Fish shoal composition: mechanisms and constraints

• Published in: Proceedings of the Royal Society. B, Biological sciences Biological sciences, 2000-10, Vol.267 (1456), p.2011-2017
• Main Authors: Krause, Jens, Hoare, Daniel J., Croft, Darren, Lawrence, James, Ward, Ashley, Ruxton, Graeme D., Godin, Jean-Guy J., James, Richard
• Format: Article
• Language: English
• Subjects: Animals; Behavior, Animal; Body length; Canada, New Brunswick; Catostomus commersoni; Cyprinidae - physiology; Cypriniformes - physiology; Datasets; Fish; Fishes - classification; Fishes - physiology; Foraging; Freshwater; Freshwater fishes; Fundulus Diaphanus; Group Composition; Group Size; Killifishes - physiology; Modeling; Notemigonus crysoleucas; Observed choices; Population Density; Population Size; Schools of fish; Shoaling Behaviour; Shoals; Social Behavior; Species Specificity; Swimming
• ISSN: 0962-8452; 1471-2954
• DOI: 10.1098/rspb.2000.1243

Observations were made on three fish species (banded killifish (Fundulus diaphanus), golden shiner (Notemigonus crysoleucas) and white sucker (Catostomus commersoni)) in a temperate lake (New Brunswick, Canada) in order to investigate the relationship between shoal choice behaviour of individual fishes and shoal composition. Encounters between shoals were observed to take place every 1.1 min per shoal and an encounter lasted 3.7s on average. The duration of shoal encounters was influenced by shoal size but not by differences between shoals in either body length or species. Conversely, the outcome of shoal encounters (i.e. whether or not an individual changes shoal) was 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 length. A simulation model demonstrated that individual shoal choice behaviour alone could account for the generation and maintenance of the observed levels of size assortedness of shoals without invoking the existence of other sorting mechanisms such as differential swimming speeds. However, the generation of species assortedness was not predicted by the model. Furthermore, our data suggest that fish density acts as a constraint on shoal choice, influencing both shoal size and composition. This work has implications for studies on information transfer and reciprocal altruism within populations.