CUMULATIVE ACUTE PHYSIOLOGICAL STRESS IN ATLANTIC SALMON SMOLTS - THE EFFECT OF OSMOTIC IMBALANCE AND THE PRESENCE OF PREDATORS

Heavy mortality experienced by sea-ranched Atlantic salmon (Salmo salar) smolts during their seaward migration is assumed to be due to predation, caused by a cumulative effect of osmotic stress and physiological stress from the fright reaction of predator-naive smolts to predators. The aim of this study was to test whether osmotic imbalance and the presence of predators have any cumulative effect on plasma chloride, blood glucose and lactate levels. All three of these physiological factors are known to yield a reliable assessment of the degree of physiological stress. Non-acclimated smolts experienced a severe physiological stress when they were exposed to either a hyperosmotic milieu or to a hyposmotic milieu. Predator-naive smolts became likewise physiologically stressed when exposed to a predator. An additive physiological effect was revealed when non-acclimated, predator-naive, smolts were exposed to a predator. A reduction in physiological stress response was noted when the smolts had previously been acclimated either to the osmotic milieu or habituated to predators. The results of this study support the view that the presence of predators and a change in osmotic milieu combined exert a cumulative stress effect, and that this effect is diminished by prior acclimation to the osmotic milieu or habituation to predators. © 1990.