Pigmentation potentially influences fish welfare in aquaculture

Pigmentation is an important factor for the survival of organisms in different environments; in fish, it plays a key role in social communication and shoal formation. Pigmentation mediates communication among colour phenotypes, as fish may temporarily change colouring to indicate their position in a social hierarchy. Although some colour phenotypes may be more common, less frequent colourations do not appear to significantly impact fish welfare, as aggression is directed more towards common, typically pigmented phenotypes than rare phenotypes. The relationship between pigmentation and fish behaviour can be clearly observed with extremely coloured phenotypes, i.e., albino and melanistic individuals. The melanocortin hypothesis states that phenotypes with more pigmentation are more aggressive, reproductively active, and resistant to stress as well as having better overall health. However, the high levels of aggression expressed by melanistic phenotypes may reduce the well-being of less intensely pigmented phenotypes if these individuals are kept together. In contrast, albino individuals are more sensitive to stress, less aggressive, and, as they are prone to physiological constraints such as impaired vision, are unable to effectively defend resources compared to pigmented phenotypes. Moreover, both these extremely coloured phenotypes are unable to signal their position in the social hierarchy by changing colours, which can result in conflict between their mental state and position in a social hierarchy and/or attempt to change their social rank. In summary, colour variation is a consequence of physiological conditions, especially stress, rather than a cause of such conditions. The breeding and propagation of albino and melanistic fish is generally unethical and should not be considered in aquaculture unless these phenotypes are bred separately under specific conditions, i.e., not kept with typically pigmented conspecifics.