The selenium method for treatment of lakes for elevated levels of mercury in fish

Abstract

Selenium in the form of sodium selenite (Na 2SeO 3) incorporated in a rubber matrix was introduced into a Swedish lake which is black-listed for fishing because of high mercury levels in fish tissues. The lake is not acidified. The level of selenium in the lake was raised from 0.4 to ∼ 3–5 μg Se l −1, and sustained at this level for a period of > 3 years. The mercury content of pike ( Esox lucius) was found to have fallen markedly after the lake had been treated for only 1 year. The mercury content was halved over the whole weight range and stayed below the black-list limit during the treatment period. After the first year the mercury content of perch ( Perca fluviatilis) muscle had also decreased. The reduction for those between 8 and 11 cm long (3–6 years old) was found to be 77%; for those between 12 and 16 cm long (7–9 years old), 65%; and for those between 17 and 25 cm long (> 9 years), 77%. After 2 years, the total reduction was 85% for perch 12–16 cm long and 75% for those 17–25 cm long. By the end of the third year, the mercury content had decreased by 84% in those that measured between 17 and 25 cm. The selenium treatment also resulted in a very marked decrease in the mercury content of the muscle of roach ( Leuciscus rutilus). The decrease, from 0.5–1.0 to 0.06–0.08 mg Hg/kg muscle, was independent of size. The treatment of lakes with selenium has been found to be one of the few known methods of lowering the mercury content of fish quickly and easily.