Phylogeny

Estimates are rough, but it is estimated that over 80 percent of all fish species exhibit schooling behavior at some point within their lives and most others aggregate to some lesser degree (Stout, Parrish 2002)

"At some point"

Parrish et al. notes that a spectrum exists: extremely territorial animals “with little need to engage in information transfer and no need for group structure” fall on one end, while highly integrated, independent, communicative species rest at the other. Fish species can oscillate between these endpoints to lesser degrees based on a huge variety of factors.

Schooling is an extraordinarily flexible behavior that can be modified throughout the fish’s lifetime (see ontogeny) and is highly dependent on environmental necessity for the behavior.

“Organisms alter their behavior with development or environmental change
so that they can enhance their fitness.” - Nakayakma

The same population can and does exhibit different schooling tendency based on
• hydrology
• age and gender
• prey distribution
• risk of predation
• territoriality
All these factors can change seasonally, daily, with associated factors or without.

For example...

mollies

Cave-dwelling (left) and surface-dwelling (right) mollies. Images courtesy of www.ngsprints.co.uk and www.elacuarista.com.

In a study of Atlantic mollies, cave-dwelling populations and surface-dwelling populations showed divergent shoaling tendencies, a less specialized aggregative behavior, based on predator presence and food availability. The cave dwelling populations faced far fewer predators and the lower food availability was conducive to decreasing shoaling behavior, which often leads to higher competition for resources (See figure below). (Plath and Schlupp 2006)

Competition

Time spent near a shoal following 1 week of starvation or 1 week ad libitum food availability. (Plath and Schlupp 2006).

A genetic difference in tendency to form shoals had been described for these cave- and surface-dwelling populations, as divergent shoaling behavior is retained after generations of laboratory testing.

Additionally, as the images above show, the cave- and surface-dwelling populations tend to show divergent pigmentation schemes. This may effect conspicuousness in the water and need for schooling behavior.

A sex difference?

The surface dwelling mollies mentioned in the example above attempted to form all-female shoals in laboratory experiments in order to avoid sexual harassment by males. Harassment by males is reduced in the cave-dwelling phenotype, as is sexual activity overall based on low food and energy availability (Plath and Schlupp 2006). Competition for mates, as with food, is to be avoided in the lower resource environment of cave-dwelling mollies.

A universal behavior?

An overwhelming majority of fish species school, but the evolutionary pressures governing this behavior are incredibly variable and fish exhibit vast life-time variation in schooling behavior.

Highly territorial species are the least likely to school, but even they may be driven to aggregation in extreme circumstances. Hammerhead sharks, for example, will school, but they are the only shark species to do so. They only do so during the day, and schools are composed mainly of small-medium sized sharks, perhaps to avoid predation by larger sharks. They are likely the only shark species to exhibit a dominance order for this reason.