For Coho salmon, study backs cheaper, faster habitat structures

Imperiled salmon populations badly need more habitat in many wood-depleted Pacific Northwest streams. A new study throws scientific weight behind an emerging method to build habitat more quickly and cheaply than traditional projects.

Compared with the traditionally hardware- and engineering-intensive methods of building woody habitat, this “accelerated recruitment” approach can be done at less than a quarter of the cost and more closely mimics natural forest processes.

It could serve as an important tool for restoration managers looking to buy time for endangered Coho salmon while waiting for protected but still relatively young redwood and Douglas fir forests to start dropping trees on their own.

“The idea is not that we would be doing these kinds of these projects forever, but it’s something that we need to do in the short term to get the fish over the hump,” said Jennifer Carah, study author and an ecologist with The Nature Conservancy’s Water Program in California.

Logging and other wood removal practices have cut the density of downed trees in Pacific Coastal streams by an average 80 to 90 percent. That’s bad news for salmon, which rely on in-stream trees to trap sediment, create pools and serve as protective cover.

For decades, people have tried to make up for the lack of natural wood through engineered projects that anchor wooden structures into streams with rebar, cable and boulders. That helps make sure the wood isn’t washed away, wasting a commodity and threatening downstream property and structures.

An example of an “accelerated recruitment” project (Credit: Jonathan Warmerdam)

But designing the structures and anchoring those logs takes time, and Carah said study co-author and Campbell Timberland Management biologist David Wright realized that the projects he was working on were just taking too long to wrap up.

“He had this epiphany one day that he wasn’t going to be able to get there from here, essentially,” Carah said. “Doing projects at the rate he was doing them, he wasn’t going to be able to treat as much habitat as he needed to treat.”

Their study found that, when done right and in the appropriate places, their unanchored logs were very likely to stay put and create good habitat at a fraction of the cost.

One method the researchers looked at was cutting and falling riparian trees directly into streams. This worked best in remote, dense forests where bringing in heavy equipment wasn’t an option and cutting one or two trees wouldn’t deprive the stream of beneficial shade. In areas with fewer trees, they brought in a rubber-tired skidder to move more distant logs into the stream. In both cases, they wedged the logs against trees and stumps on the banks for a little extra security.

An example of an “accelerated recruitment” project (Credit: Jennifer Carah)

The researchers put those techniques to work in 11 streams across Menidicino County. After two years of monitoring, three sites hadn’t lost any logs at all. In the worst case, a 4-year-old site still retained 73 percent of its logs. The projects came in at an average of $259 per log, just 22 percent of the cost of anchored projects.

Though the study was successful, Carah said the accelerated approach has its limits. The study area was largely undeveloped timberland without the bridges, culverts or bankside homes that would be at risk from loose logs. They also found that logs longer than 1.5 times the width of the bankfull stream were the least likely to float away. It was easier to find trees that tall for the small study streams than it would be on larger rivers.

Carah also stressed that this isn’t the silver bullet for restoring salmon, which are also threatened by suppressed flows in rivers and climate change at sea. But it could at least help bolster habitat until the forests can do it themselves.

“Even though the forests are protected and they’re on their way there, it’s going to take them a long time,” Carah said. “Coho salmon don’t have that long to wait.”

The study is published online ahead of print in the North American Journal of Fisheries Management.