Global Distribution of Omura’s Whales (Balaenoptera omurai) and Assessment of Range-Wide Threats

Abstract

When the Omura's whale (Balaenoptera omurai) was first described in 2003, it was known from only three locations: the southern Sea of Japan, and the vicinities of the Cocos (Keeling) Islands and Solomon Islands. Work over the following decade suggested a range limited to the eastern Indo-Pacific, but more recent discoveries in the western Indian Ocean and Atlantic Ocean suggested a more widespread range than previously thought. Here we use all available sources of information, including published papers, unpublished reports, and internet-based accounts, substantiated through genetic, morphological, photographic and acoustic documentation, to compile accounts of Omura's whales globally. Reports increased precipitously since 2015 after publication of the first detailed external description of the species, reflecting the impact of the recently elevated awareness of the species. We report 161 accounts from 95 locales in the waters of 21 range states, and found that the species is widely distributed in primarily tropical and warm-temperate locations. Currently it is known from all ocean basins with the exception of the central and eastern Pacific. The majority of accounts remain in the eastern Indo-Pacific suggesting a potentially recent range expansion from this region. There is a strong tendency toward a coastal and neritic distribution, although there are also several pelagic records. A predominantly near-coastal distribution places Omura's whale at risk from anthropogenic activities throughout its range, and its tropical distribution in often remote and poorly monitored areas makes adequately documenting and assessing threats challenging. We assess documented threats in light of the reported species' range, and found threats from, at minimum, ship strikes, fisheries bycatch and entanglement, local directed hunting, petroleum exploration (seismic surveys), and coastal industrial development. Current evidence indicates that at least some populations are non-migratory with local, potentially restricted ranges. Furthermore, there is low genetic diversity documented throughout its global distribution. Given the species may be characterized by small local populations, it may be particularly vulnerable to impacts from existing regional anthropogenic threats. We recommend that focused work be conducted to locate and study local populations, assess potential population isolation, and determine conservation status and specific anthropogenic threats across the species' range.

Multiple pygmy blue whale acoustic populations in the Indian Ocean: whale song identifies a possible new population

“…The tones of the bi- or triphonic sound all ended at the same time, 11.7 s after the beginning of the song. The second unit seems to be optional 15 , 17 , 18 , 44 . It followed after 2.8 s silence.…”

“…Diego Garcia Omura’s whale song (DGC) The ‘Diego Garcia Croak’—DGC—recently attributed to the Omura’s whales 17 was comprised of one unit (Fig. 6 c), although sometimes a second unit was present.…”

“…The first part of the Chagos song shares acoustic features with the DGC song. They are similar in duration (approximately 4 s), frequency range (approximately 15 to 50 Hz) 14 , and are both described as amplitude-modulated in structure 14 – 17 . Thus, it is possible that these songs are produced by the same whale species.…”

“…Thus, it is possible that these songs are produced by the same whale species. As the DGC song has been recently attributed to the Omura’s whale ( B. omurai ) based on acoustic similarity with the Omura’s whales recorded off Madagascar 15 , 17 , one could argue that the Chagos song is an Omura’s and not a blue whale song.…”

A new blue whale song-type described for the Arabian Sea and Western Indian Ocean

“…Although it is unfortunate that the recorder was not in the water during the observation, given the short period between the recording and observation, we consider this direct evidence of a blue whale being in the vicinity of the recorder during singing activity when no other baleen whales were sighted. Off Madagascar, Bryde's whales have never been documented off the northwest coast study region despite extensive effort working with medium-sized balaenopterids (Omura's whales), surveying inshore and offshore waters, and passive acoustic monitoring in shallow and deep water (Cerchio et al 2015(Cerchio et al , 2018(Cerchio et al , 2019; however, Bryde's whales are known to occur regionally in the Southwest Indian Ocean far to the south of Madagascar on the Madagascar Ridge (Best 2001). Conversely, blue whale song was detected extensively during passive acoustic monitoring off northwest Madagascar, including 3 different song-types (Antarctic, SWIO/Madagascar, and NIO/Sri Lanka) representing 3 separate populations in addition to the novel song-type recorded off Oman and Chagos (Cerchio et al 2018); therefore, it is clear that this region is habitat for overlapping blue whale populations.…”