Population assessment of western North Pacific gray whales (Eschrichtius robustus)

Abstract

Two geographically and genetically distinct populations of gray whales (Eschrichtius robustus) occur in the North Pacific, referred to as the eastern and western populations. Subjected to intensive modern commercial whaling during portions of the 19th and 20th centuries, the western population was proposed to be extinct during the early 1970’s. This population presently remains in small numbers and is considered one of the world’s most endangered populations of large whales. The need for increased conservation efforts indicates the appropriateness of a quantitative western gray whale population assessment. Since 1997, ongoing studies of western gray whales have resulted in a photographic dataset that can be used for mark-recapture survival estimation. A robust design model was fitted to 116 individual whale encounter histories spanning 22 monthly capture occasions from 1997 to 2002. Constant non-calf and calf (first-year post-weaning) survival and random temporary emigration were assumed. Models incorporating individual heterogeneity in residency patterns and higher temporary emigration probabilities for younger whales provided better fits to the data. Non-calf and calf survival were estimated as 0.952 (SE=0.0151, 95% CI=0.912-0.975) and 0.709 (SE=0.1178, 95% CI=0.443-0.882), respectively. These survival estimates and other life history parameters were utilized in conjunction with the Lotka equation to calculate the 1997-2002 population growth rate of western gray whales. A Monte Carlo simulation method was employed (n=10,000 trials) to account for uncertainty in the life history parameters. A range of possible fecundity values was examined to estimate a conservative, intermediate, and liberal rate of population growth. These growth rates were estimated as 0.026 (SD=0.0190, 5th-95th Percentiles=-0.008-0.054), 0.031 (SD=0.0194, 5th- 95th Percentiles=-0.003-0.061), and 0.036 (SD=0.0198, 5th-95th Percentiles=0.001-0.066), respectively. Each calculated growth rate and historical catch data were fitted to the generalized logistic equation in a 20th century back calculation of the western gray whale population. A Bayesian statistical method and the Sample-Importance-Resample algorithm (n1=2,000,000 initial samples; n2=5,000 resamples) were used to estimate model parameters and indices of population status. Back calculation results suggest that the western gray whale population is currently growing at its maximum net recruitment rate, the carrying capacity of the population is undefined, the population is currently at most between 8-9% of its original size, and the population has been highly depleted for over half of the 20th century. Current threats and low-density population effects could inhibit the recovery of western gray whales, emphasizing the necessity of concerted international protection and conservation planning for this critically endangered population.