Effects of repeated crowding on the stress response and growth performance in Atlantic salmon (Salmo salar)

A 64-day growth experiment was conducted in which two groups of Atlantic salmon parr were grown under either control conditions or subjected to a weekly crowding stressor. Subjecting fish to the stressor resulted in a 7.7% reduction in wet weight after 29 days, which was maintained at 7.9% by day 64. This reduction in weight was reflected in a 44% reduction in specific growth rate and 38% increase in feed conversion ratio over the first 29 days of the experiment. Elevation in plasma cortisol was observed in crowded fish on days 1, 29 and 64. Similarly, on days 1 and 29 an increase in both plasma glucose and lactate was detected. On day 64, however, no differences in plasma glucose and lactate were observed, with the magnitude of the cortisol response also significantly reduced. Overall, the relatively moderate impact on growth performance and reduction in magnitude of measured stress parameters at the end of the experiment suggests possible habituation to the applied stressor.