Population effect of a large-scale stream restoration effort on Chinook salmon in the Pahsimeroi River, Idaho

Stream habitat restoration is an important tool for fisheries management in impaired lotic systems. Although small-scale benefits of stream habitat restoration are commonly investigated, it is difficult to demonstrate population effects. The Pahsimeroi River Chinook salmon Oncorhynchus tshawytscha population was previously restricted to the lower portion of the river by multiple irrigation structures. To address fish passage issues, a combination of restoration projects was initiated including barrier removals, instream flow enhancements and installation of fish screens on diversions. The largest barrier was removed in 2009, more than doubling the amount of accessible linear habitat. We hypothesized restoration efforts would expand the distribution of spawning salmon in the Pahsimeroi River watershed, leading to a broader distribution of juveniles. We also hypothesized a broader juvenile distribution would have population effects by reducing the prevalence of density-dependent growth and survival. Redds were documented in newly accessible habitat immediately following barrier removal and accounted for a median of 42% of all redds in the Pahsimeroi River watershed during 2009-2015. Snorkel surveys also documented juvenile rearing in newly accessible habitat. Juvenile productivity increased from a median of 64 smolts/female spawner for brood years 2002-2008 to 99 smolts/female spawner for brood years 2009-2014. Overall, results suggested increased habitat accessibility in the Pahsimeroi River broadened the distribution of spawning adult and rearing juvenile salmon and reduced the effects of density-dependent survival. Large-scale stream restoration efforts can have a population effect. Despite the large-scale effort and response, habitat restoration alone is likely not sufficient to restore this population.

Automated summary

The restoration efforts in the Pahsimeroi River, including barrier removal, instream flow enhancements, and fish screen installations, have expanded the distribution of spawning salmon and rearing juvenile fish. The removal of the largest barrier in 2009 led to an immediate increase in reds in the river, accounting for 42% of all reds in the watershed from 2009-2015. Snorkel surveys also showed increased juvenile rearing in newly accessible habitat.