Balaenoptera omurai, Wada, Oishi & Yamada, 2003

Russell A. Mittermeier & Don E. Wilson, 2014, Balaenopteridae, Handbook of the Mammals of the World – Volume 4 Sea Mammals, Barcelona: Lynx Edicions, pp. 242-299 : 294-295

6.

Omura’s Whale

Balaenoptera omurai

French: Rorqual d'Omura / German: Omura-Wal / Spanish: Rorcual de Omura

Other common names: Pygmy Bryde's Whale

Taxonomy. Balaenoptera omurai Wada, Oishi & Yamada, 2003 ,

Sea ofJapan.

The type specimen was stranded in 1998 on Tsunoshima Island in the southern Sea ofJapan. Monotypic.

Distribution. Tropical and subtropical waters of the Indo-Pacific region extending to 34° Nand 12° S.

Descriptive notes. Total length 1000-1200 cm. No specific data are available for body weight. Existing data suggest that adult female Omura’s Whales are larger than adult males. The smallest known neonate was 320 cm in total length. Omura’s Whale has traditionally been confused with whales of the Bryde’s Whale ( B. edeni ) complex and the Sei Whale ( B. borealis ), and it was often referred to in older whaling literature as “smallform Bryde’s Whale” or “pygmy Bryde’s Whale.” Body coloration of Omura’s Whale is distinctly countershaded, with a blue-black to dark-gray back and whitish-beige to cream-colored ventral surface. Undersides of pectoral flippers and caudal flukes are also light-colored like belly. As in Fin Whales ( B. physalus ), external surface of the lowerjaws shows coloration asymmetry, being black to dark gray on the left side and whitish-beige to cream-colored on the right side. External body proportions are reported to be similar to members of the Bryde’s Whale complex. Unlike formsin the Bryde’s Whale complex, rostrum of Omura’s Whale has only a single, prominent median ridge extending from blowholes to tip of rostrum, and it lacks distinct right and left lateral rostral ridges. From above, lateral margin of rostrum of Omura’s Whale is distinctly convex and reminiscent of the U-shaped rostrum of Blue Whales ( B. musculus ). Dorsal fin is relatively small, low, and strongly falcate. Pectoral flippers are small and slender, relatively longer than in forms of the Bryde’s Whale complex. There are four digits in the manus, and phalangeal formula of the type specimen is: II-5, 111-7, IV-6, and V-3. There is no available description of size and shape of caudal flukes. Ventral groove blubber is marked by 80-90 longitudinal grooves that extend posteriorly to and slightly beyond umbilicus. Coloration of baleen laminae is bilaterally asymmetrical. On the right side, anterior 30% of rack is yellowish-white, posterior 20% is black, and middle 50% is bicolored. On the left side, anterior 85% of rack is bicolored, while posterior 15% is black. Baleen laminae are relatively wide at the base, with a reported length-to-width ratio of only 1-2:1. Largest main baleen plates are 23-28 cm in length. Number of baleen laminae in the type specimen is 203 on the right side and 208 on the left. This count represents the fewest laminae per side of any known species of rorqual. Swimming speed of Omura’s Whale has not been reported, nor has the nature ofits blow. Dorsal fin is visible when an individual dives.

Habitat. Pelagic and inshore waters of the Indo-Pacific region. Omura’s Whale has perhaps the most restricted biogeographical distribution of any known species of rorqual. Its limited distribution, however, may simply be a function of the few known specimens assigned to this species. The type specimen was an 1100cm adult female, stranded on Tsunoshima Island (34° 21° N, 13° 53’ E). Only eight more individuals have been confidently assigned to this species, all based on molecular evidence from archived tissue samples taken from individuals killed under “scientific permits” in the 1970s. Of these, six adults were captured in October 1976 in pelagic tropical seas around the Solomon Islands (9° 49’ S—=10° 17° S). An additional two individuals (one adult and one subadult) were killed in November 1978 in pelagic tropical waters around the Cocos (Keeling) Islands (10° 51°53" S). Reports of small adult Bryde’s Whales captured in waters off Western Australia during the 1958-1963 whaling seasons may represent additional records of Omura’s Whale from the Southern Hemisphere. A possible sighting of an Omura’s Whale was reported in 2012 from waters around New Caledonia (21° S) in the western South Pacific Ocean and suggests a possible range extension into southern subtropical waters. If verified, this report would support previous suggestions that specimens of Omura’s Whale may have been captured during the 1958-1963 whaling seasons off Western Australia but erroneously recorded as “Bryde’s Whales of unusually small body size.”

Food and Feeding. Little, if any, information is available concerning the diet of Omura’s Whale. There is, however, a report that a group of small rorquals, possibly Omura’s Whale, sighted in the tropical waters off the Andaman Islands were chasing tuna. Not surprisingly, another report suggests that Omura’s Whales feeds on euphasiid crustaceans.

Breeding. There is no information available for this species.

Activity patterns. There is no information available for this species.

Movements, Home range and Social organization. Little information is available concerning the home range and migration pattern of Omura’s Whale, and what is known is based almost exclusively on the re-identification of genetic samples archived from individuals killed under “scientific collecting permits” during the 1970s. These data suggest that Omura’s Whale has little, if any, seasonal migration outside of tropical to subtropical parts of the Indo-Pacific region. For this reason,it is possible that Omura’s Whale is broadly sympatric with some populations of Bryde’s Whales.

Status and Conservation. CITES Appendix I. Classified as Data Deficient on The IUCN Red List. Omura’s Whale was only recently described as a species, and it is assigned a Data Deficient category because its global population size is unknown and its true biogeographical distribution has not yet been determined. The only available estimate of regional population size is from sightings data of “small Bryde’s Whales” reported in 1980 in waters around the Solomon Islands. This report estimated the regional population of these smaller rorquals at 1800 individuals. Because Omura’s Whale was only formally identified in 2003, there is no dependable information concerning the possible impacts of commercial whaling on its populations. It is clear, however, that individuals were killed under “scientific permits” in the 1970s. There also are recent reports of accidental entanglement in Japanese set nets and captures by the Philippine artisanal whale fishery. As with other species of rorquals, the international moratorium on commercial whaling represents the primary conservation measure for insuring the survival of Omura’s Whale. Other threats include fatal encounters with vessels (ship strikes), anthropogenic impacts (commercial shipping noise), and kills for “scientific research.” Threats posed by climate change to the warm-water Omura’s Whale are probably not as great as for more pagophilic species of rorquals.

Bibliography. Best (2001), Reilly et al. (2008f), Sasaki et al. (2006), Wada et al. (2003), Yamada (2009), Yamada et al. (2008).

Figures

5. Blue Whale (Balaenoptera musculus), 6. Omura’s Whale (Balaenoptera omurai), 7. Fin Whale (Balaenoptera physalus), 8. Humpback Whale (Megaptera novaeangliae)

Distribution. Tropical and subtropical waters of the Indo-Pacific region extending to 34° Nand 12° S.