U.S. Department of Commerce


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Paper SC/61/BRG10 presented to the International Whaling Commission Scientific Committee, June 2009, Madeira,Portugal


Determining the birth-interval at which reproductive females produce calves is an indispensable component of studies on the population biology of large whales. In theory, shorter birth-intervals will result in a faster rate of population increase. Therefore, estimating this reproductive parameter is particularly important for modeling exercises designed to project the potential growth of a given population and, in the case of endangered populations, their ability to recover from a depleted state (e.g. Cooke et al., 2008). While a number of inherent biases exist with respect to determination of birth-intervals for baleen whales (Barlow and Clapham, 1997), dedicated sampling efforts and long-term field studies of known individuals have provided significant insight regarding this variable for a number of large whale populations. Knowing the ratio of males to females, including the primary, secondary, and tertiary sex ratio in a population is also essential to demographic assessments. In general, sex ratio information for baleen whale populations is patchy and, as true for birth-interval data, subject to a broad range of sampling bias.br>
Despite the aforementioned limitations, valuable information on the birth-interval and sex ratio of eastern Pacific gray whales (Eschrichtius robustus) has been published (e.g. Rice and Wolman, 1971; Rice, 1983; Jones, 1990). These studies, drawing upon biological samples collected from stranded or hunted individuals and photo-identification data from free-ranging whales, indicate that the typical birth-interval for the eastern gray whale is two-years, with a gestation period of about 418 days and lactation period of 6-8 months (Rice and Wolman, 1971; Rice, 1983). The fetal sex ratio for eastern gray whales has been estimated to be 1:1 (Rice, 1983).

The western Pacific gray whale population is critically endangered, numbering only about 130 individuals, and its ability to recover from near extinction is questionable (Cooke et al., 2008; Weller et al., 2008). Given the small size of the population and a relatively short period of study, fewer data exist regarding the birth-interval and sex ratio of western gray whales compared to the available information on eastern gray whales. This paper presents preliminary analysis of birth-intervals and sex composition of western gray whales summering off northeastern Sakhalin Island, Russia. Calving data were examined to determine the range of birth-interval values, the relative frequencies of different birth-intervals, and if calving intervals for individual females were stable or variable. In addition, determining the sex of calves and non-calves observed during the 12-year study allowed sex composition of the population to be assessed, including an examination of the proportion of male vs. female calves born to individual females.