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 g

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