Estimating How Much Restoration Is Needed to Achieve Salmon Restoration and Recovery Goals: an Example from the Puget Sound Basin

Estimating the total amount of habitat restoration needed to recover threatened and endangered salmon is a pressing management need. Using existing data from evaluations of watershed restoration, we estimate the average change in coho salmon Oncorhynchus kisutch and steelhead O. mykiss parr and smolt densities for common in-channel (culvert removal, large wood placement, boulder placement, and constructed logjams) and floodplain restoration techniques (constructed side channels and reconnected floodplain habitats). We then used these numbers and a Monte Carlo simulation to predict changes in fish numbers in a model watershed for two restoration scenarios: scenario 1) restore all accessible habitat within the watershed, and scenario 2) restore the average amount historically implemented in Puget Sound watersheds (8% of total restorable). Mean increases in coho densities from restoration ranged from 0.19 to 2.32 parr/m for in-channel techniques and 0.34 to 1.70 parr/m2 for floodplain techniques. Increases in steelhead densities ranged from −0.06 to 0.71 fish/m and from 0.03 to 0.06 fish/m2 for in-channel and floodplain techniques, respectively. Under restoration scenario 1, the predicted mean increase was 1,459,254 (117%) and 285,302 (140%) for coho parr and smolts, and 93,965 (65%) and 28,001 (125%) for steelhead parr and smolts, respectively. Under scenario 2, the predicted mean increase in parr and smolts, respectively, was 59,591 (5%) and 15,022 (7%) for coho, and 1,733 (1%) and 1,195 (5 %) for steelhead. The percentage of floodplain and in-channel habitat that would need to be restored in the model watershed to detect a 25% increase in coho and steelhead smolt production (the minimum level detectable by most monitoring programs) was 18 and 20%, respectively. However, given the large variability in fish response (changes in density or abundance) to restoration, 100% of the habitat would need to be restored to be 95% certain of achieving a 25% increase in smolt production for either species. Our study demonstrates that considerable restoration is needed to produce measurable changes in fish abundance at a watershed scale. Improvements in restoration project selection, design and monitoring may help reduce the total amount of habitat restoration needed to achieve recovery goals.