A bait ball is a tightly-packed school of fish, swimming around a common point in the middle of the ball. It’s a desperate defensive measure that (usually small) schooling fish use when surrounded by predators.
Sardines instinctively swim closely together when they’re under threat, as lone fish are more likely to be picked off than larger groups. Sardine bait balls are up to 20 meters wide and 10 meters tall. They don’t usually last more than 10 minutes before dispersing.
It is clearly visible
Bait balls are very conspicuous, though, and draw attention from predators far and wide. Aquatic predators have evolved some impressive techniques for dealing with bait balls that can scupper the defensive plans of the prey fish.
It is a defense mechanism
Pelagic fish live in open water, so they can’t take advantage of coral, or kelp, to hide in. This makes them very vulnerable to attack by predatory fish, seabirds or mammals, so these fish have developed the bait ball technique for protection. A school of fish has lots of pairs of eyes working to detect and so avoid predators. Also, their bodies tend to be quite silvery and so can dazzle predators, making it hard to pick out individual fish. The schools are also very fast-moving, and can dart up and down and sideways, and even split apart and regroup behind a predator. They can also totally split apart and disperse rapidly, which is very confusing. It seems as though the fish are synchronized and acting according to complex rules, but it’s actually more the case that they are obeying simple rules, like avoiding collisions with each other and following the same directions.
Sometimes schooling can backfire
Some species of schooling fish form huge groups at certain times of the year and at certain locations, depending on the availability of their usual food source. Vast numbers of fish in a small space attract large numbers of predators, like sharks, dolphins, seabirds and humpback whales. Bait balls arise when the fish realize that they are surrounded on all sides by predators of all sorts and the fish panic.
Predators work together
This is when predator co-operation comes in. Many predators realize that the schooling fish are easier to attack when they’re in a tight ball, if, that is, there are lots of predators and they simply “dive in”. This co-operative behavior is seen not just between members of the same species, but between members of different species, too.
How does it work?
It all starts when predators see a fish school, and they rush up to it, forcing the fish near to the surface of the water, all the while herding them into a smaller space. Once tightly packed, the fish forget their basic schooling rules and their movements become chaotic. Each fish tries to get into the safer center of the bait ball, and this creates a dense sphere of fish. The relatively low surface area of this sphere means that the predators only have to decide between a few individuals. Eventually, the outside fish are picked off, exposing the ones inside, which are then picked off until only a few remain, becoming easier and easier to capture.
Predators also sometimes charge schools or bait balls at high speeds, hoping to grab whatever they can. This is known as lunge feeding, and many whales use this method. They simply lunge, mouths agape, at the bait ball. The water pressure on the mouth of the whale opens it even further, allowing the animal to take in a huge amount of water and prey fish, which it then filters out.
Others join in too
Swordfish dash into schools, slashing with their swords, before returning to capture and eat injured or dead fish. Thresher sharks stun fish by slapping at them with their tails, before turning to scoop them up. Gannets dive in from the sky at speeds up to 86 kilometers per hour (53 mph), snatching any fish they can get.