Effects of rearing density and in-tank structure on the stress response in juvenile Chinook salmon (Oncorhynchus tshawytscha)

Current salmonid hatchery practices typically include high rearing densities without structure within the water column. In the present study, we assessed the effects of low (conservation hatchery) and high (production hatchery) rearing densities on the plasma cortisol response during simulated transport stress and whether density might interact with in-tank structure to further mitigate the stress response. After 10 months of rearing in their respective treatments, we subjected fish to a transportation simulation stressor that encompassed fish being caught, bucketed, and crowded at a lowered water depth in a smaller tank for 3 h, then caught and bucketed to a novel tank of similar size and flow. We sampled stressed and unstressed fish for plasma cortisol determination over a 23 h time period. Additionally, we determined resting cortisol secretion from head kidneys collected from unstressed fish at time 0 h from each rearing treatment to determine if resting cortisol secretion pattern was affected by the rearing conditions. There was no significant difference in size of the fish at the time of sampling; overall, they averaged 24.7 g in weight and 136.2 mm in fork length. Fish reared with in-tank structure at low density had the lowest plasma cortisol concentration during the stressor, while fish reared without in-tank structure at high density had the highest plasma cortisol concentration in response to the stressor. In-tank structure, high-density reared fish had less plasma cortisol than no-structure high-density fish, but was elevated over low-density in-tank structure fish. Rearing condition had no meaningful effect on resting cortisol secretion by the interrenals. We speculate that low density with in-tank structure rearing would ultimately lead to the highest survival rates when hatchery Chinook salmon are released into the wild due to their reduced plasma cortisol concentration in response to a stressor.