VARIABILITY IN DIET COMPOSITION AND DYNAMICS OF RADIOCESIUM IN MOOSE

1. Moose is the most important game species in Sweden and the fall-out after the Chernobyl accident has severely affected hunting of this an imal. Nine years after the accident many animals in areas that receive d high deposition still show activity concentrations that exceed the l imit for human consumption. Whilst the mechanism for uptake of Cs-137 in animals is fairly well known, the extent of variability in diet com position in wild animals is still puzzling and our ability to predict future trends in activity concentration of radionuclides limited. Stud ies on moose since the Chernobyl accident reveal considerable variatio n in Cs-137-transfer between years and between individuals even in the same area. Differences in diet composition among individuals may expl ain the high degree of observed variability. Seventy per cent of the d iet of moose in September is composed of Vaccinium myrtillus, Betula s pp. and Salix spp. Significant differences between the proportion of b irch and other components and species diversity within the diets among some years were measured. 2. We used a simple model, applying Monte C arlo simulations to create artificial diets for Cs-137 intake in moose . The model suggests that changes of diet composition or habitat use a re the main reasons for the large variation of Cs-137 observed in moos e. The variability in activity concentrations between different years obscure the decline expected from physical decay. The model shows that the standard deviation of Cs-137 activity concentration in moose meat declines with time as differences in activity concentration between f ood 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 i s expected. The model predicts that even after 30 years, the half-life of Cs-137, a high mean level and a large variation could be possible.