Assessment and assortment: how fishes use local and global cues to choose which school to go to.

Animals that live in groups are known preferentially to associate with phenotypically similar individuals. Despite this, groups of mixed phenotypic composition are the norm rather than the exception in several systems in the wild and this, combined with the large sizes of some animal groups, makes accurate global assessment by a choosing individual more difficult. In this study, we investigated the role of local and global information in mediating shoal-choice decisions in three-spined sticklebacks (Gasterosteus aculeatus) by manipulating the positions and phenotypes of stimulus fish in relation to a focal fish. Focal fish were able to assess globally mixed shoals composed of individuals of different body-length classes, preferring to associate with shoals where the majority phenotype matched their own. When local cues were manipulated this preference disappeared, although overall shoal composition remaining constant. Finally, if both stimulus shoals had the same overall composition but differed in their local cues, then the focus fish chose according to which local fish was of matching body length. These findings indicate that both local and global information play an important role in mediating assessment and shoal choice in fishes.