Recovery of Atlantic salmon smolts following aluminum exposure defined by changes in blood physiology and seawater tolerance

Acidification is acknowledged as a cause for extinction or catch reductions in numerous rivers supporting Atlantic salmon (Salmo salar L.) populations in Norway. In freshwater, labile (cationic/inorganic) forms of Al (LAl) accumulate onto and in fish gills, where high concentrations can result in mortality due to respiratory and ionoregulatory dysfunction. At lower concentrations, Al may still have major population effects, mainly through the inhibition of gill Na⁺,K⁺-ATPase activity, reducing hypo-osmoregulatory capacity and thereby affecting marine survival. Following episodic exposure, normal smolt properties are expected to be restored. In 2006 and 2007 we exposed groups of 1100 to 1200 one-year old hatchery reared, Carlin tagged Atlantic salmon smolts of the Imsa strain (South-Western Norway) to moderately acidified water (pH 5.6–5.7; 10–15μgL All⁻¹) for 2 to 14days whereupon they were transferred to a water quality assumed to be satisfactory for smolt (good water; pH 6.8–7.2 and <6μgL All⁻¹) for 2 to 14days to monitor recovery from the prior exposure. Control fish had gill–Al concentrations in the range of 5 to 10μg Alg⁻¹ gill dry weight (dw) while Al-exposed fish had gill–Al concentrations exceeding 30μg Alg⁻¹ gill dw. Following transfer to good water, gill–Al did not return to control levels within a time span of 14days. The physiological responses measured as blood ions (Na⁺, Cl⁻), blood acid-base balance (pH, pCO₂ and HCO₃ ⁻), and blood glucose improved relative to the acid/aluminum (Al) exposed groups, but not to levels measured in the control fish. Mortality was elevated in all Al-exposed/recovery groups following transfer to seawater (34ppt) and gill Na⁺,K⁺-ATPase was reduced. The results suggest that smolts had not fully recovered 14days after a short-term acid/Al exposure, where recovery depends on what traits are used as indicators of healthy fish. An Al-exposure experienced by presmolt several weeks prior to the smolt spring migration can have negative population effects, both by reducing hypo-osmoregulatory capacity and by making the fish more vulnerable to secondary stressors in the marine environment.