Humpback whales (Megaptera novaeangliae) off the U.S. West Coast are a mixture of whales from different Distinct Population Segments (DPSs) under the U.S. Endangered Species Act (ESA), predominantly Central America and Mexico. Within DPSs, demographically independent populations (DIPs) of humpback whales are delineated as ‘migratory herds’ that share both wintering and feeding areas (Martien et al., 2020). The Central America DPS, composed of those whales that winter along the Pacific coast of Central America from Panama to Guatemala, corresponds to a single DIP that migrates almost exclusively to the U.S. West Coast. This DIP’s wintering area is understood to extend into southern Mexico, and it is termed the CentAm/SMex–CA/OR/WA DIP for its wintering area and its feeding area off California, Oregon, and Washington (Fig. 1) (Taylor et al., 2021). The Mexico DPS includes multiple DIPs, with the DIP that migrates between northern mainland Mexico and the U.S. West Coast correspondingly termed the MMex–CA/OR/WA DIP (Martien et al., 2021). DIP–specific estimates of the number of whales using the U.S. West Coast Exclusive Economic Zone (EEZ), which are thus equivalent to estimates of the number of whales from each DPS using this feeding area, would be useful for management and decision–making involving these population units.
We estimated abundance for the CentAm/SMex–CA/OR/WA DIP from photo–identification data collected in their wintering area from 2019 to 2021. A randomization test suggested some fidelity in individual space use off Central America and Southern Mexico, implying that variable effort in time and space should be considered in capture–recapture estimates. We fitted a closed, one–dimensional spatial capture–recapture model to annual capture histories using a Bayesian framework. We accounted for uncertainty in the northern limit of the population and in the potential for movement of individuals across that limit by varying the northern population limit within the base model and exploring sensitivity to the northern boundary of the model domain in two alternate models.
We multiplied posterior distributions of abundance from the base and alternate models by a correction factor distribution, which was based on prior and new simulation work quantifying key expected sources of bias. The main sources of bias anticipated in estimating abundance for this dataset from a closed population model include births and deaths during the period of data collection, exclusion of first–year calves from the dataset, and sex heterogeneity in capture probability, which we assessed at a 3.4–fold (CV=0.463) greater chance of photo–identifying male individuals than females off Central America and Southern Mexico. The mean resulting correction factor is 1.35 (CV=0.143).
The base model produces a mean bias–corrected abundance estimate of 1,496 (CV=0.171), with a 20th percentile of 1,284. Alternate models with different northern model domain boundaries produce mean bias–corrected estimates of 1,313 (CV=0.167) and 1,601 (CV=0.166), corresponding to –12% and 7% differences in the 20th percentile from the base model. Comparison of the new abundance estimate for the CentAm/SMex-CA/OR/WA DIP to one from 2004-06 that omits southern Mexico (Wade, 2021) suggests that the annual population growth rate is much lower than the 8.2% rate
estimated for humpback whales off the U.S. West Coast as a whole (Calambokidis and Barlow, 2020). Population growth rate calculated directly from the current estimate including Southern Mexico and the Wade estimate is 4.8% per year (SD = 2.0%). Resummarizing our model results to exclude Southern Mexico animals results in a rate of 1.6% per year (SD = 2.0%).
Curtis, K.A., J. Calambokidis, K. Audley, M.G. Castaneda, J. De Weerdt, A.J. García Chávez, F. Garita, P. Martínez-Loustalot, J.D. Palacios-Alfaro, B. Pérez, E. Quintana-Rizzo, R. Ramírez Barragan, N. Ransome, K. Rasmussen, J. Urbán Ramírez, F. Villegas Zurita, K. Flynn, T. Cheeseman, J. Barlow, D. Steel, and J. Moore. 2022. Abundance of Humpback Whales (Megaptera novaeangliae) Wintering in Central America and Sounthern Mexico From a One-Dimensional Spatial Capture-Recapture Model. U.S. Department of Commerce, NOAA Technical Memorandum NMFS-SWFSC-661. doi: 10.25923/9cq1-rx80Download PDF
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