Limestone Slurry Reduces Physiological Stress and Increases Survival of Atlantic Salmon (Salmo salar) in an Acidic Norwegian River

Smolts of Atlantic salmon (Salmo salar) were exposed for 70 h in the acidic River Vaaraana, southern Norway (Q = 1.3 m 3 /s, pH = 4.4–5.6, Ca = 0.6–1.5 mg/L, labile Al = 50–100 μg/L), to waters neutralized by addition of limestone slurry. Chemical and biological parameters were measured upstream of the liming site and 20, 100, 1000, 2000, and 3500 m downstream. Addition of limestone slurry effectively increased pH from 4.6 to 6.9, Ca from 0.6 to 2.6 mg/L, and reduced the labile Al concentration from 59 to 35 μg/L immediately downstream of the liming site (20 m, after 30 s). In the untreated upstream waters, salmon smolts lost plasma Cl rapidly, and all fish died within 36 h. In spite of the slight oversaturation of labile Al immediately downstream of the base addition, no mortality occurred in neutralized waters. Relatively high Ca concentrations may have mitigated potential toxic effects from Al oversaturation. Plasma Cl levels did not vary with distance downstream. Detoxification of running waters with highly toxic levels of pH and Al can successfully be performed by addition of limestone slurry.