True's beaked whale
True's beaked whale (Mesoplodon mirus) is one of 21 species of beaked whales (Hyperoodontidae or Ziphiidae), medium-sized whales with distinctive, long narrow beaks and dorsal fins which are set far toward the posterior. They are marine mammals within the order of cetaceans.
True's beaked whale
True's beaked whale is a toothed whale and can be recognised as such by the single blowhole as well as the presence of teeth (rather than baleen).
Because of the highly disjunctive distribution patches of True's beaked whale, it is possible that the northern and southern hemisphere temperate ocean populations are actually distinct species or subspecies. Threats to the species include bycatch and underwater noise pollution.
This species is a member of the beaked whale family with the characteristic V-shaped crease on the throat and the short dorsal fin set relatively far back. True's beaked whale is a small beaked whale that can reach up to 5.3 meters in length.
The lower jaw has a single pair of teeth (exposed only in adult males). The smooth forehead rises at a shallow angle and is bulging in appearance. It has a distinct beak and the mouthline is curved down at rear.
True's beaked whale has a grey dorsal and lateral colouration with a lighter belly and darker areas around the eyes. Adults are often covered with scratches and scars.
True's beaked whale may be confused with Sowerby's beaked whale but can be recognised by its bulging but smooth forehead and its slightly shorter beak. Little is known about the aggregational behaviour of True's beaked whales but it is not expected to differ from similar species such as Sowerby's beaked whale (Kinze, 2002). Very few True's individuals have been weighed or measured. The longest male on record measured 5.3 m; the longest female, which had body mass of 1394 kilograms, and was 5.1 m long.
Distribution and Movements
Found along the North American coastline from Nova Scotia to the Bahamas, True’s beaked whale also inhabits temperate waters off the coast of Europe, and there are records of the species from near Australia and South Africa.
The IUCN Red List adds:
True's beaked whales appear to have a disjunct, anti-tropical distribution (Mead 1989, MacLeod et al. 2006). In the Northern Hemisphere, they are known only in the North Atlantic, from records in eastern North America (Nova Scotia to Florida), Bermuda, Europe to the Canary Islands, the Bay of Biscay, and the Azores. They also occur at least in the southern Indian Ocean, from South Africa, Madagascar, southern Australia and the Atlantic coast of Brazil (MacLeod et al. 2006). The species does not generally occur within 30° north or south of the equator, which may indicate that the northern and the southern subpopulations are isolated from one another. This is supported by morphological and colouration differences (Ross 1969, Ross 1984).
True's beaked whale is an oceanic species that may be seen at the surface but little is known on what depth they may dive to.
Squid beaks have been found in the stomachs of stranded animals, but no fish have been found in one control study.
Threats and Conservation Status
The IUCN Red List reports:
Almost no information is available on the threats and status of this species. It appears never to have been hunted. Entanglement in fishing gear, especially gillnets in deep water (e.g., for billfish and tuna), is probably the most significant threat.
This species, like other beaked whales, is likely to be vulnerable to loud anthropogenic sounds, such as those generated by navy sonar and seismic exploration (Cox et al. 2006)
As a temperate water species, the strap-toothed whale may be vulnerable to the effects of climate change as ocean warming may result in a shift or contraction of the species range as it tracks the occurrence of its preferred water temperatures (Learmonth et al. 2006). The effect of such changes in range size or position on this species is unknown.
Evidence from stranded individuals of Mesoplodon mirus indicates that they have swallowed discarded plastic items. This may eventually lead to death (e.g. Scott et al. 2001).
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