OSU researchers say fish can adapt to inflated water levels

In this winter of high water, Northwest fishing guides have been grounded, bait and tackle shops are losing customers, and steelhead reels and lures have grown dusty as chocolate-brown rivers have severely limited the number of fishable days.

But don’t worry about the fish, says Oregon State University ecologist Stan Gregory.

“Steelhead and other native fish have had thousands of years to adapt to flooding,” said Gregory, a professor of fisheries and wildlife in OSU’s College of Agricultural Sciences. “Not only can they survive, floods usually improve the habitat by scouring out river bottoms, creating new pools and cleaning out the silt.”

Right after the 1996 flood, Gregory and his colleagues surveyed cutthroat trout in Mack Creek in the H.J. Andrews Experimental Forest, where they had been conducting studies of trout populations since 1973. Despite living in a creek that was three feet over flood stage, the researchers found 30 percent of the fish stayed in the same 150-yard sections where they were found the year before — surviving in a stream that had been a whitewater torrent during the flood.

And those fish, he added, were only four to nine inches long.

“Migrating steelhead, which are much larger and stronger, should have no problem navigating most rivers, even at or above flood stage,” Gregory said. “But that doesn’t mean there aren’t challenges.“

One challenge, he points out, is that human activity has made it more difficult for fish to find refuges and holding areas. Historic natural river systems were braided and complex, with multiple channels that spread the impact of the flooding, slowed down currents and created holding places for migrating and resident fish. Now dams, development and the transition from forests to pastures and housing tracts have eliminated that complexity from many river systems.

In essence, Gregory said, human intervention has turned many rivers into single channels that are more like pipes than rivers. And though most adult and juvenile fish can survive those conditions, newly deposited eggs or young fry can be washed downriver — creating a problem that fisheries managers have begun to address, he added.

“Historically, steelhead would come back to spawn in the winter over three, four and even five months, depending on the stream,” Gregory pointed out. “They would arrive around Thanksgiving and continue through March.“

But early hatcheries took their eggs from first-arriving fish to ensure their supply, Gregory said, and fish runs began arriving early. The window for returning fish shrank to a couple of months.

“Then the impact of a single flood on the population could be much greater. Thankfully, most hatcheries today are using eggs from native fish that arrive at different times in the run.”

Flooding can help egg survival over the long haul, however, by cleaning out the fine particles from gravel and washing silt downriver, Gregory said. Clean spawning gravel allows for better flow of water over the eggs and supplies oxygen to eggs deposited in gravel redds or spawning depressions.

The short-term impact of flooding on steelhead numbers is hard to gauge, he says, because the juvenile fish live in freshwater streams for one to three years and then go out to the ocean where most spend one or two years. A flood can affect a part of the overall steelhead population, while other individuals are in the ocean phase of their life.

But in OSU studies of trout that live in streams year-round, the number of juveniles rises dramatically in the year after a flood.

“In Mack Creek, there were tons of fry in the stream the year after the 1996 flood,” Gregory said. “They were four times as abundant as anything we’d ever seen before.”

Conversely, an experiment on Berry Creek north of OSU’s McDonald-Dunn Forest created a bypass channel for fish around the small dam for more than 20 years — and the creek eventually became clogged with mud and silt. “Native fish like to lay eggs in gravel, not mud,” Gregory said.