Mesoplodon marus Gervais 1850

12. True’s Beaked Whale Mesoplodon marus French: Baleine-a-bec de True / German: True-Zweizahnwal / Spanish: Zifio de True Taxonomy. Mesoplodon mirus True, 1913, USA, “stranded in the outer bank of Bird Island Shoal in the harbor of Beaufort, North Carolina.” There are currently no recognized subspecies, but it is possible that the northern and southern forms are sufficiently distinct to be considered subspecies. Monotypic. Distribution. Anti-tropical distribution: N Atlantic Ocean, in a poorly defined region ranging from Ireland to the Canary Is in the E, and from Nova Scotia to Florida in the W, Southern Hemisphere, recorded in S Brazil, South Africa, Madagascar, and S Australia. Nevertheless, due to the small number of records, the full extent of its distribution in the Southern Hemisphere is unclear. Descriptive notes. Total length 480-540 cm; weight 1000-1400 kg. Female True’s Beaked Whales may beslightly larger than males. Body is spindle-shaped, with greatest girth around its midpoint. Flukes are wide in relation to body length, and tailstock is compressed laterally. Dorsal fin is small and set approximately two-thirds the distance between tip of the beak and end of the tail. Coloration differs between True’s Beaked Whales found in the North Atlantic Ocean and those found in the Southern Hemisphere. In the Northern Hemisphere form, individuals are medium to dark gray on top and paler underneath, with a dark patch just behind the eye. In the Southern Hemisphere form, there is an additional white or pale area on each side on the flank from the dorsal fin back toward flukes. Adult males may have a number of long white lines that are unpigmented scars from aggressive male-male interactions. Rostrum and lowerjaw form a short but distinct beak, and there are two grooves on the throat. Adult males have a pair of conical tusks at the tip of the rostrum; these may become heavily worn over time. Habitat. Primarily waters greater than 200 m deep. In common with other species of Mesoplodon, True’s Beaked Whales may be more common in areas of complex seabed topography, but this still has to be confirmed. Food and Feeding. Little is known about the diet of True’s Beaked Whale, but in common with other species of Mesoplodon, it is presumed to feed on deep-water squid and deep-water fish. As with other species of beaked whales, True’s Beaked Whales likely forage at depths greater than 500 m for much of their lives. 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. There is no specific information available for this species, but sightings at sea have been of relatively small groups of five or fewer individuals. Nothing is known about typical composition of these groups. Status and Conservation. CITES Appendix II. Classified as Data Deficient on The IUCN Red List. There are no estimates of global population size of True’s Beaked Whale. Like other species of beaked whales, True’s Beaked Whale is potentially affected by ingestion of plastic debris, bycatch in driftnet fisheries, and noise pollution. As a species restricted to cooler waters, it may also be vulnerable to effects of climate change. Nevertheless, nothing is known about the form or extent of these potential impacts. Bibliography. Dalebout et al. (2007), MacLeod et al. (2006), Mead (1989b), Rice (1998), Ross (1984).

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