The Secrets of Salish Sea’s Sixgill Sharks

Bluntnose sixgill sharks are something of an enigma, even to scientists who study them. Using data collected over more than 20 years, scientists have learned that the Salish Sea, including Puget Sound, comprises one of the world's few known pupping grounds for the species. Typically researchers see and catch only younger animals in relatively shallow, nearshore habitats. Only through unusual events, such as the stranding of female sixgills in 2007 and 2019, have researchers accessed adults for study.

Northwest Fisheries Science Center research fisheries biologist Kelly Andrews works with other scientists from the Seattle Aquarium and Washington Department of Fish and Wildlife studying sixgills in the Salish Sea. The team recently reported on the genetic analysis of 274 sixgill tissue samples collected over 16 years. It revealed that litters of pups such as those collected from the stranded females had multiple fathers. That shows that females mate with multiple males during breeding seasons, a strategy that increases genetic diversity.

They also found that sixgills collected over more than a decade were closely related. This indicates that relatives such as half-siblings, grandparents and grandchildren, and aunts and uncles return to the Salish Sea over successive years and decades. These repetitive movement patterns increase the connectivity between offshore and estuarine habitats and food webs. As top predators and their offspring migrate across ecosystems, they potentially prey on species we manage and species we are responsible for conserving. They also bring with them and transfer nutrients and contaminants across the systems. Understanding the interconnectedness of ecosystems is a key tenet of NOAA's Ecosystem-Based Fisheries Management policy that helps maintain resilient and productive ecosystems across the U.S. West Coast.

We asked Kelly Andrews what the new findings tell us about this mysterious species.

What got you started studying sixgill sharks?

Back in 2003, we didn't know anything about these sharks. Recreational fishermen started catching them off of the piers in Seattle and Tacoma. And the Washington Department of Fish and Wildlife was like, "whoa, what do we know about this species?"

We started working with the Seattle Aquarium and Washington Department of Fish and Wildlife to try to understand these top predators that were in Puget Sound. Really no one had observed them here until these fishermen started catching them off of the piers.

We used that as an opportunity to think about, from an ecosystem perspective, what types of habitats were these top predators using? And how might that influence predator-prey interactions across the Puget Sound food web?

Do we know what happened with the population that allowed people to start catching them?

The work that we did over the next decade tagging and tracking sharks gave us a working hypothesis that Puget Sound serves as a pupping and rearing ground for this species.

The way we think it works is that pregnant females come from deep offshore waters into Puget Sound and the rest of the Salish Sea, up into British Columbia and Howe Sound area. They come back, they give birth, and then those pups grow up here for several years. We don't really know how many years because no one's been able to age sixgill sharks yet. But based on growth measurements, we suspect about 4 to 6 years, though some may spend even longer growing up in Puget Sound.

And then at some point, and for some reason, they leave and head out through the Strait of Juan de Fuca and go out to deep sea where adults are generally found.

What we think happened in the late 1990s, early 2000s, was that a larger number than normal of adult females came back and pupped. And then those pups worked their way through the system and people started catching them. In those years, we could easily go out, set long lines and catch sixgills that we could then take samples from. We could tag them, track them, and characterize their movement patterns. Then around 2008 we couldn't catch any more and the aquarium couldn't attract any more at their bait stations underneath the aquarium, and they just kind of disappeared.

Since then there have been a couple of episodes in 2015-2016, and in the early 2020s, where scuba divers started to see sixgills more frequently. It's sort of a repeating pattern, but it doesn't really seem to have a clear cause and effect or driver.

Did this new research come from the samples you've been collecting this whole time?

The early samples were from work that we did with the Aquarium and Washington Department of Fish and Wildlife.

And then in the Strait of Georgia, a pregnant female washed ashore up in British Columbia in 2019. They were able to collect tissue samples from that mother and her 70 pups. Those new samples allowed the aquarium to run more genetics and combine the older samples with these newer samples. Some of the new individuals showed close relationships, like half siblings with some of the sharks we caught more than a decade ago! That tends to suggest that either the same females are returning to the Salish Sea, or at least close relatives of those sharks we sampled 15 years ago are coming back.

What does that tell us about the importance of the Salish Sea to the species as a whole?

It's definitely a pupping ground and a rearing ground for this population. It's similar to the role the Salish Sea plays for other species, like salmon. Young salmon smolt are using the estuaries of Puget Sound as early rearing habitats.

Spiny dogfish is another shark that comes from the outer coast into Puget Sound annually between June and September. They repeatedly migrate into Puget Sound for the summer and then go back out to the outer coast as well.

That all fits the idea that it's a productive system at different times of the year and maybe different years of the decades. If you think about sixgill sharks using it over this longer period, there's a reason they keep coming back.

So even though we've learned quite a bit about sixgill sharks, at least in the Salish Sea, they're still kind of an enigma?

We've also collected tissue samples and performed research on what we think they're feeding on when they are in Puget Sound. Their muscle tissues have these isotopic signatures that give us insight on what they're feeding on, whether they're scavenging off of dead animals or they're eating crab or flatfish. But there's still a lot we don't know.

From our tracking work, we found that their daily routine is to use deeper habitats during the day and then they come up into shallower waters at night. Seasonally, they tend to shift back and forth between a couple different areas in Puget Sound. And then as they transition from juveniles and sub-adults to adults, they leave (and sometimes come back to) Puget Sound. Then they finally leave Puget Sound for good, until they come back to pup.

Do we have a sense of whether they live a long time?

Yes, they live a long time, but we really don't know how long. Just a few years ago, the Greenland shark was aged and they're similar to sixgill sharks. Those Greenland sharks have been aged to hundreds of years. The oldest recorded Greenland shark was estimated to be roughly 392 years old.

The Greenland shark is in the Arctic, in really cold waters, but the sixgill shark also goes into really deep, cold waters in nearly every ocean of the world. They're encountered more than 3,000 meters deep in some places.

We don't know specifically. But if it's anything like some other large benthic shark species, they could live to be really old.

Do you keep trying to fit more pieces into the picture over time?

For me, the next pieces of unknown information are: How long is the gestation period for these sixgill females? How often do they give birth? Does the same individual come back to Puget Sound? This all leads to predicting when would we expect the next pulse of juveniles to be coming up through Puget Sound? This not only helps us understand their population dynamics, but there is also a large following of recreational divers that organize, recreate, and bond over these sharks that could be informed with this information-a win-win!