U.S. Department of Commerce


Date of this Version



Paper presented to the International Whaling Commission Scientific Committee, 62nd Annual Meeting, Agadir, Morocco, June 2010


Age at first reproduction (AFR) is a basic component of age-structured whale assessment models, but direct estimates of this parameter do not exist for either the abundant eastern or critically endangered western population of gray whales (Eschrichtius robustus). Instead, assessments of both populations have utilized either of two recognized estimates of eastern gray whale age at sexual maturity (ASM) that are adjusted by a year to account for fetal gestation. These ASM estimates are: 1) 9 years median, 6-12 years range, and 2) 6 years median, 5-9 years range, but there are biases and discrepancies associated with these estimates. Over a decade of individual monitoring of western gray whales on their primary feeding ground off the northeastern coast of Sakhalin Island, Russia, has identified 17 female whales first sighted as calves or yearlings that were potentially sexually mature by the 2009 field season, ranging in age from 5 to 11 years. However, only two of these whales have been observed to have produced a calf, establishing the first observed values of western gray whale AFR as seven and 11 years. While limiting, that only two AFR observations were made is also informative, suggesting that until more information is available, the first eastern gray whale ASM estimate is the more appropriate to use in western gray whale assessments. Overall, eastern and western gray whale assessments would benefit from a concerted effort to collect AFR observations from each population.