Multiple determinants of welfare in farmed fish:: stocking density, disturbance, and aggression in Atlantic salmon (Salmo salar)

Adult Atlantic salmon (Salmo salar) were held in seawater tanks at stocking densities of 15, 25, and 35 kg center dot m(-3) (with five replicates). After 51 days, four measures of welfare (body condition, fin condition, plasma glucose, and cortisol) were collected and combined by principal components analysis into an integrated welfare score for each fish. Fish with high and low welfare scores were found in all conditions, but overall welfare was best at the intermediate density, supporting the view that low densities as well as high densities can compromise welfare in salmon. The effect of stocking density interacted with the stronger effect of disturbance, creating a complex pattern of variation in welfare among the experimental tanks. Counter-intuitively, welfare scores were higher in tanks with a high level of disturbance, while welfare scores were strikingly low at the lowest density. Aggression in all tanks was low, but rose after feeding, when aggression was negatively related to both disturbance and stocking density. These data highlight the complex determinants of welfare in fish in production systems and, taken together, suggest that negative effects of husbandry practice and density on welfare in Atlantic salmon might be mediated by their influence on social interactions.