A fish mystery solved using genetic testing

ANCHORAGE (KTUU) - The population of cod in the Northern Bering Sea has increased immensely since 2010, and scientists are using fish DNA to find out why.

(NOAA)

Think of it like a genetic ancestry test, but for fish.

Until recently, pacific cod were rarely found in the Northern Bering Sea. A 2010 survey showed cod made up only three percent of the entire fish population. That's been changing, fast.

A survey in the summer of 2017 showed that number shot up 900 percent.

Ingrid Spies is a research fisheries biologist who led the way on this research to determine whether the population spike is evidence of a growing population or of an existing population migrating from elsewhere?

One thought was that cod could have migrated from Russia or the Gulf of Alaska, where they observed cod numbers decline significantly in 2017. Scientists were able to come to a conclusive answer to the question using genetic testing.

"We were lucky to have baseline genetic data on spawning populations throughout the whole range of pacific cod in the Bering Sea, Gulf of Alaska, and the Aleutian Islands," said Spies. "We could take the sample from the Northern Bering Sea that we collected in 2017 and compare it to those baselines."

What they found proved to be surprising for Spies. It turns out the population was not growing, but had moved from the Southeastern Bering Sea, in line with a survey that shows a 37% decline in cod population in the same area.

"I expected that these fish in the Northern Bering Sea would have been from somewhere else. Maybe from the Gulf of Alaska, maybe distinct stock that had recently expanded. I didn't expect them to be from the Eastern Bering Sea."

Which led researchers to the next question -- why?

"We're not used to seeing these big shifts in fish distribution, and our first thought was that it could be lack of a cold pool, lack of ice in the Bering Sea," said Spies. "The cold pool is a body of water that is less than two degrees Celsius that is left behind after the ice melts in the Bering Sea."

Spies explains the cold pool used to serve as a natural barrier between the Northern and Southern Bering Sea, but has greatly diminished in recent years, leading to the cod migration.

This discovery isn't just a novelty of migration, it provides critical data when it comes to sustainable fishery management.

"We have to rethink the sources of data we have for doing our assessments," said Spies, "In the past, we could do research surveys from the Eastern Bering Sea alone. Now we realize we need to extend our surveys farther north to find the fish."

Cod are not the only species that are seeing changes. NOAA scientists will now shift their focus to other species like walleye pollock and yellowfin sole that are also showing movement in the Bering Sea.

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