Sowerbys Beaked Whale, Beaked Whale

Sowerby's beaked whale (Mesoplodon bidens), also known as the North Atlantic or North Sea beaked whale, is a species of toothed whale. It was the first mesoplodont whale to be described. James Sowerby, an English naturalist and artist, first described the species in 1804 from a skull obtained from a male that had stranded in the Moray Firth, Scotland, in 1800. He named it bidens, which derives from the two teeth present in the jaw, now known to be a very common feature among the genus.^[3]

Physical description

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  Profile of an adult female

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  Adult male

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  Adult male head, showing teeth.

Sowerby's beaked whale has a typical body shape for the genus, and is mainly distinguished by the male's dual teeth positioned far back in the mouth. The whale's beak is moderately long, and the melon is slightly convex. The colouration pattern is a grey with light countershading on the bottom, and frequently has cookie cutter shark bites and scars from teeth (in males). The whale reaches 5 metres (16 ft) in females and 5.5 metres (18 ft) in males, with a weight of 1000-1300 kilograms (2200-2900 lb). The gestation period lasts for 12 months and the young are born at a length of 2.4 to 2.7 metres (8 to 9 ft) with a weight of around 185 kilograms (400 lb).

Diet

The diet of Sowerby's beaked whales consists primarily of small mesopelagic and benthopelagic fish, with cephalopods accounting for a much smaller proportion of the diet.^[4]

Population and distribution

Sowerby's beaked whale ranges from Nantucket to Labrador in the western North Atlantic and from Madeira to the Norwegian Sea in the eastern North Atlantic. They typically range in waters 200 to 1,500 metres (650 to 5,000 ft) deep. No population estimates have been made. In 1991, there were about 90 records of the species, 80 from the eastern North Atlantic and less than ten from the western North Atlantic; the majority of the records are from around the British Isles.^[5]

On 10 January 2009, a female Sowerby's beaked whale was found at the port of Fethiye on the Aegean coast of Turkey, far away from her natural habitat. The whale was successfully saved and released back to the open sea.^[6]

On 25 July 2015, biologists with the New England Aquarium investigated the death of a beaked whale in Massachusetts. The carcass of the 17-foot (5.2 m) long female, which weighed almost 1 short ton (0.91 t), was found on Plymouth Long Beach in Plymouth, Massachusetts. Biologists from the aquarium and the International Fund for Animal Welfare said they would perform a necrospy at the Woods Hole Oceanographic Institution. The whale was initially identified as a Sowerby's beaked whale, but the aquarium said a more thorough examination and consultation with additional experts was needed as staff had not seen a beaked whale since 2006.^[7]

On 26 October 2018, a 4.2-metre (14 ft) whale was found beached near the town of Saltdean, on the UK coast. The carcass was subsequently taken to the Natural History Museum, London for post-mortem.^[8] On 29 August 2019, a stranded whale was rescued from Dungarvan Bay, in southeast Ireland.^[9] On the 4th July 2020, a whale became disorientated and strayed into Wicklow, harbour on the east coast of Ireland but was later discovered dead on Wicklow, beach.^[10]

On 21 August 2020, a 3.86-metre (12.66 ft) female Sowerby's beaked whale washed up on the shore of a beach in Caister-on-Sea, Norfolk. Emergency services assisted the whale out to sea in an attempt to encourage the animal to live. Unfortunately the next morning a report was received to suggest the whale was stranded and deceased at Lowestoft, on the UK coast, following reported sights of 2 whales in the nearby towns of Brancaster and Blakeney in Norfolk earlier in the month.^[11]

On successive days in October 2020, two Sowerby's beaked whales were washed up on separate beaches in East Lothian. Both animals died.^[12] The necropsy revealed that one of the whales had an unusually high density of small gas bubbles in the lung tissue, and both whales showed signs of gas emboli in the mesenteric arteries. These symptoms are often associated with decompression sickness, however it was not clear if this was the cause in this case. The MOD later revealed that, one day before the first whale stranded, airborne ASW sonar exercises had been carried out within 80nm of the stranding sites. There was not enough evidence from the necropsy to confirm decompression sickness as the cause of death, so the Scottish Marine Animal Stranding Scheme still consider these as live strandings. However, given the otherwise good physical health of the beaked whales combined with the unusual proximity in both time and location of their strandings, the use of sonar in the surrounding area should still be considered a possible factor in the stranding.^[13]

Behaviour

A whale breaching

Sowerby's beaked whales are reclusive creatures that stay away from ships and are rarely sighted. The whales are occasionally seen in groups of up to 10 individuals (males, females, and calves) and have been known to strand in groups as well. They hunt at depths greater than 500m, and dive to more than 1000m while foraging. Their swimming and diving behaviours are more similar to deep-diving delphinids such as Risso’s dolphins and pilot whales than to other mesoplodonts. Compared to Blainville’s beaked whales, a mesoplodont of similar body size, Sowerby’s beaked whales swim faster while descending and while actively foraging. Their dives are also relatively short and generally last between 30 and 40 minutes. Sowerby’s beaked whales cover long distances between dives by swimming at speed just below the surface of the water, as opposed to the typical beaked whale strategy, where time near the surface is minimised by travelling on shallow dives at depths between 100 and 200m.^[14]

Conservation

Skull of Sowerby's beaked whale.

The species has been hunted infrequently by Norwegians, but such practices have long since been abandoned. There are some deaths due to entanglement in fishing gear, but it is unlikely to be very damaging to the species. Sowerby's beaked whale is covered by the Agreement on the Conservation of Small Cetaceans of the Baltic, North East Atlantic, Irish and North Seas (ASCOBANS)^[15] and the Agreement on the Conservation of Cetaceans in the Black Sea, Mediterranean Sea and Contiguous Atlantic Area (ACCOBAMS).^[16] The species is further included in the Memorandum of Understanding Concerning the Conservation of the Manatee and Small Cetaceans of Western Africa and Macaronesia (Western African Aquatic Mammals MoU).^[17]

See also

• List of cetaceans

Notes

References