page_title: Bait ball
section_title: Predator cooperation
hierarchical_section_title: Bait ball / Predator cooperation
caption_attribution_description: Humpback whales in North Pass between Lincoln Island and Shelter Island in the Lynn Canal north of Juneau, Alaska. This is a group of 15 whales that were bubble net fishing on 18 August 2007.
context_page_description: A bait ball, or baitball, occurs when small fish swarm in a tightly packed spherical formation about a common centre. It is a last-ditch defensive measure adopted by small schooling fish when they are threatened by predators. Small schooling fish are eaten by many types of predators, and for this reason they are called bait fish or forage fish.
  For example, sardines group together when they are threatened. This instinctual behaviour is a defense mechanism, as lone individuals are more likely to be eaten than an individual in a large group. Sardine bait balls can be 10–20 metres in diameter and extend to a depth of 10 metres. The bait balls are short-lived and seldom last longer than 10 minutes.
  However, bait balls are also conspicuous, and when schooling fish form a bait ball, they can draw the attention of many other predators. As a response to the defensive capabilities of schooling fish, some predators have developed sophisticated countermeasures. These countermeasures can be spectacularly successful, and can seriously undermine the defensive value of forming bait balls.
context_section_description: The most effective strategy predators use against schooling fish is first to scare them into forming a bait ball. Strategies such as those outlined in the previous section can work to a degree against freely streaming fish schools, but work much better if the fish school is first compacted into a bait ball. It is difficult for predators working individually to scare a fish school into a bait ball, and they usually work together.
  Thresher sharks compact their prey by swimming in circles around them, splashing the water with their long tails, often in pairs or small groups. They then strike sharply at the bait ball to stun the fish with the upper lobe of their tails.
  Schools of forage fish can draw silky sharks in large numbers. Silky sharks have been documented "herding" such schools into a bait ball trapped against the surface, and then consuming the entire school. When attacking tightly packed fish, silky sharks charge through the ball and slash open-mouthed, catching the prey fish at the corners of their jaws. Although multiple individuals may feed at once, each launches its attack independently.
  Pods of many dolphin species commonly herd a school of fish into a bait ball while individual members take turns ploughing through and feeding on the more compacted shoal. Corralling is a method where fish are chased to shallow water where they are more easily captured. Some dolphins take this further with strand feeding, driving prey until they are stranded on mud banks, where they can be easily accessed. Dolphins have also been observed blowing bubbles to startle and separate individual fish from a bait ball.
  The humpback whale uses a feeding technique called bubble net feeding. A group of whales swim in a shrinking circle, blowing bubbles below a school of prey fish. Forage fish show a strong fear of bubbles and can be easily contained within a bubble curtain. The shrinking ring of bubbles encircles the school and confines it in an ever smaller cylinder. The whales then lunge feed, often as a synchronised group, suddenly swimming upward through the "net", mouths agape, swallowing thousands of fish in one gulp. The ring can begin at up to 30 metres (98 ft) in diameter with perhaps a dozen whales cooperating. The use of a crittercam attached to whale's back has revealed that some whales blow the bubbles while others dive deeper to drive fish toward the surface, and yet others herd the prey into the net by vocalizing. Some humpback whales also scare schooling fish by slapping their tails (lobtail). Although many whale species lunge feed, only humpbacks use bubble nets.
  Killer whales usually hunt larger fish, such as salmon, individually or in small group of individuals. However, forage fish, such as herrings, are often caught using carousel feeding. The killer whales force the herrings into a tight ball by releasing bursts of bubbles or flashing their white undersides. They then slap the ball with their tail flukes, either stunning or killing up to 10–15 herring with a successful slap. The herring are then eaten one at a time. Carousel feeding has been documented in only the Norwegian killer whale population and with some oceanic dolphin species.
  Sperm whales may also herd prey.
caption: Group of humpback whales lunging through the centre of their bubble net
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