Skin spots play an essential role as a visual stimulus in the social behaviour of fishes, particularly in gregarious species. Several studies have demonstrated that the characteristics of some flank signals are used as conspicuous visual cues for different aspects of intraspecific communication, as well as shaping the interactions among individuals, and reflecting the features of individuals. Here, we analyse the spatial pattern of spots in the European grayling Thymallus thymallus (Actinopterygii, Salmonidae) (Linnaeus, 1758) (Salmonidae, subfamily Thymallinae), which are usually gregarious. Adult individuals are characterised by conspicuous black spots on their sides, generally restricted to the front half of the flank. By sampling 55 individuals in Slovenia, Croatia and Norway, we tested the possible influence of environmental factors (e.g. water velocity, water depth, position in the pool/stream) and/or individual characteristics (sex and length) on the characteristics and spatial pattern of graylings’ spots. Spottiness did not show any relationship with sex or body size, but the numbers of both spots and spotted lines appeared to be correlated with some physical properties of the river: more highly spotted graylings seemed to inhabit faster and deeper waters, and were positioned at the head of pools and streams. We suggested that in gregarious fish, spottiness may signal the status (e.g. quality and/or dominance) of individuals, which has a role in determining the spatial arrangement of individuals along the river bed and, consequently, within the group.

Peer reviewed