Acid runoff hampers recovery of endangered Atlantic salmon.

McCormick, SD, A Keyes, KH Nislow and MY Monette. 2009. Impacts of episodic acidification on in-stream survival and physiological impairment of Atlantic salmon (Salmo salar) smolts. Canadian Journal of Fisheries and Aquatic Sciences 66(3):394-403.

Atlantic salmon exposed to short springtime bursts of acidic runoff water are less able to migrate to the ocean because of important changes in their ability to balance salt.

Although heavily acidified streams have fewer salmon, this is the first study to show that health effects occur in streams with only brief periods of acidification. The decline in water quality occurs when the salmon are at a critical period in their development and may contribute to the continued decline of this endangered species.

Despite large restoration programs, Atlantic salmon populations have continued to decline in many rivers in the northeastern US and eastern Canada. Although potential causes include dams, loss of spawning beds, and overfishing, acid rain and acidic water conditions also affect salmon conservation efforts, despite reduced emissions of acidifying substances in North America.

Acid rain occurs when sulfate and nitrate -- mainly generated from coal-fired power plants and other combustion of fossil fuels -- mix with the atmosphere and are carried by precipitation to the Earth. This deposition increases the acidity of and aluminum content in water and soil.

Fish, other wildlife and plants cannot always tolerate the high acid levels and can succumb to its health effects. Acid rain is blamed for exterminating Atlantic salmon populations in certain rivers in Nova Scotia and Norway.

Many streams in the northeastern U.S. have periods of lower pH and higher aluminum concentrations after snowmelt and heavy rains. This short-term change in water quality occurs at the time when young salmon move to the ocean (smolts). Atlantic salmon live their adult lives in the ocean and return to their natal freshwater streams to reproduce.

In this field study in southeastern Vermont, Atlantic salmon smolts were caged in 5 streams with varying levels of acidification and aluminum typical for this region of the US. The streams had low (pH 7.1 – 7.4, Al 4.6 – 11.2 ppb), intermediate (pH 5.8 – 6.2, Al 22 – 38 ppb) and high (pH 5.5 – 6.0, Al 48 – 74 ppb) acid and aluminum levels.

After very short exposures of 3 and 6 days, fish in the more acidic waters (with pH less than 5.6) had higher mortality, had elevated levels of stress hormones in their blood and Al in their gills and were less able to regulate the salt levels in their gills when compared to fish in waters with pH greater than 7.0.

The ability to regulate salt levels (osmoregulation) is critical for this species because it allows fish to survive in the ocean after they migrate from fresh water.