Animals have evolved various methods of preventing predators from eating them. A fish that can change the color of its skin to blend in with its surroundings will be less likely to be killed than another fish that is unable to camouflage itself this way. However, there are no fool-proof defenses. There will always be a predator that has adapted to the protective behavior by being sharp-eyed enough, fast enough or in some other ways able to overcome a potential prey’s defenses. If this wasn’t the case then the prey species would essentially become invincible, their numbers would skyrocket and the balance of the ecosystem would be upset. This, in essence, is what happens with some animals that are introduced into an alien environment and live in an area where no predator has co-evolved, but that’s a subject for another diary.
Schooling is one of the most common defensive mechanisms used by fish, especially for those types that live in the open water. In fact, a full 80% of all fish species school to some degree during at least part of their life cycle. The theory behind schooling behavior is that there is safety in numbers. A single fish swimming alone stands little chance against a larger predator. That same fish, when moving in unison with several hundred or thousand other fish, has just greatly increased its odds of surviving. Since a predator can’t possibly eat the entire school, many of the individual members will be spared once the attack begins.
But what happens when a school is faced with multiple predators? Or even worse, an interspecies coordinated attack? Some of the more intelligent oceanic predators, such as tuna and sea lions, have figured out a way to jujitsu the schoolers.
Normally the structure and heirarchy of a school is strictly adhered to by its members. The school moves through the water as if directed by a collective brain, with each individual maintaining both its place in the schooling pattern as well as the distance it keeps from those fish that are next to it. If a group of predators respond to the schooling behavior not with a chase and attack method, but instead with a herding technique, then the structural pattern of the school breaks down. Panic results and each individual in the school goes into a self-preservation mode by abandoning the collective good of the school and diving right in towards the relative safety of the center of the pack. The fish left on the outside follow suit and soon every fish is diving for the center and the school no longer forms a school but a hovering ball of bait fish clammoring for the middle of the mass. In other words what we’ve got here is piscine anarchy, otherwise known as a bait ball.
The predators continue to herd and harrass the school and one by one the bait ball members are picked off and eaten. Soon there is nobody left, as you can see from this video of dolphins creating and then attacking a bait ball.
National Geographic has some great footage of a large mackerel bait ball being decimated in a coordinated attack by bluefin tuna, dolphins and even shearwaters diving in from above. View it here.
Other diaries in this series can be found here.