“Mesoplodon densirostris”
Blainville’s Beaked Whales sometimes known as the “Dense-beaked Whale”, are little known members of the beaked whale family Ziphiidae. The French zoologist Henri de Blainville first described the species in 1817 from a small piece of jaw, the heaviest bone he had ever come across which resulted in the name densirostris (Latin for “dense beak”). Off the northeastern Bahamas, they are particularly well documented and a photo identification project started sometime after 2002. As adults, Blainville’s Beaked Whales can reach estimated lengths of approximately 15-20 feet (4.5-6 meters) and weigh 1,800-2,300 pounds (820-1,030 kilograms). Males can be easily distinguished from females and juveniles by a pair of large visible tusk-like teeth that erupt and point forward from the heavily arched lower jaw. Females & juveniles have teeth as well, but they remain hidden beneath the gum tissue of the mouth and their jaw line is less-curved. Blainville’s Beaked Whales have a relatively medium-sized round body with a small, wide-based, slightly “falcate” dorsal fin located far down (about two-thirds) the animal’s back. The whale’s head has a low, sloping forehead and indistinct ”melon”. Their coloration varies from dark gray to brownish and bluish. The face & underside of the animal is pale gray or white, giving it a counter-shading appearance. The skin may appear wrinkled on the dorsal area and is covered with linear & oval-shaped scars and other markings. Individuals especially mature males, accumulate scars & scratches with age also, “diatom” infestation may discolor areas of the skin. Many species of beaked whales (especially those in the genus Mesoplodon), are very difficult to distinguish even when dead. At sea, they are challenging to observe & identify to the species level due to their cryptic, skittish behavior, a low profile and a small, inconspicuous blow at the water’s surface; therefore, much of the available characterization for beaked whales is to genus level only. Uncertainty regarding species identification of beaked whales often exists because of a lack of easily discernible or distinct physical characteristics. Blainville’s Beaked Whales are usually found individually or in small social pods averaging between 3-7 individuals, but have been occasionally seen in larger pods of up to 12 animals. Groups may consist of various combinations and/or be segregated depending on age or sex. Adult populations in productive waters over the continental shelf like the Bahama Islands, may be grouped in harems and consist of several adult females with a single adult mature male. Males commonly battle over access to females, which is probably the cause of the long linear scars seen on individuals.
Blainville’s Beaked Whales’ distribution is considered the most extensive of the Mesoplodon genus. They have a cosmopolitan distribution throughout the world’s oceans and range from the Mediterranean, England, Iceland, Nova Scotia (Canada), Brazil and South Africa in the Atlantic; to California, Chile, Japan, New Zealand and Australia in the Pacific. Like other beaked whales, these whales are deep divers. Regular dives range from 20-45 minutes, and commonly reach depths of at least 1,600-3,300 feet (500-1,000 meters), but dives of over 54 minutes and up to 4,600 feet (1,400 meters) have been recorded. While diving, they use suction to feed on small fish & cephalopods (e.g., squid) in deep water. Blainville’s Beaked Whales may reach sexual maturity at about 9 years of age. A sexually mature female will give birth to a single newborn calf that is about 6-8.5 feet (1.9-2.6 meters) long and weighs about 130 pounds (60 kilograms). The estimated lifespan of this species is unknown. Blainville’s Beaked Whales occur in tropical to temperate waters worldwide. They are generally found in deep, offshore waters of the continental shelf. This species is often associated with steep underwater geologic structures such as banks, submarine canyons, sea mounts and continental slopes. This species is commonly sighted in the northeastern Bahamas, Caribbean Sea, Gulf of Mexico, Hawaiian Islands, Sea of Japan and the Society Islands of the South Pacific. During studies, individuals have been identified and re-sighted in certain geographic areas that may have resident populations. In productive waters, adult animals are more commonly found in waters over the continental shelf & submarine canyons and sub-adult animals are found both inshore & offshore. Their distribution may vary depending on the movements of warm-water currents however, there are no known seasonal movements or migrations.
For management purposes, Blainville’s Beaked Whales inhabiting U.S. waters have been divided into 3 stocks: the Hawaiian stock, the Northern Gulf of Mexico stock and Western North Atlantic stock. The estimated number of animals in the Hawaiian Stock is 1,200-2,200, the Northern Gulf of Mexico is about 100 animals and there is no current estimate for the Western North Atlantic Stock of the 350-600 total unidentified Blainville’s Beaked Whales. The Northern Gulf of Mexico and Western North Atlantic stocks are considered “strategic” because of uncertainty regarding stock size and evidence of fishery-related mortality & serious injury. There are insufficient data to determine the population trends for this species, but they are probably not rare because estimates do not include a correction factor for submerged animals with long dive times which may be substantial & underestimate actual abundance. Unidentified beaked whales which may include Blainville’s Beaked Whales, have been incidentally taken in the pelagic drift gill-net fishery off the U.S. Atlantic coast. Blainville’s Beaked Whales are often undifferentiated in the field and at sea due to a lack of distinct characteristics and the difficulty of positively identifying animals to the species level. In the Indian Ocean off of the Seychelles and western Australia, Blainville’s Beaked Whales have been incidentally taken by Japanese fishing boats. This species has been occasionally taken in hunts targeting small cetaceans. Blainville’s Beaked Whales may be sensitive to underwater sounds & anthropogenic noise. Recently, strandings of this species in the Bahamas due to acoustic trauma have been associated with active sonar during naval military activities & exercises. Anthropogenic noise levels in the world’s oceans are an increasing habitat concern, particularly for deep-diving cetaceans like Blainville’s Beaked Whales that use sound to feed, communicate & navigate.