Schooling in fish  (Page 3/10)

Evolutionary basis of schools

Although schools themselves operate as a single, cohesive unit that collectively makes decisions, the evolutionary study of schools focuses on the benefit reaped for each individual, as aggregation theory explains that individuals must school for selfish reasons (Hamilton 1970). Individuals school only because it is better for themselves and their genes. As an example of the advantages of schooling, studies of the spottail shiner ( Notropis hudsonius ) show that individuals within a school spend less time engaging in antipredator behavior (which is energetically costly) and allow them to locate more food because more individuals are searching for it (Seghers 1981).

Predation avoidance

Schools help fish avoid the risk of predation by getting away more easily in a group, evaluating predators more effectively, and sharing learned behaviors. The use of schools to avoid predation is one of the most studied evolutionary explanations for schooling in fish and may very well be one of the main reasons schooling evolved.

At the most basic level, schools protect the individual members from predation by confusing a predator (Parrish 1991). When many fish swim together, it becomes harder to focus on one fish and make a catch, especially since the school tends to continuously move. Without a visual target, most predators are unsuccessful in catching their prey. As an individual, unless one is on the edge of the school, one is safer from predators simply because there is a barrier between one fish and another. This is definitely a source of conflict, as if a predator catches a fish from the school, it will be a fish on the edge (Hamilton 1970), however these fish still get some level of protection because the school still can confuse the predator.

When approached by a predator, solitary herring ( Clupea harengus L. ) react much more quickly than those in a school ( [link] ). While it is possible that herring in schools move slower so as to avoid collisions with other members, the slower speed likely indicates decreased urgency (Batty and Domenici 1997), indicative of the benefit to individuals in the schools. Evolutionarily, fitness is optimized when the energy invested in any behavior is just good enough: in this case, it takes less energy for fish that are schooling to avoid the predator than it does for solitary fish, because the school confuses the predator. This allows more energy to be invested in other behaviors, such as finding food or a mate.

Schools dilute the effect of predators: by taking individuals from an environment and concentrating them into one unit, this decreases the probability of ever meeting a predator (Parrish 1991). Additionally, fish commonly approach a threat the first time they encounter it in order to decide how much energy needs to be exerted to avoid the threat in the future. Evaluation behavior, though useful, can be risky as it requires a fish to get close enough to the potential predator to evaluate size, shape, or other factors. The hypothesis that schools increase survivorship during predator evaluations was tested in the Trinidadian guppy ( Poecilia reticulata ) by observing the mortality of individuals within schools compared to solitary individuals who attempted evaluations of predators. Individuals in a school had virtually no mortality (Magurran 1994), because no one individual was likely to be attacked due to confusion effects, allowing a collective examination without the threat to any individual.