This taxonomic unit is treated as a single species even though there is extensive and growing evidence that it is, in fact, a complex of multiple forms with morphological, genetic, ecological, and behavioural differences that merit subspecies if not also species designations. At the time of writing (June 2017), the Committee on Taxonomy of the Society for Marine Mammalogy (<a href=""https://www.marinemammalscience.org/species-information/list-marine-mammal-species-subspecies/"">www.marinemammalscience.org/species-information/list-marine-mammal-species-subspecies</a>), which is generally regarded as the authority for marine mammal taxonomy, recognized a single killer whale species, Orcinus orca (Linnaeus, 1758), and two unnamed subspecies in the eastern North Pacific, the ENP resident killer whale (O. o. un-named subsp.) and the ENP transient killer whale (O. o. un-named subsp.) also known as Bigg's killer whale. The Committee noted, however, Other forms of killer whales in the North Pacific, North Atlantic and Antarctic [Southern] Ocean may warrant recognition as separate subspecies or even species, but the taxonomy has not yet been fully clarified or agreed (Morin et al. 2010; Foote et al. 2009, 2013). One population (a distinct population segment) of the ENP resident subspecies (the southern resident population) was listed as Endangered under the Canadian Species at Risk Act in 2003 and the U.S. Endangered Species Act in 2005.