Salmon lice or suboptimal water quality ? reasons for reduced postsmolt survival?

While high levels of H+ and aluminum (Al) are lethal to Atlantic salmon (Salmo salar L.) smolts, low levels of Al still affect growth, physiological status and the subsequent survival of postsmolts after entry into the marine environment. Further, fish that are sub lethally stressed may be more sensitive to disease and parasite attacks and can have a reduced tolerance to additional stressors. Salmon populations on the western coast of Norway may experience both moderately acidified rivers and salmon lice (Lepeophtheirus salmonis Krøyer) attacks. The present study addresses the question of interactive effects of acidification and salmon lice infestation on postsmolt survival. Five groups each of ~1500 one-year old Atlantic salmon smolts of the Imsa strain, South-Western Norway, were exposed to one of four suboptimal water qualities. Three groups (Acid, moderate acid and episodic acid) experienced moderate acidic water differing with respect to pH (5.6 to 5.8), aluminum (15 to 30 µg Al/l) and exposure duration (3 to 10 days). A fourth group exposed to pH>6.5, superoxygenated>moderate acid>episodic acid>control). Plasma chloride was within normal levels in the non-infected groups, while all infected groups (except episodic acid) had elevated levels of plasma chloride. Year to year variations in acidification pressure and salmon lice densities can in combination explain some of the year to year variation in postsmolt survival and hence the variation in Atlantic salmon year-class strength in Norwegian rivers.