TESLIN, Yukon Territory — Ask scientists about the decline of the Yukon River’s chinook salmon run, and they’ll tell you they know one thing for sure — it’s for real.

After that, it gets harder.

“There’s all sorts of different possibilities and probably not one single reason why the fish decline or rebound,” said Peter Hagen from Juneau, Alaska, where he works with the National Oceanic and Atmospheric Administration, which conducts much of the U.S. fisheries research.

Chinook salmon lead a long, complicated life.

Hatched in small, gravel-bedded streams in river headwaters, chinooks remain in freshwater for up to 18 months before heading downstream. After a few months near river mouths, they head into the deep, cold waters of the Bering Sea, where they make their living for up to eight precarious years before beginning their epic migration back to the creeks where they began.

That’s a long time and a lot of environments. Research hasn’t yet teased out when and where the problem lies.

Two common culprits — industrial development and increased fishing — haven’t been factors along the river, so many scientists believe the problem is in the ocean.

Climate change may be playing a big role, said Hagen. While the temperature of the Bering Sea has been up and down over the last decades, sea ice has generally been melting earlier in the year.

Life in the Arctic is crucially dependent on timing, and that earlier ice retreat is throwing off the bloom of algae and plankton that drive the whole ecosystem.

“A big driving factor is ice retreat,” Hagen said.

Plankton blooms need both daylight and sea ice, which the tiny plants use to anchor themselves. If the sea ice is gone by the time the sun is strong, the bloom is delayed and weakened.

“Where the ice is already gone and the sun’s shining down, the bloom takes longer to happen and in some cases doesn’t seem to provide the big energy boost that propagates through the system.”

Others suggest that new species migrating into the area as the climate changes may introduce new competition or even new predators.

Hagen said climate change has impacts on land, too.

“Things are changing. It’s a combination of factors, marine and freshwater.”

The commercial fishery in the Bering Sea has also had an impact on salmon. Although restrictions are now in place, as recently as 2007 there were 130,000 chinooks lost as bycatch in Alaska’s huge pollock fishery.

There are signs of hope.

This year’s run is slightly larger than last year’s. As well, Hagen said, surveys are showing significantly more juveniles swimming around in the ocean. That raises hope that more of them will make it back upstream.

“We think there will be an uppick,” he said. “But it’s going to be a tough one in the long run.”