OSMOTIC-STRESS, ANTIPREDATOR BEHAVIOR, AND MORTALITY OF ATLANTIC SALMON (SALMO-SALAR) SMOLTS

Groups of Atlantic salmon (Salmo salar) smelts were exposed to full-st rength seawater for 1, 6, 12, 24, 48, 96, and 144 h or were left in fr esh water (control). Their physiological performance was monitored as changes in levels of plasma chloride, hematocrit, and muscle water con tent. At 12 and 24 h, smelts showed a transient increase in plasma chl oride levels concurrent with a reduction in hematocrit, indicating a s ignificant osmotic stress during this period. After 38 h, values fell to normal resting levels in seawater and no further changes were obser ved. In a second experiment, similar groups of smelts were exposed to seawater for 0 (control), 12, 24, 48, and 96 h and then confronted wit h predatory cod (Gadus morhua). Groups exposed to seawater for 12 and 24 h showed a change in antipredator behaviour, i.e., reduced escape d istance and schooling behaviour, and suffered significantly higher pre datory mortality than control fish or groups fully acclimated to seawa ter. These results suggest that Atlantic salmon smelts that suffer phy siological stress while transferring from fresh water to the sea may a lso be exposed to increased predation as a consequence of less effecti ve antipredator behaviour.