school of thousands of tiny bait fish were swirling in a large tornado-like vortex, while just beneath, a large group of mackerel were rising from below and striking the ball of fish with lightening speed. Also in the mix, albeit on the periphery, were a small group of rainbow runner and few grey reef sharks. Basically, we had just entered nothing short of a total feeding frenzy, with the school of bait fish at the center of it all….

A 3-shot sequence showing mackerel coming from below, striking the school of bait fish.

Fish congregate together in schools for a variety of reasons, ultimately to enhance an individual fish’s chance of survival in a competitive and dangerous fish-eat-fish world.

First and foremost, fish school as a “safety in numbers” strategy; the more fish there are in a group, the less likely a predator will be able to eat any single fish. Generally speaking, it’s much easier for a larger predatory fish to track down and eat a solitary smaller fish, whereas schooling fish can distract and disorient a predator, making it more difficult to snatch a fish from the group.

Danny Merritt goes for an up-close view of the bait ball.

Fish will also school to increase the chances of finding food. Essentially, the more fish there are in a group, the greater an area those fish can cover when searching for prey, or, detecting an area in the ocean which may be more suitable for finding prey. The downside of this strategy is rather obvious; an individual fish must share whatever food they find with the rest of the school. When I write these fish “share”, it’s not as if a fish will offer half of their dinner to another, more that they must compete within the school for the food they do find, making it highly possible for a number of fish to miss out on a meal when the rest have eaten more than their fair share.

A swirling blur of fish.

It is also thought that fish will school for a hydrodynamic advantage. Similar to how professional bicycle riders will draft behind other riders so as to reduce wind resistance and friction, fish swim in schools, following behind one another for the same reasons; to reduce friction in the water thereby conserving energy.

Not all fish school, but many do, particularly those which live in the open ocean which is devoid of hiding places or any form of protection from predators. In general, it is all about trading costs for benefits, and for certain fish, it just makes sense to come together to eat, and to hopefully avoid being eaten.