Effect of Reduction in Water Salinity on Osmoregulation and Survival of Large Atlantic Salmon Held at High Water Temperature

Abstract

AbstractWe examined the possible ameliorating effects of reduced salinity (28‰) on the physiological performance of large Atlantic salmon Salmo salar initially reared at 35‰ and exhibiting osmo‐ionoregulatory disturbances in high seasonal water temperatures (>18°C). Considerable differences were observed in the behavior, survival, and plasma values for cortisol, osmolality, and chloride after 3 d, with fish from the 28‰ salinity regime approaching basal levels (cortisol: 8.0 ± 1.7 mmol/L; osmolality: 374.3 ± 6.8 milliosmols per kilogram [mosmols/kg]; [Cl‐]: 170.5 ± 3.8 mmol/L [mean ± SE]) and fish from full‐strength seawater showing a further decrease in osmoregulatory capacity (cortisol: 33.7 ± 5.0 mmol/L; osmolality: 411.4 ± 9.7 mosmols/kg; [Cl‐]: 189.1 ± 6.8 mmol/L). Two weeks later, fish kept at 28‰ still had a normal osmoregulatory capacity and exhibited a significant increase in condition factor, whereas many fish (70%) in full‐strength seawater had died. These results indicate a limited capacity for maintaining osmo‐ionoregulatory homeostasis in full‐strength seawater during long periods at high temperature. Access to water of lower salinity appears to be essential for the survival of Atlantic salmon in this environment.