Exposure to acid water and aluminium reduces Atlantic salmon postsmolt survival

Sammendrag

Severe acidification is acknowledged as the cause for the extinction of numerous Atlantic salmon (Salmo salar L.) populations in Norway. Acidification mobilizes aluminium (Al) from soils resulting in elevated Al concentrations in the freshwater ecosystem. In freshwater, inorganic (labile or cationic) forms of Al (Ali) accumulate onto fish gills, where high concentrations will result in mortality due to respiratory and ionoregulatory dysfunction. At lower concentrations, fish health can still be affected, resulting in e.g. reduced growth, although blood ion levels can remain within the normal range for smolts in fresh water. However, at these concentrations Al may interact with gill Na+K+-ATPase, inhibiting the enzyme activity, thereby reducing marine survival. While there is an abundant literature on the interactions between water chemistry and fish responses at the physiological level, there is less data on the interaction between low levels of acidity and population responses. Over the years 1999, 2000, 2002 and 2004 we exposed 13 groups of 1000 to 1500 one-year old hatchery reared, Carlin tagged Atlantic salmon smolts of the Imsa and Suldal strains (South-Western Norway) to moderately acidified water (pH 5.8; <10 µg Ali/l) from 3 (episodic) to 60 (long term) days. Fish exposed to River Imsa water having a pH >6.5 and <20 µg total Al/l acted as controls. In 2003 an additional group exposed to River Imsa water added O2 (<200% on inlet water) was included to address a more novel stressor. Exposure to acid water resulted in an initial period of depressed plasma ionic concentrations, which recovered to normal levels with increased exposure time. Glucose levels, however, remained high over the whole exposure period and gill Na+K+-ATPase levels were depressed. These responses were all related to Al exposure, not pH. Long time acid exposed fish grew poorer than non-exposed control fish. Acid exposed smolt released into River Imsa migrated out of the river together with control fish despite having lower seawater tolerance. Return rates were, however, reduced in all exposed groups, although marine growth and timing of return were unaffected. Gill-Al in the range of 50-70 µg Al/g gill dw caused a 20% reduction in returns, at higher concentrations return rates were further reduced. The results suggest that even moderately and/or episodically acidified rivers containing Ali concentrations around the detection limit can still be the cause for the substantial reductions in returns of Atlantic salmon.