RADIOCESIUM TURNOVER IN ARCTIC CHARR (SALVELINUS-ALPINUS) AND BROWN TROUT (SALMO-TRUTTA) IN A NORWEGIAN LAKE

(1) The radioactivity of brown trout (Salmo trutta L.) and Arctic charr (Salvelinus alpinus (L.)) was monitored in a Norwegian lake from 1986 to 1989. A distinct difference was observed between brown trout and Arctic charr in the accumulation of radiocaesium (Cs-134 and Cs-137) from the Chernobyl fallout, and the study focused on the understanding of this difference. (2) Brown trout had a large food consumption and a corresponding high intake of radiocaesium. Excretion was 20% faster in brown trout than Arctic charr as brown trout lived at high temperatures in epilimnic water. Arctic charr had a lower food consumption (less than one-third of trout) and lived in colder meta- and hypolimnic water. Arctic charr therefore had a lower intake and slower excretion of radiocaesium. (3) Brown trout and Arctic charr had different diets. For brown trout zoobenthos was the dominant food item, whereas Arctic charr mainly fed on zooplankton. The radioactivity in the stomach contents of the two species was different in 1986, but similar for the rest of the period. (4) Higher levels of radiocaesium in brown trout than Arctic charr in 1986 were due to a higher food consumption and more radioactive food items in its diet. (5) The parallel development in accumulated radiocaesium through summer 1987 was probably formed by brown trout balancing a higher intake with a faster excretion. (6) The ecological half-lives of radiocaesium in brown trout (357 days) and Arctic charr (550 days) from Lake Hoysjoen indicated a slow removal of the isotopes from the food webs.