Birth-Intervals and Sex Composition of Western Gray Whales Summering off Sakhalin Island, Russia

• Main Authors: Weller, David W., Bradford, Amanda L., Lang, Aimée R., Brownell, Robert L., Jr., Burdin, Alexander M.
• Format: Text
• Language: unknown
• Published: DigitalCommons@University of Nebraska - Lincoln 2009
• Subjects: Environmental Sciences; Barlow; Pacific; Weller; baleen whale; baleen whales; Sakhalin
• Online Access: https://digitalcommons.unl.edu/usdeptcommercepub/128; https://digitalcommons.unl.edu/context/usdeptcommercepub/article/1125/viewcontent/Brownell_SC61BRG10_Birth_intervals_and_sex.pdf

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 ...