The Guidelines for Preparing Stock Assessment Reports Pursuant to the 1994 Amendments to the Marine Mammal Protection Act specify that a stock under the Act should, whenever possible, comprise a demographically independent population (DIP). Considerable new data suggest the existence of potential DIPs within some distinct population segments (DPSs) of the North Pacific subspecies of humpback whales (Megaptera novaeangliae kuzira), however this document considers evidence for a case where the DPS contains only a single DIP. This putative DIP is composed of the animals that winter in the waters offshore of Central America, nearly all of whom spend summers off of the U.S. west coast, called the CentAm/SMex-CA/OR/WA unit. A status review conducted in 2015 concluded that, based on genetics and movement data, this group of animals met the criteria of a DPS) (Bettridge et al. 2015), and they were subsequently listed as an endangered DPS under the Endangered Species Act (ESA).
Martien et al. (2019) identify three ‘strong’ lines of evidence for delineating DIPs – movements, genetics, and morphology. Robust data from a single strong line of evidence are sufficient to meet the DIP definition, where ‘robust data’ means that there has been appropriate evaluation of all relevant factors (e.g. age and sex difference, sample size, analytical methods, etc.) such that the observed difference is real, not a sampling or analytical artifact.
For the CentAm/SMex-CA/OR/WA unit, there are robust data consistent with demographic independence for two strong lines of evidence: genetics and movements. There are no lines of evidence to suggest that further DIPs exist within this unit.
Data collected since a 2015 status review (Bettridge et al. 2015) indicate that the wintering area for the Central American DPS extends into southern Mexico. Few data were available from the Pacific coast of southern Mexico at the time of the status review and the resulting description of the DPS (2016). However, genetic and movement data collected in recent years suggest that individuals that winter along the Pacific coast of southern Mexico off the states of Oaxaca and Guerrero (Figure 1) are likely part of the Central America DPS instead of the Mexico DPS, and therefore also part of the CentAm/SMex-CA/OR/WA unit (Audley et al. 2016, García Chavez et al. 2016a,b, García Chavez et al. 2017, Steiger et al. 2017, García Chavez et al. 2018, Auladell Quitana et al. 2019, Ramirez et al. 2019, Ortega-Ortiz et al. 2021). Some whales photographed in the area between Bahia Banderas off the state of Nayarit and the northern border of the state of Guerrero have been matched to the CentAm/SMexCA/OR/WA unit, while others have matched to whales photographed to the north along the mainland within the range of the Mexico DPS. The proportion of whales in the area between Nayarit and Guerrero that belong to the MMex-CA/OR/WA unit may vary among years, and with substantially more effort in this area being made in 2020-2022, the extent of the geographic range of the MMex-CA/OR/WA unit should be reconsidered in the near future.
Taylor, B.L., K.K. Martien, F.I. Archer, K. Audley, J. Calambokidis, T. Cheeseman, J. De Weerdt, A.F. Jordán, P. Martínez-Loustalot, C.D. Ortega-Ortiz, E.M. Patterson, N. Ransome, P. Ruvelas, and J. U. Ramírez. 2021. Evaluation of Humpback Whales Wintering in Central America and Southern Mexico as a Demographically Independent Population. U.S. Department of Commerce, NOAA Technical Memorandum NMFS-SWFSC-655. doi: 10.25923/sgek-1937Download PDF
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