With a family tree that goes back about 150 million years, some might call the Bluntnose sixgill shark (Hexanchus griseus) a swimming dinosaur. And since they live most of the year in water too deep for scuba divers to access, they have been shrouded in mystery.
During a brief period in the summer, Bluntnose sixgill sharks rise to waters shallow enough for divers to see. In the 80's and 90's, a good number of these sharks were seen around Hornby Island, on the east side of Vancouver Island, and right off the pier of the Seattle Aquarium. Since then, these sightings have dropped to almost zero.
An experimental fishery reduced the numbers in Barkley Sound on the West coast of Vancouver Island but, unlike other areas, Bluntnose sixgill sharks are still seen on a regular basis.
A little shark curiosity
When I took over a dive charter lodge in Barkley Sound, it was a natural fit between myself and the sharks. I’ve been diving since 1982 and was always fascinated by marine creatures. So when I had my first encounter with a 12-foot Sixgill shark in 1996, I was hooked. With my wife Kathy Johnson, we moved to in Barkley Sound and saw Sixgill sharks right in front of our lodge.
I started to read up on them, ask questions and was surprised how little was known about these sharks. Through one of our many volunteer stewardship and conservation projects I connected with Dr. Chris Harvey-Clark, one of the co-discoverers of Greenland sharks in Quebec. We quickly became close friends, and when I suggested setting up a citizens' science project to research distribution and abundance he immediately agreed to get involved. So with my company Rendezvous Dive Adventures, we set up what I call “Shark Survey Week” and we’ve been diving for these sharks ever since.
Each year during the last two weeks of August, divers come to the resort and enjoy great dives, good food, and presentations by various shark researchers. We do see sharks between July and September but the last half of August seems to be the peak.
A little shark biology
What makes the Sixgill shark so different from other better known sharks is the fact that they have six gill slits (sharks commonly only have five). Together with another species of Sixgill shark, and two Sevengill shark species, they constitute the Cow shark family, one of the oldest shark families.
The shark's reproduction method is also different, called “aplacental viviparity”. The eggs hatch inside the female but are not fed through a placenta. Once they are finished their development inside the mother, they are born live, as miniature versions of the adults.
Sixgill sharks are both scavengers and hunters, feeding on a wide variety of prey. The list includes the Spotted rat fish (Hydrolagus colliei), another deep water fish closely related to ancient sharks and can also be seen at diving depths during the summer.
A little shark exploration
On a foggy August morning, ten divers geared up for a dive on Tyler Rock, ready to explore the waters and view some amazing sharks. After spending 40 minutes underwater, they surface to tell me they have spotted a small shark, probably an immature male. They have taken pictures to log scars, gender, and length. We also log other variables - temperature, time of day, and behaviour. Seeing immature sharks is fairly common for our expedition, ranging between seven and ten feet.
DNA research suggests that sharks that are seen in the same area are often siblings. On some occasions we see larger sharks up to 14 feet - a real treat to see. Two years ago, we found a tiny two and a half foot long shark which is about the length at which they are born. Do they rise to shallower waters in the summer to give birth and find mates? No one knows for sure, but we hope our data helps uncover the answer.
Although our project is in its four official year, we have been collecting similar data for over a decade. We share the information with researchers with the Seattle Aquarium, the Department of Fisheries and Oceans, and many others. We hope this data assists many other groups in uncovering more information about these mysterious and fascinating sharks that live right in my own backyard.
To learn more about Rendezvous Diving Adventures' citizen science project, please visit their website.