Because not all species school consistently, however, individuals of these species are more inclined to become aggressive to other in their school. Schooling increases competition for food and resources, especially when these are scarce (Magurran 1991), and cross species comparative studies show that there is less aggression among various species of commonly schooling fish as compared to guppies who do not necessarily school regularly (Magurran 1991). Selfish herd theory predicts this behavior in terms as each individual is selfishly joining the school when it is in trouble, and is therefore more inclined to fight for limited resources (Hamilton 1970).
Alternatively, juvenile chum salmon, ( Oncorhynchus keta ) are a schooling fish until presented with a large food source (Olla and Ryer 1991). In the presence of this very extensive source of food, they abandon the school, become aggressive and hoard food. This behavior is usually one responsive to scarcity or increased competition, but in this case it is responsive to a resource-rich environment. Individuals are be expected not to school so as to not attract predators, and because individuals with lower fitness may actually get less food in a school than individually, but the aggression and hoarding is a seemingly unnecessary energy expenditure. However, because this species is rarely presented with a resource rich environment, it rarely roams solitarily. It is hypothesized that this behavior is simply a reaction to an altered environment that has not been refined by selection (Olla and Ryer 1991).
Unexplained aspects of schooling
In addition to the behaviors of a school that are easily explained in terms of individual fitness, many other aspects of schooling are harder to explain. Northern bluefin tuna ( Thunnus thynnus ), in particular are often seen milling around unidentified core, first attracted to the area by a particulate or other object, but the school soon grows to a point that this could not be the motive for aggregation (Edestein-Keshet and Parrish 1999).
North esk salmon ( Salmo salar ) smolts introduced to a new environment school for the first 24 hours, and then disperse. New introductions to these tanks immediately try to seek out existing schools, or form their own, suggesting that fish may school as a way of alleviating stress (Johnstone et al 1995), however no evidence exists to support this hypothesis.
It has also been observed across species that schools tend to contain individuals with uniform phenotypes such as size or color, and studies indicate that fish within a school also have comparable foraging abilities, leading to the idea that fish self-segregate based upon phenotypes (Ranta et al 1994). Although this may indicate fitness advantages for the more skilled fish, it does not explain how the less skilled schools are selected for or why phenotypes such as color also aid aggregation. This does make sense from the perspective of the selfish herd because you would only school with groups in which you have an opportunity to get to the center, and this is most likely to occur with individuals similar to you. Cross species studies show that small schools can also be selected for in instances where large resources are monopolized by larger or more skilled schools, allowing for the added protection of the school without the necessity for increased competition (Guimaraes 2007).