In the 1960s, king salmon were abundant in Alaska, and it stayed that way through the 90s. After the new millennium, though, Chinook numbers fell — and they've remained low since.
"People have scratched their heads and said, 'Where are all the kings? What happened to all the kings?'" said Andy Seitz, an associate professor at the University of Alaska Fairbanks School of Fisheries and Ocean Sciences.
At a lecture in Unalaska this week, Seitz explained how his research team has studied adult Chinook in the Bering Sea for the last three years. The project relied on pop-up satellite tags, which attach to salmon and measure the water temperature, depth, and ambient light of their environment.
Seitz has collected data from 17 tags and discovered a pattern: Chinook usually spend nights in the warm, shallow waters of the Bering Sea and then dive to deep, cold waters during the day.
But looking at the data more closely, Seitz said he noticed something strange: seven instances where king salmon recorded high temperatures in deep water — water that should be cold.
"Where is it 70 to 75 degrees Fahrenheit yet diving to 50 meters deep? It's a tropical little oasis in the Bering Sea, and it's in the stomach of this thing. This is a salmon shark," he said.
Seitz and his team think warm-blooded salmon sharks ate the kings and their tags, and the odd data was recorded when fish were trapped in the sharks' guts. He also said they found five instances where marine mammals and other unidentified predators could have killed Chinook.
So while he can't be certain, Seitz said his team may have found another possible explanation for the decline of king salmon. It may not be the result of an evolving species or an evolving climate, but a previously unknown source of marine mortality that should be investigated further.
"The take-home message from this is not: Salmon sharks and marine mammals are causing the decline of king salmon," said Seitz. "We're not saying that."
"But it was previously thought that salmon sharks hang out in the Bering Sea in the summertime and then skedaddle out of there because of cold water in the winter. But most of these salmon shark mortality events occur smack-dab in the middle of the winter," he said. "So this makes us rethink a few assumptions about king salmon and their causes of death."
Seitz and his team are now looking for funding to continue their project, which — at $4,000 per salmon tag — has already cost more than $375,000.
The original purpose of the study was to see if pop-off tags are good tools for studying Chinook. And now that they've collected some compelling data, Seitz said they're hoping to increase their sample size — and keep studying the lives and deaths of king salmon in the Bering Sea.