SOURCES OF VARIATION IN RADIOCESIUM LEVELS BETWEEN INDIVIDUAL FISH FROM A CHERNOBYL CONTAMINATED NORWEGIAN LAKE

1. We evaluate sources of individual variation in Cs-137 Of Arctic cha rr (Salvelinus alpinus) and brown trout (Salmo trutta) from a Norwegia n lake contaminated by fall-out from the Chernobyl accident. Samples w ere collected on 14 occasions between July 1986 and October 1989. 2. T he relationship between Cs-137 concentration, and fish weight, age and growth rate, changed with time. In 1986 the Cs-137 concentration in A rctic charr was negatively correlated with age and positively correlat ed with growth rate. Later, after a transition period, Cs-137 concentr ation in Arctic charr became positively correlated with weight and age , and negatively correlated with growth rate. The timing of this chang e was associated with changes in prey relative to fish radioactivity. 3. From September 1987 and throughout the study, the increase in Cs-13 7 concentration with fish weight could be described by a power-functio n with weight exponents of 0.39 and 0.11 for Arctic charr and brown tr out, respectively. We could detect no diet changes with size in brown trout, while Arctic charr showed a diet shift from zooplankton to zoob enthos with increasing age and weight. This diet shift probably accoun ted for the larger increase in Cs-137 concentration with weight in cha rr than trout. 4. Variation in fish weight accounted for 29 and 11% of the individual variation in Cs-137 concentration in Arctic charr and brown trout, respectively. Most of the variation in caesium concentrat ions can be attributed to size independent factors, of which individua l variation in diet composition is suggested to be the most important one. 5. Frequency distributions of Cs-137 concentration in fish were p ositively skewed, leptokurtic and differed significantly from the norm al distribution, but not from the log-normal distribution. Individual variation in caesium concentration was rather large, and larger in Arc tic charr than in brown trout, probably because of a wider niche utili zation by Arctic charr than by brown trout. These large individual var iations necessitate large sample sizes if representative values of Cs- 137 concentration for the populations shall be obtained.