Shoaling is a widely important mechanism used by fish in freshwater and marine environments. Although used rather loosely in conjunction with the term “schooling,” the two words differ in definition by nature of organization. Where shoaling refers to a congregation of conspecific fish that stay together, schooling indicates that those fish are swimming in a coordinated fashion. Shoaling is not a learned behavior, but one in which a species is born to participate. This conduct evolved in as many as 50% of all fish species for a number of reasons. The incentives for shoaling mostly stem from the simple premise that all animals have three jobs in life: eat, reproduce, and avoid being eaten. From this, we can derive the motives for shoaling behavior. To facilitate these life goals, natural selection has chosen for shoaling behavior to increase foraging success, reproductive convenience, hydrodynamic efficiency, social interaction, and predator avoidance (Moyle and Cech 2003, Brahams and Colgan 1985).
Fish that engage in shoaling have developed several methods for facilitating the interaction with others. Individually, shoaling fish have eyes on the sides of their head to better see others in the shoal. Furthermore, it has been demonstrated that vision is important to cohesion because of the inability of most species to maintain schools at night, rather just shoals (Shaw 1961). “Schooling marks” such as prominent stripes or spots help fish recognize others as members of their own species. As a mechanoreceptor, the lateral line system is present in all shoaling fish, assisting in the detection of proximity and movement of other individuals in the shoal. Aside from physiologically based communication adaptations, chemical communication in the aquatic environment plays a large role in fish interaction.
There is ample evidence that chemical composition, concentration, flux, and hydrodynamic transport all have profound effects on chemically mediated ecological processes (Zimmer and Butman 2000). The application of chemical communication in shoaling behavior is vastly important. Most essentially, pheromones serve to aid in association preference of shoal mates, allowing the benefits of shoal behavior to be reaped. Furthermore, pheromones are pivotal to the advantages of foraging, reproduction, and predator avoidance while in a shoal or school. In this paper, I will describe how pheromones serve to assist shoaling fish in capitalizing on these advantages. Kin and Shoal Mate Association Preferences
In order to benefit from the advantages of shoaling, an individual must first associate with a group. Association among fish is often described in terms of cohesion and familiarity. In the case of pheromones, odors are most closely tied to familiarity between fish and chemical recognition of kin. It is important to understand that in the determination of chemical cues as directors of association preference, visual recognition and chemical recognition are distinguished. Additionally, for some species, chemical cues are more important in shoal cohesion than visual cues. For example, in a study on the effects of diet based cues on shoaling preferences of Threespined Sticklebacks, fish within visual contact of groups of unfamiliar conspecifics showed a significant preference toward the group that had undergone the same diet treatment (p = 0.001). This strongly suggests that diet based chemical cues released by Threespined Sticklebacks are a larger consideration than visual cues when selecting shoal association (Ward et al. 2004).
Association preference may also be directed by kinship and response to predator stimuli. Association with kin is largely managed by pheromones. In Rainbowfish, Melanotaenia eachamensis, kin recognition is clearly documented and variation in association was significant among different sexes. Males and females both showed a preference to associate with kin of the same sex (p < 0.05). However,...