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

Summary

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, <5 µg Al/l acted as control (reference group). A fifth group was treated by increasing O2 (145 % saturation in inlet water), in control water. After freshwater exposure, smolts (n=150) from each of the 5 exposure groups were moved into tanks containing seawater (33 psu) and given two days of recovery before being infected with salmon lice copepodids. Five non-infected groups (n=100) from the same exposures acted as controls. Over a 42 day period, postsmolts were regularly inspected and sampled for mortality, lice density and physiological status. The lice smolt-1 density was highest in the superoxygenated and episodic acid groups, followed by the acid, moderate acid and the reference groups. Mortality was low in the 5 non-infected control groups, and significantly elevated in the lice infected groups (Acid>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.