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Wildlife Detectives

Sixgill Sharks: Puget Sound's Open Secret

A discussion with biologists from Seattle Aquarium

June 16, 2016
This blog post was created by KCTS 9 marketing and communications intern Xavier G. in collaboration with KCTS 9 staff.
A living mystery is present in our own backyard, yet many are completely unware of it. Typically found at several thousand feet beneath the ocean's surface, the sixgill shark thrives in the dark, quiet depths. However, the shark’s discovery several years ago in waters as shallow as just 20 feet in the Puget Sound has given researchers an extraordinary opportunity to learn more about these elusive creatures. What does the appearance of one of the world’s largest sharks in the Puget Sound say about our waters?  Why has the number of sharks in the area diminished over the past few years? 
On October 26 at 8:00 p.m., KCTS 9 will encore the Wildlife Detectives documentary, Mystery Sharks of Seattle, which explores these questions. We recently caught up with biologists from the Seattle Aquarium to learn more about the research that became the basis for this film.
Mystery Sharks of Seattle takes a look at the sixgill shark and its peculiar presence in (and disappearance from) the Puget Sound.  Why is the Puget Sound such an odd home for these creatures?  
Seattle Aquarium: We don’t know if the Puget Sound is an odd home for sixgill sharks. The Salish Sea (Puget Sound and the Strait of Georgia/inland waterways of British Columbia) just happens to be one of the only inland fjord waterways where large numbers of subadult sixgills have been documented and studied thus far. There may be other sixgill nursery areas throughout the world — likely there are — but they just have not been identified yet.
How did the aquarium researchers first become aware of sixgill sharks in the area?
Seattle Aquarium: While the presence of the sixgill shark in local waters was not widely known to the general public, the existence of sixgill sharks has been an open secret among Puget Sound divers, recreational and commercial fishermen, scientists and naturalists. The aquarium has collected reports of sporadic encounters between divers and sharks since the mid-1980s. In the late 1990s to early 2000s, an increase in the frequency of these encounters to a level where they were consistent and in larger numbers prompted the aquarium to begin sixgill research in our local waters.
What kind of research is the Seattle Aquarium doing on the sixgill shark? What has the research discovered so far?
Seattle Aquarium: The Seattle Aquarium has been conducting research on sixgill sharks, Hexanchus griseus, in Puget Sound since 2002. The research involved every-other-month research events where we placed bait, lights, cameras and divers (safe within a protected contact cage) underneath Pier 59 to video-document, visually tag (movement and abundance analyses) and biopsy sharks (genetic analyses) attracted to the research station.
From 2002 to 2005, this work was very successful and attracted an estimated 273 sixgills! Of that group, we were able to visually tag 45 individuals and take tissue biopsies from 29 individuals. This research generated three scientific papers with two more in preparation, as well as one book.
In 2005 we had to suspend this bimonthly research because of the Pier 59 piling replacement project. Once the piling work was completed and we rebuilt our protected-contact shark research cage, we restarted the every-other-month research events in 2008. We replicated our methods as closely as we could but the results were vastly different. Since 2008 we have documented very few sixgills on the research cameras during events (range 0-3) and none of the sixgills documented stayed for any period of time at the research station to feed, such that divers could get close enough to tag and biopsy. To this date the observations have become more and more rare, with no sixgills seen under pier 59 since 2012. We aren't the only ones who have noticed this scarcity of sixgills. Our research partners, the National Oceanic and Atmospheric Administration (NOAA) and the Washington Department of Fish and Wildlife (WDFW), ceased their active research on sixgills in 2007 because the sharks were so scarce.
So why the relative absence of sixgills now compared to 2002 to 2005? The most current data available indicate we experienced a boom cycle of sixgill shark abundance locally from around 1999 to 2006. So where did they go?  Results from the work done by our partners with acoustic tags suggest that between 2006 and 2008, all of the acoustically tagged sixgills left Puget Sound and have not returned. 
Some conclusions that we can make from the research we and our partners conducted is that sixgill sharks may utilize Puget Sound as a pupping and nursery habitat. We believe this for two reasons.  The first reason is that adult females have been documented in Puget Sound in the process of giving birth or immediately afterwards. The second reason is that the vast majority of the sixgills documented by ourselves and our partners (over 300 animals) were subadult juveniles — smaller than recognized size at maturity (9 feet for males and 12 feet for females).
We also learned that these subadult sixgills in Puget Sound have relatively small home ranges (about 10 km) that shift between adjacent summer and winter ranges. In addition, we learned that these subadult sixgills are often found in groups made up primarily of related individuals — full or half siblings. These groups of sixgills comprised of related animals may remain together in these relatively small home ranges until they reach a size at which they begin to migrate into their adult habitat of the open ocean. The processes that drive the animals' movements while in Puget Sound and the triggers that stimulate outmigrations are unknown.
The aquarium has monitored sixgill shark presence in the region to act as gatekeepers for the study of this species since 2002. It has been over a decade since we saw sixgills in numbers great enough to study, thus we are suspending our active sixgill research under pier 59. Sharks have been designated by the Association of Zoos and Aquariums (AZA) as a Signature Species in their Saving Animals From Extinction (SAFE) program,  and we remain active in the shark community as follows: membership on the International Union for Conservation of Nature (IUCN) Northeast Pacific Shark Specialist Group that recommends the conservation status of shark species for the IUCN; leading efforts to write a book on shark conservation published by Elsevier Scientific; organizing and sponsoring biennial shark symposiums and conservation workshops; accepting samples for genetic analysis from our partners the International Pacific Halibut Commission (IPHC) and others; giving talks about sixgills to dive clubs and other interested parties; participating actively in AZA Marine Fish Taxon Advisory Group discussions and recommendations regarding shark populations worldwide; and monitoring diver sightings of sixgills in Puget Sound. We are committed to staying active in shark conservation and the shark community in Puget Sound so that we can be ready to restart active research when sixgills are back locally in greater numbers.
What do these findings say about our waters? Do you expect to see sixgills return to the Puget Sound in similar numbers as before?
Seattle Aquarium: These findings can only suggest that Puget Sound can support relatively large numbers of apex predators such as sixgill sharks for relatively long periods of time (six to seven years), which means the ecosystem has enough food, or biomass, to support them — which is good news. We have no idea when they will be back in the numbers that we saw in the early 2000s, as we do not know enough about their life history to predict the next boom cycle of sixgills. There still are sixgills in Puget Sound today, but in relatively low numbers when compared to the early 2000s. Local divers and fishermen are encouraged to be a part of the research effort by reporting any sixgill sighting to the aquarium using the online reporting form.
Sixgill sharks have a reputation for being tamer than other shark species.  Is this true? How were the researcher’s interactions with these sharks?
Seattle Aquarium: Sixgill sharks behave as most sharks do — they are non-aggressive towards divers unless provoked. There are approximately 500 known species of sharks in the world; about 25% of those are listed as near-threatened or wose. Sharks kill an average of four humans per year. Humans kill well over 100 million sharks each year. Sharks are not the aggressor in this relationship.
Focusing on the shark itself, what aspects and features make the sixgill distinct from many other common and popular sharks?
Seattle Aquarium: Bluntnose sixgill sharks, or Hexanchus griseus, are members of the cow shark family. Most sharks have five gill slits, but the aptly named sixgills have six. Sixgill sharks represent a modern manifestation of what could be ancient animals. Sharks very similar to the sixgill are thought to have been present on earth as far back as the Jurassic era. An ecological implication is that these sharks could have occupied a unique niche at the ocean bottom that may have been present as a stable habitat since that era. Their continued success shows the potential that no more modern sharks have any selective advantage that would make them more successful in this habitat.
The Puget Sound boasts very diverse sea life. Besides the sixgill, what are some other species that you can find beneath the surface that may surprise people?
Seattle Aquarium: Puget Sound is home to many other elasmobranchs (shark-like fishes that have no bones but rather cartilage skeletons) such as the normally deep dwelling ratfish, a type of chimera; longnose skates and big skates, which are a type of ray; and spiny dogfish, which are sharks that are four to five feet long. The Puget Sound is also home to the largest octopus in the world — the giant Pacific octopus — as well as the most well-studied Orca population in the world, the iconic southern resident killer whales (SRKW).
You can learn more about sixgill sharks on the Seattle Aquarium’s website. Be sure to tune in for Wildlife Detective’s documentary, Mystery Sharks of Seattle, which encores Wednesday, October 26, at 8:00 pm on KCTS 9.
Read our conversation with Wildlife Detetcives: Mystery Sharks of Seattle filmmaker Michael Werner.
Images courtesy of Seattle Aquarium. 



KCTS 9 staff consists of experienced journalists and videographers, producing local stories on the issues that shape the greater Seattle area. More stories by KCTS 9