Sources of Variation in Radiocaesium Levels Between Individual Fish from a Chernobyl Contaminated Norwegian Lake

Abstract

1. We evaluate sources of individual variation in l37 Cs of Arctic charr (Salvelinus alpinus) and brown trout (Salmo trutta) from a Norwegian lake contaminated by fall-out from the Chernobyl accident. Samples were collected on 14 occasions between July 1986 and October 1989. 2. The relationship between 137 Cs concentration, and fish weight, age and growth rate, changed with time. In 1986 the 137 Cs concentration in Arctic charr was negatively correlated with age and positively correlated with growth rate. Later, after a transition period, 137 Cs concentration in Arctic charr became positively correlated with weight and age, and negatively correlated with growth rate. The timing of this change was associated with changes in prey relative to fish radioactivity. 3. From September 1987 and throughout the study, the increase in 137 Cs concentration with fish weight could be described by a power-function with weight exponents of 0.39 and 0.11 for Arctic charr and brown trout, respectively. We could detect no diet changes with size in brown trout, while Arctic charr showed a diet shift from zooplankton to zoobenthos with increasing age and weight. This diet shift probably accounted for the larger increase in 137 Cs concentration with weight in charr than trout. 4. Variation in fish weight accounted for 29 and 11% of the individual variation in 137 Cs concentration in Arctic charr and brown trout, respectively. Most of the variation in caesium concentrations can be attributed to size independent factors, of which individual variation in diet composition is suggested to be the most important one. 5. Frequency distributions of l37 Cs concentration in fish were positively skewed, leptokurtic and differed significantly from the normal distribution, but not from the log-normal distribution. Individual variation in caesium concentration was rather large, and larger in Arctic charr than in brown trout, probably because of a wider niche utilization by Arctic charr than by brown trout. These large individual variations necessitate large sample sizes if representative values of 137 Cs concentration for the populations shall be obtained.