Birth-Intervals and Sex Composition of Western Gray Whales Summering off Sakhalin Island, Russia
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...
| Main Authors: | , , , , |
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| Format: | Text |
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DigitalCommons@University of Nebraska - Lincoln
2009
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| Online Access: | https://digitalcommons.unl.edu/usdeptcommercepub/128 https://digitalcommons.unl.edu/context/usdeptcommercepub/article/1125/viewcontent/Brownell_SC61BRG10_Birth_intervals_and_sex.pdf |
| Summary: | 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 ... |
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