Inter-individual and -family differences in the cortisol responsiveness of Atlantic cod (Gadus morhua)

Cortisol is released into the bloodstream of fish in response to exogenous stimuli referred to as stressors, and long-term or repetitive exposure to stressors can lead to poor growth and disease susceptibility in aquaculture-reared fish. Thus, as has been shown for other fish species (e.g. rainbow trout), the identification of fish with an attenuated cortisol response to stress may be beneficial to the emerging cod aquaculture industry. In this study, we: 1) examined if differences in the magnitude of the cortisol response to a standardized stressor (30 s handling) could be consistently detected within a multi-family population of cod; 2) determined if these differences were related to growth, and independent of stressor type; and 3) assessed whether single nucleotide polymorphisms (SNPs) identified by the Atlantic Cod Genomics and Broodstock Development Project (CGP) were correlated significantly with the magnitude of cortisol response. In this study, we were able to consistently separate fish with a high (HR) cortisol response from those with a low cortisol response (LR) to handling and heat stress. We showed that the percentage of HR and LR responders differed dramatically among the 10 cod families (with some families having no LR or HR individuals), and that significant negative correlations existed between cortisol responsiveness (total Z score) and wet mass, specific growth rate and condition factor; LR fish were approx. 55% heavier at the end of the year long study. Finally, we identified four polymorphic SNPs (QTLs) that were significantly associated with the magnitude of cortisol response to handling stress. This research suggests that there is a genetic component to this phenotype (HR vs. LR) in Atlantic cod, and that responsiveness to stress at the juvenile stage may be an important trait to consider in broodstock selection.