UW Aquatic & Fishery Sciences Quantitative Seminar

Alex Zerbini

National Marine Mammal Lab, Alaska Fisheries Science Center

A Bayesian assessment of the humpback whale stock in the western South Atlantic Ocean


The population of humpback whales (Megaptera novaeangliae) wintering off the eastern coast of South America is referred to by the International Whaling Commission (IWC) as ‘breeding stock A’. This population was heavily exploited in 20th century modern commercial whaling operations. After more than 30 years of protection, the present status of this population remains unknown. A deterministic sex and age-aggregated population dynamics model was fitted to modern whaling harvest data, absolute estimates of abundance and indices of relative abundance, with the goal of estimating the pre-exploitation population size (K), the maximum net recruitment rate (Rmax), the maximum depletion level (Nmin/K), and other status indices. A Bayesian statistical method was used to calculate probability distributions for the model parameters and other quantities of interest. Prior distributions were set on Rmax - an uninformative (Uniform [0, 0.106]) and an informative (Normal [0.067, 0.042]) - and on the population size in 2005 - N2005 (Uniform [ln(550); ln(20000)]). A total of 5000 samples were used to compute the joint posterior distribution of the model parameters using the Sampling-Importance-Resampling (SIR) algorithm. Sensitivity of model outputs to the priors on Rmax, a genetic constraint, data inclusion and catch allocation scenarios was investigated. Posterior probability distributions of quantities of interest for the base case scenario were: Rmax = 0.07 (95% credibility intervals [CI] = 0.013-0.099), K = 24461 (95% CI = 25031-31220), Nmin/K = 1.9% (CI = 0.3%-11.9%), N2006/K = 27.0% (CI = 18.0%-38.7%), N2020/K=61.7% (CI = 23.4%-88.0%), and N2040/K=97.5% (CI = 30.4%-99.9%). Despite apparent recovery in the past three decades, the western South Atlantic humpback whale population is still low relative to its pre-exploitation size and requires continued conservation efforts.

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