Introduction

The Omura’s whale (Balaenoptera omurai) was described as a new species distinct from and sister to the clade formed by the Bryde’s whales (B. edeni brydei and B. e. edeni) and sei whale (B. borealis), through mitochondrial DNA sequencing, skeletal morphology and a limited description of external characteristics (Wada et al., 2003). A species-level status was first proposed by Wada and Numachi (1991) and Yoshida and Kato (1999), and the phylogenetic relationship supported by work using additional markers (Sasaki et al., 2006; Rosel and Wilcox, 2014). Estimated time of lineage divergence was initially 17 million years (Sasaki et al., 2006), but more recent work suggests a more recent divergence at 7–9 million years (Marx and Fordyce, 2015; Slater et al., 2017). The initial species description was based on a 1998 stranding in the Sea of Japan (the holotype specimen, which included the complete skeleton and baleen plates) along with tissues, morphological data and descriptions of eight specimens taken in the late 1970s by Japanese research whaling vessels near the Cocos (Keeling) Islands and Solomon Islands (Wada et al., 2003). Prior to the collection of the holotype specimen, the eight whaling specimens had been attributed to Bryde’s whale (B. edeni) but recognized as distinct due to their diminutive size, divergent allozyme profiles and partial mitochondrial DNA (mtDNA) haplotypes; these were referred to as “small” or “small-form” Bryde’s whales (Ohsumi, 1980; Wada and Numachi, 1991; Yoshida and Kato, 1999). Relatively concurrent with work on these aforementioned specimens, indigenous whaling specimens from the Bohol Sea, Philippines, were noted to have atypically small skulls for B. edeni and divergent mtDNA cytochrome b sequences (Perrin et al., 1996; LeDuc and Dizon, 2002) and were later identified as belonging to B. omurai (Sasaki et al., 2006; Yamada et al., 2008). Between 2003 and 2012, several specimens were identified primarily through examination of skulls throughout the Southeast Asian region (Yamada et al., 2006a, 2008; Yamada, 2009), which was possible due to the detailed description of skull morphology available from Wada et al. (2003). Through this work, the range of the species was thought to be restricted to the eastern Indo-Pacific region, more specifically the tropical eastern Indian Ocean and western Pacific Ocean.

In 2013 a population of Omura’s whales tractable for long-term monitoring was discovered off the northwest coast of Madagascar, extending the range into the western Indian Ocean (Cerchio et al., 2015). Following shortly on this report, a stranding off the coast of Mauritania was genetically identified extending the range much further into the tropical eastern North Atlantic Ocean (Jung et al., 2015). Prior to 2015 there was only a limited description of the species drawing similarities to the asymmetrical pigmentation of fin whales, based on the Sea of Japan stranding and Cocos (Keeling) Islands and Solomon Islands whaling specimens (Wada et al., 2003); however, no good photographic evidence or any documentation of free-ranging live animals with confirmed species attribution were available. Through the use of detailed surface and underwater still photographs and video, combined with genetic verification (mtDNA) of simultaneously collected skin biopsies, Cerchio et al. (2015) described in detail the external physical appearance and pigmentation patterns. Thereafter, numerous reports have been made in peer-reviewed scientific literature, gray literature and internet-based accounts, primarily in shallow coastal waters and expanding the range to most ocean basins from the western South Atlantic Ocean to the Indian Ocean, the northwest Pacific Ocean and Oceania (Cerchio and Yamada, 2018).

Here we summarize all records that we could find relating to the global distribution of Omura’s whales, drawing upon all available sources of information, in an effort to better establish the known range and begin to assess range-wide threats to the species, especially at the regional level.