Sixgill Sharks

The sixgill shark (Hexanchus griseus) is normally found in water thousands of feet deep. However, between May and September, sixgills enter shallow water at Flora Islet, near Hornby Island. It is still unknown why the sharks come to the area.

SCUBA divers have a rare and unique opportunity to observe these sharks in the wild. The sharks are not baited or attracted in any way. Every encounter is a special bit of luck!

Little is known about the sixgill's feeding habits, migration patterns, reproduction or longevity. Ongoing research hopes to reveal answers to many mysteries about the sixgill's life.

Natural History of the Sixgill Shark

Range

Sixgills are found in all of the world's oceans. In most locations, they live at depths reaching 2500 m (8000'), so they are inaccessible to humans under most circumstances.

At Flora, divers ususally see sixgills at depths of 25 - 30 m (80 - 100'). They have even been sighted as shallow as 10 m (30').

Sixgills are also reported to migrate inshore seasonally at Monterey Canyon, San Francisco Bay, inlets in Norway, and other locations in British Columbia.

Behavior

We don't really know why sixgill sharks return to Hornby each year, or why they are observed in only one small area.

Divers have been encountering sixgills at Flora Islet for over two decades. During that time, the sharks have not been observed feeding, mating, or doing any activity that would help explain why they come to the area.

Sixgills are almost always solitary. When more than one shark is observed in the same area, they are normally travelling in different directions at different depths. They do not appear to be engaged in any common activity.

The sharks are comfortable in the presence of divers, as long as they are given some space. They swim slowly, close to the bottom, along ledges of the wall.

Sixgills do not like to be touched or to be surrounded by divers. If they feel uncomfortable, their response is to increase their swim speed and head into deeper water.

No attacks on humans have been attributed to sixgills. The sixgill shark is a large, powerful animal, but it need not be feared, only respected. Human divers are privilidged visitors to the shark's world.

Physical characteristics

Sixgills commonly attain a length of 4.8 m (16'), and there have been reports of them reaching 7.6m (25'). Most sharks observed at Flora Islet are 1.8 - 4.2 m (6 - 14') in length.

The sixgill gets its name from a physical feature which sets it apart from other sharks. Instead of the more advanced five-gill arrangement which has evolved in most sharks, the sixgill has six gill slits. The sixgill shark that exists today is virtually unchanged from fossil forms dating back 150 million years.

The sixgill has a very wide body. It is also called "cow shark" and "bulldog shark". It has only one dorsal fin which is placed very far back towards its long, raked tail. (Most sharks have two dorsal fins).

Sixgills range from dull grey to brown in colour dorsally, with lighter white to yellow ventrally.

Another identifier of the sixgill is that its mouth contains two different types of teeth. Its upper teeth are triangular in shape, and the lower ones are edged with saw-like serrations.

Diet

Stomach content analysis suggests that sixgills eat both bony and cartilaginous fishes. Preferred items in the latter group include rays, ratfish (Hydrolagus colliei) and dogfish (Squalus acanthias).

The remains of crustaceans and sea mammals such as seals, sea lions and whales have also been reported in the sixgill's gut.

Sixgills apparently scavenge and they are known to take bait from longlines set for dogfish and for halibut.

Although they are generally slow moving, sixgills can move with incredible speed for short periods, and may use this ability coupled with their drab coloration to capture their prey using an element of surprise.

Reproduction

Female sixgills reach sexual maturity at 4 m/400 kg (13' /880 lb). Male sixgills reach sexual maturity at 3 m/200 kg (10' /440 lb).

Many of the sharks seen near Hornby are not of a size that would indicate sexual maturity. No reproductive activities between sixgills have been reported at Flora.

Other research indicates that these sharks give birth to pups 65 - 70cm (25 - 28") in length, in litters which can number from 20 to over 100.

Research

We collect information about each encounter that we have with sixgills. We also collect data from other charter operators who bring divers to the Hornby Island area. We cooperate with scientists conducting research on sixgills and other species. We are happy to share our information.

Diving Around Hornby Island
Sixgill Sharks
Sea Lions
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