The quietly essential salmon stream

The quietly essential salmon stream

UPPER CHENA RIVER - On this rainy September afternoon, Erik Schoen vacuums water from a backwater slough. The clear flow runs through a plastic tube and into a jar. The liquid will tell him if toothy finned predators swim in this body of water.

Earlier this summer, more than 50 juvenile Chinook (king) salmon hovered in front of the road culvert that leads from this dark water to the Chena River. Sticks and leaves had plugged the culvert, allowing only water to pass. Those obstacles seemed to be frustrating the pinkie-size salmon that had hatched from spherical eggs earlier in the summer.

"I wonder how big they would get if they could swim up into that slough," said Schoen, a biologist with the International Arctic Research Center of the University of Alaska Fairbanks. "Or if they would get eaten."

Schoen is here with graduate students Raven Dawson and Kristen Reece to study intriguing nooks of river habitat like this. They want to see how they might enhance life here for the king of fish, once abundant and now not so much.

The Chena River is a 100-mile waterway that winds through Fairbanks, a town of about 100,000 people. The smooth water flowing beneath town bridges is one of the richest waterways for king salmon along the entire drainage of the Yukon River.

According to a study by biologist Randy Brown of the U.S. Fish and Wildlife Service, the Chena is a close second in production of Yukon River kings along its entire thousand-mile-plus length. The number one king nursery of the Yukon is the Salcha River, just 35 miles southeast of Fairbanks.

These two rivers - one of them urban by Alaska standards - have pumped out billions of salmon. Those fish are born in cobbles of the upper rivers, spend their first year in freshwater, and then blast out to the ocean via the Tanana and then Yukon rivers.

Once in the Bering Sea and the Gulf of Alaska, they gain 99 percent of their peak body weight by feeding on smaller things like crab larvae and krill. The fish swim there for a few years, then somehow know it is time - at about age 5 - to return to their birth streams to spawn.

This pattern has repeated for millennia, but it is stalling.

Though many Alaskans thought it would never happen as it has elsewhere in North America, both king and chum salmon numbers have crashed in the Yukon and Kuskokwim river drainages. In 2022, king salmon returned to the Yukon in record low numbers, followed by poor returns the next few years. Chum salmon, even more abundant than the kings, followed suit.

In April 2024, both Alaska and Canada government leaders agreed a seven-year moratorium on subsistence fishing for Yukon River salmon.

How did such abundant creatures drop in numbers so fast? The answer is complicated. It includes warmer ocean temperatures that produce less salmon food, as well as competition from hatchery-born salmon.

There are also freshwater factors, including disease or excessive autumn rainfall that speeds up rivers enough to nudge eggs from their gravel nooks.

On that September day on the upper Chena, Schoen was using an instrument that helped him measure environmental DNA in the slough. All living creatures shed molecules that the "eDNA" tool can identify.

Schoen hopes to be able to detect the presence or absence of northern pike in the old river channel interrupted by the construction of a road that leads from Fairbanks to Chena Hot Springs. If pike are thick within the slough, they might eat most of the young salmon if they were to somehow make it past the clogged culvert.

Salmon-predator overlap is one of many questions the researchers are seeking to answer. The goal of the team - led by Schoen and master's students Dawson and Reece - is to find out the most effective steps to restoring salmon habitat in the Yukon River Basin, which includes the upper Chena River.

"What's the biggest bang for our buck - replacing certain culverts?" Schoen said. "We want to take everything we learn and figure out the most effective actions we can take to help salmon recover. There's a lot we can fix."

Since the late 1970s, the University of Alaska Fairbanks Geophysical Institute has provided this column free in cooperation with the UAF research community. Ned Rozell [email protected] is a science writer for the Geophysical Institute.