Sixgill sharks in Seattle
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The Seattle Aquarium began conducting research on sixgill sharks, Hexanchus griseus, in Puget Sound in 2002. Most Seattle area residents didn’t realize that the third largest predatory shark in the world was spending time in our local waters, sometimes even swimming right under the Seattle Aquarium! The bluntnose sixgill shark, Hexanchus griseus, which grows up to 15 feet in length, is the largest of 11 species of sharks seen in Puget Sound by divers and fishermen.
Shark conservation is an important topic around the world. Of the roughly 500 known species of sharks worldwide, the International Union for Conservation of Nature lists the status of roughly 25 percent as threatened. Local sharks and conservation came to the forefront in the Seattle area when several sixgill sharks were caught in Elliott Bay. Since little was known about these animals, state regulators responded to requests from the Seattle Aquarium and concerned citizens by placing a permanent sport fishing ban for sixgill sharks in May 2001.
A joint research team with representatives from the NOAA Fisheries Service, the University of Washington, Seattle Aquarium, Point Defiance Zoo and Aquarium, and Washington Department of Fish and Wildlife (WDFW) began studying sixgill sharks in local waters in 2002. At the Seattle Aquarium, researched involved bimonthly research events below Pier 59, where a permanent research station had been constructed on the seafloor. Using bait, lights and cameras, Aquarium divers (safe within a protected-contact cage) were able to video document, visually tag and biopsy sharks sixgill sharks attracted to the research station. Data collected was used to study the habits, biology and local abundance of sixgills.
From 2002 to 2005, this work was very successful and attracted an estimated 273 sixgills. Of that group, Aquarium divers were able to visually tag 45 individuals and take tissue biopsies from 29 individuals. In 2005, active research was suspended to accommodate the Pier 59 piling replacement project.
When work resumed in 2008, Aquarium researchers replicated their previous methods as closely as possible—with vastly different results. In the ensuing years, very few sixgills were documented on the research cameras during the bimonthly events, and none of the sixgills documented stayed at the research station long enough for divers to tag and biopsy them. Sightings became more and more rare; our research partners at NOAA and WDFW ceased their active research on sixgills in 2007 because the sharks were so scarce, and 2012 was the last year a sixgill was seen below Pier 59.
So why the relative absence of sixgills now compared to 2002–2005? The most current data available indicates we experienced a boom cycle of sixgill shark abundance locally from around 1999 to 2006. And 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 is that adult females have been documented in Puget Sound in the process of giving birth or immediately afterwards. The second is that the vast majority of the sixgills—over 300 animals—documented by the Seattle Aquarium and our partners were sub-adult juveniles, smaller than the recognized size at maturity (9 feet for males and 12 feet for females).
We also learned that these sub-adult sixgills in Puget Sound have relatively small home ranges (about 6 miles) that shift between adjacent summer and winter ranges. In addition, they’re 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 their migrations are unknown.
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.