Arctic charr (Salvelinus alpinus) re-established in a formerly acidified and highly dilute mountain lake under declining acidic deposition

Arctic charr in Lake Ronvatn, a mountain lake in southern Norway was re-established through stocking. The population went extinction during the early 1980s due to acidification, when the lake was highly acidified with a mean pH of 5.2-5.4 with occasional declines to 4.3-4.7. However, from the mid to late 1990s, the pH and acid-neutralising capacity (ANC) of the lake rose to 5.8-5.9 and 13-15 µeq L-1, respectively. The lake is extremely dilute with a mean conductivity and calcium concentration of 7.7 µS cm-1 and 0.35 mg L-1, respectively. The lake was stocked with 250 Arctic charr from a neighbouring lake between 1998 and 2000. These introductions were highly successful, as test-fishing in 2004, 2008 and 2012 revealed a relatively dense population of Arctic charr, and the presence of several young age groups. Water quality has remained stable since the late 1990s, or has slightly improved.