Muskrat (Ondatra zibethicus) demography and spatial structure in reservoirs in Central Russia

We studied the demography and spatial structure of the muskrat population at the Nizhnekamsk Reservoir in central Russia. Ca-45 radioactive prenatal labelling of muskrats was performed over a period of eight years. A total of 3,876 individual muskrats were captured from a 12 km(2) experimental plot and analysed on the basis of age, sex and presence of radioactivity. During the first year of life, the highest mortality was among muskrats from the latest litter, whilst during the second year of life it was amongst individuals from the earlier litters. Spring and autumn muskrat migration were studied and we observed a cyclical type of spatial structuring within the population. The proportion of individuals migrating in autumn varied from 18-57%, but significant differences in the migratory activities of different age groups were not observed. We found several tendencies throughout the eight years: a decrease in the proportion of the first litter (not overwintering), an increase in the proportion of third litters, and a decrease in the proportion of yearlings born in the first litter - we assume these represent responses to flooding.