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). One putative DIP is composed of whales that winter in the waters off mainland Mexico (MMex) and summer off of the contiguous U.S. west coast (referred to here as the MMex-CA/OR/WA unit). The Mexico DPS also includes whales that winter in mainland Mexico and the Revillagigedo Archipelago and that feed in more northerly waters (mainly in Alaska and to a lesser extent in Russia). We refer to these whales as the Mexico-Northern Pacific unit (Mex-NPac). We consider whether there are data to suggest that these Mex-NPac whales represent a single or multiple DIPs, and if so whether there are adequate data to delineate them.
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.
The MMex-CA/OR/WA unit has robust data consistent with demographic independence for two strong lines of evidence: genetics and movements. The MMex-CA/OR/WA whales meet the DIP definition. There are no lines of evidence to suggest that further DIPs exist within this unit.
NMFS conducted a status review of humpback whales in 2015 that resulted in the identification of 14 DPSs of humpback whales. These include the Mexico DPS and the Central America DPS, which overlap to some extent on the feeding grounds along the U.S West Coast (Bettridge et al. 2015). Few data were available from the Pacific coast of southern Mexico at the time of the status review to include within the assessment and resulting description of the two DPSs (81 Federal Register 62660; September 3, 2016). Data collected since the 2015 status review indicate that the wintering area for the Central America DPS extends into southern Mexico. For example, 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 unit1 (Castillejos-Moguel 2015, Audley et al. 2016, García Chavez et al. 2016a,b, García Chavez et al. 2017, Steiger et al. 2017, García Chavez et al. 2018, López-Aquino et al. 2018, Auladell Quintana 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/SMex-CA/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 2014-2022, the extent of the geographic range of the MMex-CA/OR/WA and CentAm/SMexCA/OR/WA units should be reconsidered in the near future.
Available movement data are consistent with the existence of multiple DIPs within Mex-NPac. However, the photographic data have not been stratified in the way required to formally evaluate additional putative DIPs. Thus, for the Mexico DPS, there is evidence to delineate one DIP (MMex-CA/OR/WA) but insufficient analyses (and perhaps data) to resolve the multiple DIPs within the Mex-NPac unit.
Martien, K.K., B.L. Taylor, F.I. Archer, K. Audley, J. Calambokidis, T. Cheeseman, J. De Weerdt, A.F. Jordan, P. Marinez-Loustalot, C.D. Ortega-Ortiz, E.M. Patterson, N. Ransome, P. Ruvelas, J.U. Ramirez, and F. Villegas-Zurita. 2021. Evaluation of Mexico Distinct Population Segement of Humpback Whales as Units Under the Marine Mammal Protection Act. U.S. Department of Commerce, NOAA Technical Memorandum NMFS-SWFSC-658. doi: 10.25923/nvw1-mz45.Download PDF
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