Increasing sixgill and sevengill sightings provide new research opportunities in the Salish Sea
Researchers with the Seattle Aquarium, Washington Department of Fish and Wildlife, Oregon State University Big Fish Lab and Point Defiance Zoo & Aquarium have tagged eight bluntnose sixgill sharks this summer with a budget for 12-15 total through September.
Growing up to 14 feet if not bigger, sixgill sharks are some of the largest sharks in the world. They tend to live in deep waters, nearly two miles below the surface, which makes them difficult to study. However, the Salish Sea provides a unique opportunity — they’ve been sighted year-round in shallow waters.
When a broadnose sevengill shark was caught in 2021 in Hammersley Inlet, researchers set off to study the potentially new population in Puget Sound, seeking to learn more about their role in the local ecosystem.
“We’re looking at the connectivity,” said Lisa Hillier, a shark researcher with Fish and Wildlife. “Why those are in South Puget Sound, what they’re doing, what they’re eating. We do know that they are really targeting seals, which is great because seal populations right now are kind of at capacity.”
Gastric lavage — making the sharks puke — has revealed the seal population may be one reason for the sudden appearance of sevengills. While seals are protected by humans, sharks can help make the population sustainable in ways humans can’t.
“Sharks don’t need to have permission from NOAA or from the ESA or somebody else to eat their dinner,” Hillier said. “And if there’s a lot of prey in the area, it’s obviously having a cascading effect in the ecosystem. Sharks will help balance out that very productive seal population that’s now taking a lot of the other animals out of the system.”
According to Daniela Escontrela, a researcher with the Seattle Aquarium, sixgill sharks also cycle nutrients — divers find them at sunken whale carcasses, where the sharks then spread these nutrients to other parts of the ocean.
“We presume they’re apex predators,” she said. “We don’t actually know the exact role they play or how strong their top-down effects are.”
She describes the ecosystem as a Jenga tower: remove one block, and the whole thing could topple.
“While we don’t know what would happen with sixgills when we remove them, I think we can learn from other species that it normally isn’t good,” she said.
Seattle Aquarium’s sixgill research first began in 2003, Escontrela said. At that time, researchers put bait under the pier and deployed tags when sharks came by. Around 2015, sharks stopped showing up, and research dwindled. In recent years, increasing sightings have led researchers to presume this population has returned.
“Obviously it’s time sensitive, because we don’t know when the sharks are going to leave,” she said.
With this new wave of research, scientists are posing all kinds of questions, she said: How well do sharks ventilate, or breathe? By collecting blood samples, they can learn how long they remain at the surface.
Another question is the impacts of baiting on shark movement — how humans and sharks interact in the ecosystem. This is done by attaching acoustic tags and collecting stomach samples. The next question looks at how sharks’ diet may have changed since the last samples collected in 2007. Lastly, the aquarium is also launching a photo identification method to follow specific individuals.
Methods
Researchers nowadays use a dropline with a weight at the bottom and circle hooks to reduce the chance of foul hooking. They bring the shark onto the boat, collect measurements, blood and muscle samples as well as photos.
Acoustic tags are implanted surgically by a veterinarian when the shark’s gills are ventilated, and a damp cloth is placed over the eyes to calm them. Researchers also attach external floy tags, which have a large placard and a number for divers to be able to see and report the shark via the aquarium’s report form. Afterward, the shark is released.
“They’re really easy to work with,” Escontrela said. “I used to do this with the University of Miami, and warm water sharks can be pretty thrashy. The sixgills and sevengills, because they’re cold water sharks, they have a slower metabolism, so they calm down a lot faster than others.”
Tag batteries last about three years, she said. These tags ping frequently and must be picked up by at least three receivers to know the exact location of the shark. In about five years, researchers will switch to tags that last 10 years, she said, to study longer term migration once the sharks leave Puget Sound.
It will take a couple of years of data — assuming sharks return to the same sites each year — to get a sense of their movement patterns.
The three targeting sites for this first year are off Bainbridge Island, Redondo and Elliott Bay, Escontrela said, and they have found success catching sharks at each of the sites.
These sites were chosen for their different baiting behavior, she said. At Redondo, by the fishing wharf, divers likely bait sharks often. Compare this to Bainbridge, where it’s likely no one does. Once a month, researchers bait beneath the Seattle Aquarium as a control so they can guarantee how much is put in the water. At Elliot Bay, they deploy what’s called an adaptive monitoring package which uses sonar to pick up when something large swims by to test if they come even when there is no bait.
“We had one shark show up in June,” she said. “And not shark related, we also saw a humpback whale on it, which is cool.”
Targeting sixgills
In July, a fisherman was charged with nine counts of unlawful fishing and one gross misdemeanor after state officers say he hauled sixgill sharks out of the water and posted photos and videos of the catches on social media.
The penalty is steep, Hillier said: $1,000 per count, with additional penalties for lying to the police.
Hillier said they were likely not targeting sharks because they were assumed harmful, but simply because they are fun to catch.
“Sharks are a super important part of Puget Sound and the ecosystem around here, so we are hoping that this enforcement action really highlights how important the sharks are and that people should not be breaking the law. Specifically for sixgills, you can’t remove them from the water. The enforcement action right now is targeting those people that are intentionally damaging and harming and removing them from the water.”
—By Sam Fletcher