Variability in Diet Composition and Dynamics of Radiocaesium in Moose

Abstract

1. Moose is the most important game species in Sweden and the fall-out after the Chernobyl accident has severely affected hunting of this animal. Nine years after the accident many animals in areas that received high deposition still show activity concentrations that exceed the limit for human consumption. Whilst the mechanism for uptake of 137 Cs in animals is fairly well known, the extent of variability in diet composition in wild animals is still puzzling and our ability to predict future trends in activity concentration of radionuclides limited. Studies on moose since the Chernobyl accident reveal considerable variation in 137 Cs-transfer between years and between individuals even in the same area. Differences in diet composition among individuals may explain the high degree of observed variability. Seventy per cent of the diet of moose in September is composed of Vaccinium myrtillus, Betula spp. and Salix spp. Significant differences between the proportion of birch and other components and species diversity within the diets among some years were measured. 2. We used a simple model, applying Monte Carlo simulations to create artificial diets for 137 Cs intake in moose. The model suggests that changes of diet composition or habitat use are the main reasons for the large variation of 137 Cs observed in moose. The variability in activity concentrations between different years obscure the decline expected from physical decay. The model shows that the standard deviation of 137 Cs activity concentration in moose meat declines with time as differences in activity concentration between food plants diminish. The coefficient of variation varies considerably among years in the real population and the model suggests that even at comparatively low mean values a large variation between individuals is expected. The model predicts that even after 30 years, the half-life of 137 Cs, a high mean level and a large variation could be possible.