The Mystery of the Missing DTAG: Biologists Narrow Search for Tag as Time Runs Short

November 10, 2020

Scientists climb mountains and ride ferries to pick up radio signals from the lost tag.

Southern Resident Connections - Post 13, Part 2 of 2

Sheila Thornton holding VHF receiver Researcher Sheila Thornton of Fisheries and Oceans Canada searches for VHF signals from the missing tag in thick wildfire smoke in September. She later learned that the tag had floated past the same location near the Vancouver airport about 12 hours before. Photo credit: Fisheries and Oceans Canada.

The missing tag eluded scientists like a diving killer whale. Research scientist Sheila Thornton of Fisheries and Oceans Canada learned that she must have been within a stone’s throw of the tag. When she trained her tracking antenna on the water from a breakwater near the Vancouver airport, she saw it had passed by just 12 hours prior.

Fisheries and Oceans Canada has largely funded the international research on the overnight behavior of killer whales. A Canadian team studies animals from Canada’s Northern Resident killer whales, an orca population that has done well in recent years. Meanwhile, U.S. biologists tag endangered Southern Residents, which in contrast, have struggled and declined.

The collaborative research seeks to document killer whale behavior for a full 24 hours. This helps researchers understand how and when the whales expend energy, and when they are feeding. That could help reveal when additional protections could most benefit the whales.

Comparing the two populations may be revealing, but comes with risks, said Thornton, who leads killer whale research at DFO.

“Putting tags out overnight means exactly this—you run the risk of losing the tag,” she said. Each tag costs many thousands of dollars, she said, but also represents many hours of planning and research that together have much greater value.

The team that affixed the tag to K37 called it a “good stick” because the suction cups attached quickly and cleanly. It provided the best opportunity of the field season to document the overnight activity of a Southern Resident, storing its data for later downloading. So they had to get it back.

U.S. scientists had permission to operate on the water and track the killer whales in Canada. They did not have permission to land in Canada because of border closures related to the COVID-19 pandemic. So they could not climb to a higher elevation to search for the tag’s VHF radio signal over a greater range.

At sea level, VHF signals may travel no more than a kilometer or two.

Image
Killer whale at sea with visible DTAG
The suction cups of the DTAG held it onto Southern Resident killer whale K37 in September. An accelerometer in the tag tracks movement of the whale, and hydrophones record sounds of the whales and the ocean. Photo credit: Candice Emmons/NOAA Fisheries

Hiking, Climbing, and Ferry-Riding

They had plenty of help from their counterparts in British Columbia, however.

“Even a small bit of elevation helps,” Thornton said. “For every 10 meters higher in elevation, you can dramatically increase your range.”

She estimated that over the 6 days of the search, she hiked at least 30 kilometers and drove more than 800 kilometers. She needed to reach higher elevations where she could detect the tag’s radio signal at a greater distance. She sent one team member on a gondola 2,800 feet, or 850 meters, up Grouse Mountain on the north side of Vancouver. She also dispatched members of her team to ride BC Ferries with antennas that look like they are from Star Trek. On most days, the wildfire smoke was too dense for their own tracking boats.

“Sometimes, you feel like you’re chasing your tail,” Thornton said. “It was a real concerted effort from both sides of the border.”

At one point, the radio signal vanished for almost 2 days. Scientists suspect the tag’s antennas may have become tangled in kelp. All the while, the tag kept moving. They worried it would become stranded on vast mudflats near the mouth of the Fraser River, which would have been nearly impossible to search.

“Like all of 2020, this tag kept throwing us curveballs,” said research biologist Marla Holt of NOAA Fisheries’ Northwest Fisheries Science Center.

Illustration of the missing DTAG search timeline of events

Narrowing the Possibilities

Each new detection of the VHF signals narrowed the approximate swath of the Salish Sea where the tag was floating. Scientists would later find that the tag spent most of the week in Canada. Then currents carried it south and west back into U.S. waters and toward the open ocean.

On the morning of September 18, seven days after affixing the tag to an endangered killer whale, the team headed north from Friday Harbor. They had little to go on beyond some distant signals heard in Canada. Sometimes the beeps get scrambled, a sign that the signal may be bouncing off a nearby hill or cliff.

“Radio tracking is one of those things that is one part art and one part science,” said biologist Brad Hanson.

Just before the vessel hit a fog bank off the east side of Waldron Island, Hanson switched on the VHF receiver. Everyone heard the beep. The tag was close. The tones from the receiver got stronger, and the “cone of detection,” which is the swath of ocean where the signal is coming from, narrowed.

The three scientists on the boat watched for the foot-long black antenna rising from the water as they passed tidal collections of flotsam, kelp, and other debris.

“We were all looking,” said Candice Emmons, a NOAA Fisheries research scientist who was steering the boat. “You can go past it in a second.”

Researcher Jeff Hogan first spotted the boxy shape 60 to 80 yards away. He pulled the tag from the water with the same net biologists use to collect prey and fecal samples. The team quickly swabbed the suction cups for traces of killer whale skin. Microbiologists at the science center will examine the samples for microbes that may affect the health of the whales.

“We were fortunate that everyone had the right equipment in the right place to pick it up,” said Greg Schorr of Marine Ecology and Telemetry Research, who assisted in the search.

Holt determined from the data on the tag that it came off only an hour early, meaning that it had recorded precious overnight data while affixed to K37. It was as valuable as she had hoped.

“I think we would all like to know why it came off,” she said.

U.S. and Canadian scientists both stressed that they would never have found the tag without the help from their partners across the border. “We talk a lot about collaboration,” Holt said. “In this case, it was essential to the research, and it saved our data.”


Southern Resident Connections

Southern Resident Connections

Southern Resident killer whales are icons of a vibrant but struggling marine ecosystem that is important to us all. Join us in exploring the ecological connections that tie this system together, and the ways we are protecting and working to recover the whales we all care so much about.

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Last updated by West Coast Regional Office on November 13, 2020