Distinct responses of sympatric migrant and resident Atlantic cod phenotypes to substrate and temperature at a remote Gulf of Maine seamount

Life-history strategies often vary within motile marine species, affecting morphometry, growth, diet, and fecundity. Atlantic cod (Gadus morhua) in the Gulf of Maine display marked variation in a number of life-history traits, exemplified by differences in body colour. Migratory behaviours are suspected to differ among these colour types, but have yet to be shown definitively. Here, we used the combination of an acoustic telemetry system and fine-scale benthic habitat maps to reveal that the red phenotype cod adhered to an isolated kelp forest covering <2 km2 of a seamount in the central Gulf of Maine. Meanwhile, the olive phenotype cod adopted diel vertical migratory behaviour, possibly in response to a temperature gradient. Use of shallow, structured habitat was influenced by temperature and may be enabled by dynamic conditions related to internal waves that persist throughout the summer and early fall. Detections decreased in response to changing thermal conditions, although phenotypes reacted to these changes in distinct ways: the olive phenotype abandoned shallow habitat prior to peak summer temperatures, while the red phenotype remained until mid-fall when temperatures and temperature variability declined. Our findings support a link between morphometry, colour, behavioural strategies, and habitat preferences that may be widespread in Atlantic cod.