Fisheries thrive under specific habitat conditions that are maintained under the watchful eye of resource managers who monitor water quality and protect the system. Some fish are more particular about habitat structures and conditions than others, meaning that a comprehensive understanding of what fish look for is essential to guaranteeing consistent recruitment and a healthy fishery.
Streams and rivers are essential pathways for migratory fish, and a host of environmental stressors have caused many of these waterways to become unsuitable for the fish who navigate up and downstream each year. Researchers have spent years studying what keystone species like salmon prefer in both their spawning and migratory habitats. Ensuring that the fish can safely make their way upstream to spawn and downstream to return to the ocean is one way to ensure higher populations.
While habitat improvements are often associated with high costs and the introduction of fishing restrictions and potentially costly monitoring programs, a study published in the North American Journal of Fisheries Management throws scientific weight behind an emerging method to build habitat more quickly and cheaply than traditional projects.
Habitat reconstruction traditionally focuses on hardware- and engineering-intensive methods of building woody habitats, whereas the “accelerated recruitment” approach assessed in this study can be done at less than a quarter of the cost and more closely mimics natural forest processes.
The accelerated process could 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 project is intended to be a temporary solution rather than investing much more time and money into a more long-term fix.
“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 of 80 to 90 percent. That’s bad news for salmon, which rely on in-stream trees to trap sediment, create pools and serve as a protective cover. In short, the removal of these trees led to further instability in the streams.
While earlier solutions focused on constructing artificial structures that might bridge gaps left by the deterioration of the surrounding habitat, this new solution proposes a more natural solution. As opposed to making up for the lack of natural wood through engineered projects that anchor wooden structures into streams with rebar, cable and boulders, unanchored logs serve a similar function at a fraction of the cost.
Anchoring the structures ensures the wood isn’t washed away, wasting a commodity and threatening downstream property and structures. However, the study found that, when done right and in the appropriate places, unanchored logs were very likely to stay put and create good habitat at a fraction of the cost.
Ultimately, designing the structures and anchoring those logs takes time. Carah said that David Wright, study co-author and Campbell Timberland Management biologist, realized that the projects he was working on were 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.”
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.
The researchers put those techniques to work in 11 streams across Mendocino 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.
While the study was successful, the accelerated approach still has its limits, according to Carah. The study area was largely underdeveloped timberland without the bridges, culverts or bankside homes that would be at risk from stray 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.
Additionally, though the method could solve one problem, other environmental stressors will continue to impact salmon migration and fisheries. For example, suppressed river flows and climate change at sea will continue to impact salmon health and recruitment, even if the unanchored logs improve stream health. At the very least, the strategy can help bolster habitats 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.”