Population level differences in aggressiveness and their relationship to food density in a stream salmonid (Salvelinus fontinalis)

The hypothesis that stream salmonids from habitats with low food density will be more aggressive than conspecifics from habitats with higher food density was tested through behavioural observations of wild‐caught young‐of‐the‐year brook charr Salvelinus fontinalis in aquaria. Standing stocks of benthic invertebrates in a high food density habitat were more than double those in a low density habitat and charr from the latter were significantly smaller than those from the former. Charr from the low food density habitat were significantly more aggressive, as indicated by nipping attempts within pairs, than were those from the high food density habitat. Both populations were genetically isolated and food density differences seemed to be related to persistent features of their habitats. This suggests that the observed behavioural differences may have both genotypic and phenotypic Components