Photo/Illutration Chum salmon, also known as white salmon (Provided by Masahide Kaeriyama)

Chum salmon, a migratory species widely consumed in Japan, is losing out to a relative in a battle over food resources in the Northern Pacific Ocean triggered by global warming, a study found.

The fish, which are born in Japan, migrate through the Sea of Okhotsk, the Bering Sea and the Gulf of Alaska before returning to spawn in Japanese rivers after three to five years.

In recent years, only 20 million to 30 million have come back each year compared with about 70 million around 2005.

Masahide Kaeriyama, an expert in salmon ecology and professor emeritus at Hokkaido University, and fellow researchers analyzed stocks, catches and other data about the world’s three major salmon species: chum salmon, pink salmon and sockeye salmon.

They found that pink salmon have migrated in large numbers to the Bering Sea in the northern part of the Northern Pacific Ocean in recent years, creating intense competition for nutritious plankton that chum salmon feed on.

The researchers said climate change has raised temperatures of coastal waters in high-latitude Northern Pacific regions since the 2010s.

While those regions used to be too cold for juvenile pink salmon to thrive, the warmer environment has allowed them to survive in greater numbers, the researchers said.

The research team believes that the increase in pink salmon numbers in high-latitude Northern Pacific regions has led to the shrinking chum salmon population in Japan.

“Rising seawater temperatures have transformed the overall ecosystem of the Bering Sea,” Kaeriyama said. “We’ve been surprised to find that the impact on Japanese salmon has been greater than expected.”

The team’s findings will be carried in a report published by the North Pacific Anadromous Fish Commission, an international organization.

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