Adaptive Value

Why do fish school?

"Thus, few school members suffered as much harrassment as loners."
- (Robertson et. al 1976)

Schooling provides several clear benefits to those fish who choose to indulge in it. When several hundred, even several thousand, fish are packed into a relatively small area, each individual fish is more protected from muscle usepredation than if he were a lone fish. Robertson et. al observed that when a school of S. croicensis was attacked by territorial members of the same species, the attackers were on average less persistent than the same territorial fish attacking lone fish. They note that lone fish are forced to flee, possibly for long distances, and abandon their feeding activity in order to escape, while fish in a school can usually count on the attacker's attention being diverted to another member of the school. This results in the schooling fish being better able to continue their feeding unmolested. If by chance a particular fish is targeted repeatedly by a predator, it can "hide in the crowd", so to speak, and lose the predator's attention that way.

Better Living Through Hydrodynamics

It has been widely suggested that another reason for the widespread prevalence of schooling is the considerable energy savings each fish is able to achieve due to the hydrodynamic characteristics of a school. This effect is explained in detail in Liao et al. 2003; a fish swimming in a vortex is able to use less muscle power, as illustrated to the right, with red equalling more muscle use. (Image credit: Liao et al. 2003) Liao et. al do not connect these energy savings to schooling, but the connection has been made as early as Wiehs 1973.

Landa 1988 suggests that many fish school selfishly; that is, they join schools in order to recieve the most benefits at the least cost. These benefits, according to Landa, are primarily hydrodynamic and protective in nature. Fish do not "ride for free"; there is a minimum amount of effort they must put in to not be left in the collective dust of the school they are attempting to join. However, this effort, at least for fish in the middle of the school, is much less due to hydrodynamics than would be spent by a lone fish.

An interesting study of prioritization in school benefits is provided by Abrahams and Colgan 1985. Their study determined that schools of Notropis heterodon would, upon exposure to a predator, alter their schooling structure from the most hydrodynamically favorable structure to the structure with the highest visibility for individual fish. This was accomplished by an extension of the school along the vertical, allowing each fish to see without obstruction by its neighbors but decreasing overall movement efficiency. This result indicates that fish value predator avoidance more highly than efficiency, which suggests that the primary purpose of schooling may in fact be to avoid predation. The authors note, however, that hydrodynamic efficiency is never completely sacrificed.
height v predator
Image source: Abrahams and Colgan 1985.

This graph illustrates the results obtained by Abrahams and Colgan. Schools marked with a C are control schools; the others were exposed to predators. The black bars are pre-exposure, the white post-exposure. Asterisks indicate significant extensions in verticality. We see that several schools, 1, 11, and 13 particularly, extended by several hundred percent.