The Bluntnose Sixgill Shark, Hexanchus griseus, often simply referred to as the cow shark, is the largest hexanchoid shark, growing to more than 18 ft. (6 m) in length. Many of the sixgills’ closest relatives are extinct, and the living species that are the closest genetically include the dogfish, Greenland Sharks, as well as other six and seven-gilled sharks. There are more closely related relatives in the fossil records than living species, some dating back as far as 200 million years.

What makes this shark a notable species is due to its primitive physical characteristics. Most common sharks today have only 5 gill slits, where as the aptly named Sixgill still has six gills. Their skin colour ranges from tan to brown, and can be as dark as black with a light colored lateral line down the sides and on the fins’ edges, along with dark colored spots on the sides. The general body shape is a heavy, powerful body with a broad head and small eyes. The pupils are black and the eye color is a fluorescent blue green. As an adult, the Bluntnose Sixgill Shark can grow to a massive size: males can grow to up 11 ft. (3 m) in length while females, which tend to be larger, can reach 14 ft. (5 m).

Sixgill Sharks typically inhabit depths greater than 300 ft. (90 m), and have been recorded as deep as 6150 ft. (1,875 m). Like many deep-sea creatures, the Bluntnose Sixgill Shark is known to undertake nightly vertical migrations (travelling towards the surface at night, and returning to the depths before dawn). This migration is done in order to find food living closer to the surface.

Very little is known about the reproductive process of Bluntnose Sixgill Sharks. What little is known is actually more like scientific speculation. Female Bluntnose Sixgill Sharks reach sexual maturity between the ages of 18 and 35 years. Males usually reach sexual maturity much younger, between the ages of 11 and 14 years old. Scientists are unsure of how the Bluntnose Sixgill Shark reproduces, but it is thought that males and females meet seasonally between the months of May and November. Gestation among these sharks is also unknown, however scientists believe that it’s longer than two years. The Bluntnose Sixgill Shark is ovoviviparous, which means that the young are carried within the mother’s body until the eggs hatch. The litter size ranges from 20 – 110 pups, which are quite large and well developed. This high birthing rate is thought to reflect a high mortality rate amongst the pups.

The largest threat to Bluntnose Sixgill Sharks is commercial fishing. Recent observer data indicates that this species is caught regularly as by-catch by fisheries pursuing halibut and spiny dogfish.

There is no current Bluntnose Sixgill Shark fishery, although the species has been the focus of at least three known directed fisheries in Canadian waters. The first occurred in the early 1920s with a focus on skins used to make shark leathers. The second took place between 1937 and 1946 with a focus on the shark livers for vitamin A. The third commercial fishery for Bluntnose Sixgill Sharks commenced under an experimental basis in the late 1980s to early 1990s but was terminated due to conservation concerns. The IUCN has assessed the Bluntnose Sixgill Shark as lower risk/near threatened.

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The images below were taken in the summer of 2009 at Ogden Point a group of us recovered a dead Bluntnose Sixgill from Fonyo Beach in Victoria BC. The shark was taken to the University of Victoria for a necropsy to determine the age, diet and cause of death.

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Retention and selling of Bluntnose Sixgill Sharks captured by hook and line fisheries, both commercial and recreational, in British Columbia is prohibited. In Puget Sound waters, there has been a permanent closure on the recreational and commercial take of Sixgill Sharks since 2001. Beginning in April 2006, all commercial hook and line fisheries operating in Canada’s Pacific waters became subjected to 100% at-sea monitoring coverage in the form of observers and electronic monitoring (video surveillance). Monitoring will allow for highly reliable catch estimates of non-target species including Bluntnose Sixgill Sharks in the future. The issue with the current method of observation does not individually identify each shark by species, which then creates incomplete data and facts concerning this and other shark species in the Pacific Northwest. Interacting with and photographing one of these unique animals is truly a special event.

Be that diver

Scott