Population Biology and Status Of Eastern Pacific Gray Whales: Recent Developments

Gray whales (Eschrichtius robustus) of the eastern Pacific stock are perhaps the best-known population of baleen whales, but uncertainties remain concerning many aspects of their population biology. The most recent abundance estimate (1987/88) of 21 113 (SE 688) may be the most accurate and precise available for any baleen whale population; but the estimated confidence interval is problematic, most likely not reflecting all relevant sources of error. The population is estimated to have increased at an annual rate of 3.2% (SE 0.5%) during the 20-yr period 1967/68 to 1987/78, during which annual harvests of about 174 whales were taken by the Soviet aboriginal fishery. Data from this fishery indicate a steep decline in pregnancy rates during 1987 to 1990, from a relatively stable level during the preceding 20 yr. Three approaches have been used to determine the status of this population: back-calculation, dynamic response, and Bayesian estimation of parameters for a simple population dynamics model. None of these approaches produced unequivocal results, but the back-calculation produced reasonable trajectories of abundance through time with either a tripling of carrying capacity since 1846 or a 50% minimum increase in recorded catches. All uncertainty considered, the population is probably above its 1846 level, when commercial exploitation began, and may now be approaching current carrying capacity. The combination of net productivity exceeding 4% and indications that the population is nearing carrying capacity are consistent with the hypothesis that for most large mammals, maximum net productivity occurs at high population levels.