Near record-breaking rain last winter resulted in a bumper crop of Chinook salmon, but not of steelhead trout.
The Napa County Resource Conservation District recently released the results for its annual fish monitoring program. Researchers during the spring catch, count and release fish at a Napa River location north of the city of Napa.
California environmental regulators want to see more steelhead and salmon. Millions of public and private dollars have been spent over the past decade restoring Napa River segments and trying to reduce sediment runoff, in part to create a more fish-friendly environment.
The Napa County Resource Conservation District’s ninth annual fish count provides a clue to how salmon and steelhead are doing. The focus is on juvenile fish as they migrate from the Napa River to the ocean.
Local researchers counted 2,315 salmon smolts in 2017, compared to 580 in 2016 and none 2014 and 2015. Only the 2011 count of 7,377 was higher.
The count comes on the heels of the third-highest annual rainfall total – 45 inches – recorded at Napa State Hospital since 1892. Those rains broke a five-year drought.
Lots of water resulted in lots of Chinook salmon, though it also posed a potential threat.
“The concern was all those big rains would wash off the eggs and kill the fry,” said Jonathan Koehler, senior biologist for the Napa County Resource Conservation District.
Instead, the Chinook salmon appear to have thrived. But the steelhead count was low for the fourth consecutive year.
Researchers caught 70 steelhead trout smolts, six fry and six adults, for a total of 82. That compares to 3,105 counted in 2013.
Koehler said the effects of the five-year drought are lingering for the steelhead in a way that hasn’t happened for the salmon. He attributes this to a difference in the way the two species live.
Chinook salmon leave the river for the ocean the same year they are born, so last winter’s rain benefits showed up immediately in last spring’s high count. Steelhead can stay one to three years in the river system before leaving, so higher counts of migrating smolts may come in future springs.
“It’s kind of a delayed effect, compared with the Chinook,” Koehler said.
An unknown for next year’s count is how the recent Napa County wildfires will affect the salmon and steelhead populations. Wildfires burned vegetation along slopes, leading to an increased risk of mudslides that can wash fish-harming sediments into waterways.
Koehler said the potential exists for ash and more sediment to end up in local creeks that feed the Napa River. The fires burned around sections of Milliken, Redwood and Dry creeks.
The spring 2017 count yielded about 93 percent native fish, a figure Koehler said reflects a healthy ecosystem. In contrast, non-native fish dominate in the Sacramento-San Joaquin Delta.
Much of the Napa River turns into a raging torrent in the winter and the non-native fish don’t like those high flows, Koehler said. He sees that as the most plausible explanation of why the river in its Napa Valley stretch has largely escaped invasive species.
“I think it’s just the nature of the Napa River,” Koehler said. “It’s certainly modified. It’s not a pristine river by any stretch. But it’s still wild enough that the natives do well compared to the non-native.”
The annual fish count is done with a rotary screw trap. This is an eight-foot-diameter metal funnel that is floated half-submerged on the Napa River north of Trancas Street. It turns with the current and guides fish into a water-filled compartment trap to await counting.