Condition-associated modulation of melanin-based skin pigmentation in juvenile Atlantic salmon ( Salmo salar )

Abstract

Melanin-based pigmentation in vertebrates has often been regarded as a stable trait, inexpensive to produce and highly heritable. Accumulating evidence, however, links melanin patterns to behavioural, physiological and endocrine variation. In salmonids, eumelanin-based skin spottiness has been associated with stress responsiveness and coping style, but it remains unclear to what extent environmental challenge induces within-individual state-dependent changes in spot expression. Here, we examined whether feeding dynamics during acclimation to novelty are associated with longitudinal changes in visible skin spot density in juvenile Atlantic salmon (Salmo salar), in which dark patterns are expressed through eumelanin-containing melanophores. Fish were transferred from group rearing to individual housing, inducing transient feeding suppression followed by large inter-individual variation in feeding recovery and growth. Although visible spot density declined overall, the magnitude and direction of change varied among individuals and covaried with feed intake, growth and condition factor. Individuals with higher feed intake and growth, and smaller declines in condition factor, exhibited reduced spot loss and, in some cases, increased spot density, whereas lower-performing individuals showed larger reductions. Pre-existing spot patterns predicted post-transfer performance. These findings suggest that salmonid skin spots track resource acquisition and coping capacity, consistent with regulated phenotypic plasticity in condition-associated visible pigmentation.