Researchers record pool depth, water temperature, and velocity in the Entiat River, Washington. Photo courtesy of Heather Porter.
Salmon are cherished in the Pacific Northwest. They help sustain various food webs, are an economic driver, and are culturally important to Tribes and other in the Northwest. Because salmon populations have greatly declined, a significant amount of time, money and effort has been spent restoring salmon habitat. This begs the questions: Are our current restoration efforts working, and if so, how do we know? Current research by Pacific Northwest (PNW) Research Station scientists along the Entiat River in central Washington address these questions.
Stream restoration efforts often include the installation of structures in the water to create or expand pools that are essential habitat for native fish, particularly salmon. Research shows the fish to be attracted to these pools, but these structures have yielded mixed results in terms of increasing the number of fish in restored streams.
Research fisheries biologist Carlos Polivka and aquatic ecologist Shannon Claeson and colleagues wondered if these mixed results resulted from the scale at which post-restoration evaluations are typically conducted. Because the goal of stream restoration is to improve the life cycle of the fish across the entire length of a stream, post-restoration monitoring usually assesses fish abundance and the number of fish per square meters (density) across large areas of a stream to determine restoration effectiveness. Highly localized effects of restoration may be masked.
The PNW scientists found that in-stream restoration structures in the Entiat River have localized effects on fish density that may be undetectable just 5 meters (about 16 feet) away from the structure. Even when a restored pool is included in the analysis, increasing the survey area to just twice that of the pool can make it difficult to observe the effects of the restoration on the fish. Thus, an increase in fish density within the immediate area of a restoration structure may be underrepresented when post-restoration monitoring evaluates fish density over large areas of the stream.
Knowing this, the researchers developed a technique to confirm that the high number of fish observed in the enhanced habitat represented an increase in the number of fish the restored reaches can support. They applied this to all reaches with restoration structures and concluded that restoration in the Entiat River was largely successful in improving rearing habitat for sub-yearling Chinook salmon, which bodes well for similar restoration projects in other rivers if post-restoration monitoring is conducted with the same methodological approach.
These findings can inform stream monitoring programs and may be useful to conservation agencies when assessing the efficacy of stream restoration efforts in the future. Effective stream restoration will help increase salmon populations, which will benefit communities and ecosystems throughout the Northwest.