ESTIMATION OF MULE DEER WINTER MORTALITY FROM AGE RATIOS

Because most mortality occurs during winter, estimation of overwinter survival rates is needed for sensible management of winter-stressed un gulates. We developed general estimators and determined overwinter sur vival rates of mule deer (Odocoileus hemionus) in southcentral Wyoming , from pre- and post-winter age ratio samples of the population, and a ge ratios of carcasses found in the spring. Both adult and fawn overwi nter survival rates can be estimated if all 3 of these ratios are avai lable. Adult survival varied from 0.798 (SE 0.103) in 1985-86 to 0.908 (SE 0.030) in 1988-89. For these same winters, fawn survival was 0.66 5 (SE 0.103) and 0.631 (SE 0.037). Critical assumptions are that estim ates of age ratios are unbiased or have the same bias in all 3 surveys , and that reliable estimates of the variances of these ratios are obt ained. Sample size calculations based on a binomial distribution are p resented for the estimators provided.