This eight-foot-diameter metal funnel floats half-submerged and rotates as currents hit the inner baffles. Fish swim inside and end up trapped in a water-filled compartment.

“It’s sort of like the revolving door at a department store, where you step into it and you have to step inside the store,” Koehler said.

It almost seems unsporting, this device called a rotary screw trap that floats on a narrow section of river about a mile north of Trancas Street. But then, this is in the name of science, not a fish dinner, and the fish stay alive.

Koehler is senior biologist for the Napa County Resource Conservation District. One of his springtime chores is morning trips to clear out the trap compartment using a net.

He’s found baby Chinook salmon aplenty this spring. And, if that seems logical following the super wet winter – fish like water, right? – it’s not necessarily so. Drought may be bad, but too much water coursing down the river during a monster storm can be bad, too.

“Eggs are in the gravel,” Koehler said. “The force of the water just physically displaces them, gets them out in the water column, and they get crushed or eaten by predators or die in some other way.”

But Koehler and other trap-checkers have counted about 2,000 young salmon this year. The fish are released to continue their journey to the Pacific Ocean, from whence some will hopefully return to the Napa River as adults in few years to spawn.

The district has been counting fish in spring using this floating fish trap for nine years and spring 2017 is proving to be a bumper year for salmon.

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