The Estimation of Survival and Litter Size of Polar Bear Cubs

The mark/recapture analyses that are currently used to study polar bears (Ursus morifimus) generally underestimate the average number of cubs per litter and assume that the annual survival of cubs is unity. In this paper, a relationship among the number of yearlings per litter and 2-year-olds per litter is used to derive the survivorship of yearlings, which is then used to solve for the number of cubs per litter. When this relationship was applied to published population data from North America, the resulting estimated survival rates of yearlings ranged from 0.70 to 0.75, and the estimated average number of cubs per litter was between 1.70 and 1.98. These findings indicate that current estimates of sustainable yield for polar bear populations may be in error because the reproductive rate of adult females was likely to be underestimated, and the survival rate of cubs of the year was likely to be overestimv+d. Int. CoM. Bear Res. and Manage. 5:%0-%3 Obtaining estimates of a sustainable yield requires information on age-specific rates of reproduction and survival. For polar bears, the agespecific rate of reproduction equals the product of the average number of litters produced per year times the average number of cubs per litter for a female of age x. Satisfactory estimates of litter production rates are derived from marklrecapture information (Stirling et al. 1975; unpubl. rep., Environ. Manage. Serv., Edmonton, 1978; Lentfer et al. 1980). However, current estimates of the average number of cubs per litter are unreliable. For example, the estimate by Lentfer et al. (1980) of the average number of cubs per litter was less than their estimate of the average number of yearlings per litter. In addition, they found fewer litters of cubs than of yearlings. Obviously, there are sampling biases because the average number of cubs per litter cannot be less than the average number of yearlings per litter and there must be at least as many litters of cubs as of yearlings. However, in lieu of suitable alternatives, the average number of cubs per litter is assumed to equal the average number of yearlings per litter. Inherent in the assumption that the number of cubs per litter equals the number of yearlings per litter is the assumption that mortality of cubs is zero. Unfortunately, survival rates for cubs have not been estimated. Standard age class estimates of survival of cubs are not possible because, in ' Present address: National Marine Fisheries Service, P.O. Box 271, La Jolla, CA 92038. most studies, a greater number of yearlings are captured than cubs, and sample sizes are typically too small for mark/recapture studies to be of use. An additional problem with assuming that mortality of cubs is zero is that the average number of yearlings per litter is consistently greater than the average number of 2-year-olds per litter (Stirling et al. 1975; unpubl. rep., Environ. Manage. serv., Edmonton, 1978; Lentfer, unpubl. rep., Alaska Fed. Aid Proj. W-17-4/5, 1976; Lentfer, et al. 1980). This implies that yearling mortality is greater than zero; therefore, it is unlikely that the mortality rate of cubs is zero. The purpose of this paper is to present a method that estimates yearling survival from the difference between the number of yearlings per litter and the number of 2-year-olds per litter, and then to show how to use this rate to estimate the number of cubs per litter. The first draft of this manuscript was substantially improved by changes suggested by the editors of the National Fish and Wildlife Laboratory, R.L. Brownell, W. Calvert, A. Johnson, and D.B. Siniff. A second draft was improved by C. Meslow and 2 other editors. We thank R. Rice for typing the first draft, and L. Prescott for typing the final version.