Estimation of calf:cow ratios of Pacific walruses for use in population modeling and monitoring



We report on a method of visually classifying Pacific walruses (Odobenus rosmarus) to sex and age class with the goal of estimating calf:cow ratios. Development of this method began in 1958 by Dr. F. H. Fay and was used during six surveys in the Chukchi Sea between 1981 and 1999. We estimate calf:cow ratios using beta-binomial models to allow for overdispersion and use Monte Carlo simulations to assess the reliability of prior surveys and quantify sample sizes required for future surveys. Calf:cow ratios did not vary by region, date, or by the number of cows in a group. However, higher ratios were observed in the morning and evening than during the day, indicating haul out behavior of cows varies by reproductive status. Adjusted for solar noon, few calves were observed in 1981 (3:100), 1984 (6:100), and 1998 (5:100), while substantially more were observed in 1982 (15:100) and 1999 (13:100). Classifying between 200 and 300 groups with cows (~1,600–2,300 individual cows) will yield calf:cow ratios with ~20%–30% relative precision. Tagging studies that examine hauling-out behavior of cows with and without calves relative to time-of-day are necessary to better understand how to interpret calf:cow ratios.