The Mystery of the Missing DTAG: Biologists Search for Lost Tag with Vital Killer Whale Data

November 04, 2020

International teams race against time to save overnight record of killer whale behavior.

Southern Resident Connections - Post 12, Part 1 of 2

Biologists attaching a tag to a killer whale at sea Biologists prepare to attach a tag with suction cups to Southern Resident killer whale K37 in September. The tag tracks the whale's movements and depth, and records sounds of the whales and the ocean. Photo credit: Candice Emmons/NOAA Fisheries

Biologists used to studying endangered Southern Resident killer whales spent almost a week in September on a whole different kind of effort. They were searching for a sophisticated tag with a rare overnight record of a whale’s behavior and acoustic world.

Researchers climbed mountains in Washington’s San Juan Islands and southern British Columbia in search of distant radio signals. Others took their tracking antennas aboard ferries because they were the only vessels operating in the dense wildfire smoke blanketing the Northwest.

All while the tag’s battery life ticked away.

The whales were feeding off the southwest side of San Juan Island when a team from NOAA Fisheries’ Northwest Fisheries Science Center attached the tag. It was a skeleton crew of the minimum three people to reduce the risk of coronavirus. They used a long pole to attach the tag with suction cups to the flank of Southern Resident killer whale K37. It was the afternoon of September 11.

Just tagging the whale was, in itself, a major accomplishment. The biologists need to reach a whale in suitable ocean conditions, without too much other boat traffic and with space to maneuver.

They had set the tag to fall off at about 9 a.m. the next morning. A timer would send an electrical signal that corrodes a nickel-chromium wire and releases the suction cups. Then the tag would float to the surface so that its antennas could transmit its location.

Biologists knew that the whales would likely head toward the mouth of the Fraser River in British Columbia. The Fraser is the largest single source of Chinook salmon in the Salish Sea.

Usually, the crew would have two research boats: one for tagging, the other to recover tags that fall off. Pandemic restrictions held them to one. The boat headed toward the Fraser River the following morning. The scientists had essential help from other agencies, including the State Department. The Canada Border Services Agency helped clear the U.S. research vessels to work in Canada.

When they found K37, though, the tag was already gone—it had come off somewhere in the international waters of the Salish Sea. The sea was shrouded in wildfire smoke so thick that researchers could hardly see the bow of their own boat.

Thus began an unprecedented cross-border search for the digital acoustic recording tag, or DTAG, which is about the size of a cell phone. The tags often fall off early, so if this one had stayed on overnight, it could hold an extraordinary record. Transcribed and translated, it could inform protections for the endangered population of killer whales.

“We all were aware that it could have some of the most valuable data we’d get this year,” said Marla Holt, a NOAA Fisheries research biologist leading the study for the Science Center.

Battery Time Running Out

The tag held complex instrumentation: Two hydrophones recorded the sounds of the ocean and whales, including their echolocation while feeding; a three-dimensional accelerometer tracked movement; and a pressure and temperature sensor determined depth. A “salt switch” activated the tag when it hit the water on a whale by detecting the salinity.

The battery powering the tag’s satellite signals would last only about 48 hours. Its weaker radio signals should last longer, but even those would cease in little more than a week. Then the tag would disappear like a scrap of wood in the ocean.

“You take an already challenging situation, and you add smoke and the pandemic, and we had to really work together to find it,” said research biologist Brad Hanson.

Illustration of the missing DTAG search timeline of events

Almost 24 hours after the tag disappeared, Holt picked up one of its satellite signals. It put the tag near the Tsawwassen Ferry Terminal on the south side of Vancouver, British Columbia. The catch was that the timestamp of location data was already several hours old. Also, the location came with a large margin of spatial error.

Another expert, Greg Schorr of Marine Ecology and Telemetry Research, headed north from Hood Canal by boat in thick fog and smoke. He brought a special receiver called a goniometer that could pick up the tag’s satellite signals without a satellite. When he could not detect a signal, the team called their colleagues north of the border.

“It was literally like finding a needle in a haystack when you consider how many things were stacked against us,” said Sheila Thornton, a research scientist who leads orca studies at Fisheries and Oceans Canada and collaborates with Holt on the DTAG project. “We did not have much to go on.”

Part 2: Biologists close in on missing tag as radio signal goes dark


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 10, 2020