Fish shoal composition: mechanisms and constraints

Observations were made on three fish species (banded killifish (Fundulus di aphanus), golden shiner (Notemigonus crysoleucas) and white sucker (Catosto mus commersoni)) in a temperate lake (New Brunswick, Canada) in order to in vestigate the relationship between shoal choice behaviour of individual fis hes and shoal composition. Encounters between shoals were observed to take place every 1.1 min per shoal and an encounter lasted 3.7 s on average. The duration of shoal encounters was influenced by shoal size but not by diffe rences between shoals in either body length or species. Conversely the outc ome of shoal encounters (i.e. whether or not an individual changes shoal) w as influenced by body length and species differences but not by shoal size. Together, these results suggest that encounter duration itself is unlikely to have an important influence on encounter outcome. The collection of ten entire fish shoals showed that they were assorted by species and body leng th. A simulation model demonstrated that individual shoal choice behaviour alone could account for the generation and maintenance of the observed leve ls of size assortedness of shoals without invoking the existence of other s orting mechanisms such as differential swimming speeds. However, the genera tion of species assortedness was not predicted by the model. Furthermore, o ur data suggest that fish density acts as a constraint on shoal choice, inf luencing both shoal size and composition. This work has implications for st udies on information transfer and reciprocal altruism within populations.