Genetic variation and evidence for population structure in eastern North Pacific false killer whales (<i>Pseudorca crassidens</i>)

False killer whales ( Pseudorca crassidens (Owen, 1846)) are incidentally taken in the North Pacific pelagic long-line fishery, but little is known about their population structure to assess the impact of these takes. Using mitochondrial DNA (mtDNA) control region sequence data, we quantified genetic variation for the species and tested for genetic differentiation among geographic strata. Our data set of 124 samples included 115 skin-biopsy samples collected from false killer whales inhabiting the eastern North Pacific Ocean (ENP), and nine samples collected from animals sampled at sea or on the beach in the western North Pacific, Indian, and Atlantic oceans. Twenty-four (24) haplotypes were identified, and nucleotide diversity was low (π = 0.37%) but comparable with that of closely related species. Phylogeographic concordance in the distribution of haplotypes was revealed and a demographically isolated population of false killer whales associated with the main Hawaiian islands was identified (Φ ST = 0.47, p &lt; 0.0001). This result supports recognition of the existing management unit, which has geo-political boundaries corresponding to the USA’s exclusive economic zone (EEZ) of Hawai‘i. However, a small number of animals sampled within the EEZ but away from the near-shore island area, which is defined as &lt;25 nautical miles (1 nautical mile = 1.852 km) from shore, had haplotypes that were the same or closely related to those found elsewhere in the ENP, which suggests that there may be a second management unit within the Hawaiian EEZ. Biologically meaningful boundaries for the population(s) cannot be identified until we better understand the distribution and ecology of false killer whales.