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NOAA Technical Memorandum NMFS-AFSC-207

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Population status of the eastern North Pacific stock of gray whales in 2009


An age- and sex-structured population dynamics model is fitted using Bayesian methods to data on the catches and abundance estimates for the eastern North Pacific (ENP) stock of gray whales (Eschrictius robustus). The prior distributions used for these analyses incorporate revised estimates of abundance for ENP gray whales and account explicitly for the drop in abundance caused by the 1999-2000 mortality event. A series of analyses are conducted to evaluate the sensitivity of the results to different assumptions. The baseline analysis estimates the ENP gray whale population to be above the maximum net productivity level (MNPL), because the posterior mean for the ratio of 2009 abundance to MNPL, termed the optimal sustainable population ratio, is 1.29 (with a posterior median of 1.37 and a 90% probability interval of 0.68-1.51), indicating the population is estimated to be well above MNPL. The baseline analysis estimates a probability of 0.884 that the population is above its MNPL, which means there is a 0.884 probability that it is at its optimum sustainable population size as defined by the U.S. Marine Mammal Protection Act. These results are consistent across all the model runs. The baseline model also estimates the 2009 ENP gray whale population size (posterior mean of 21,911) to be at 85% of its carrying capacity (posterior mean of 25,808), and this is also consistent across all the model runs.

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