A five-year monitoring effort has shown that the Fisher Slough restoration project has benefited juvenile chinook even more than predicted in the Skagit River Salmon Recovery Plan.
The Nature Conservancy led the project to restore tidal marsh habitat and connectivity in the Skagit watershed using a dike setback and a floodgate. Once the project was completed, the Skagit River System Cooperative (SRSC), the natural resources extension of the Sauk-Suiattle and Swinomish tribes, monitored the site between 2009 and 2013.
The Fisher Slough project is one of seven estuary restoration projects completed in the Skagit, totaling more than 750 acres of restored habitat intended to help recover chinook salmon. These seven projects mainly used four different designs: dike setback, dike breach, intertidal fill removal, and self-regulating tide gates (floodgates) to restore habitat.
“Our monitoring has generally found that dike setback and fill removal works the best for fish benefits,” said Eric Beamer, SRSC research director. “However, what is interesting at Fisher Slough is the use of both dike setback and a self-regulating floodgate.”
The restoration created about 46 acres of juvenile rearing habitat, and the combined effects of the dike setback and floodgate significantly changed the seasonal dynamics of dissolved oxygen and water temperature, benefiting estuarine resident chinook, Beamer said.
“We were able to statistically detect the effects of both dike setback and floodgate operation on habitat conditions and juvenile chinook salmon abundance and size,” Beamer said. “This has never been done before and overall the result is positive for Fisher Slough and the Skagit.”
SRSC sampled fish with beach seines and fyke traps, and found an increase in habitat use by juvenile chinook upstream of the floodgate. Compared with an estimate of fewer than 17,000 smolts in 2009, the high estimate for 2013 was nearly 39,000 – an increase in carrying capacity of about 22,000 fish.
“This increase in smolt carrying capacity is 1.3 times a greater benefit to Skagit chinook salmon than predicted by the Skagit Chinook Recovery Plan,” Beamer said. “The dike setback clearly increased the amount of rearing habitat upstream of the floodgate, but juvenile chinook salmon are able to access the new habitat only because the floodgate was operated in sync with at least part of the natural tidal cycle.”
The monitoring also showed that variability in floodgate operation, such as how long the floodgate doors are opened compared to being closed, influences how many juvenile chinook salmon used the new habitat.
“A big secondary lesson from monitoring this project helps us know how much influence structures like floodgates can have on the fish,” Beamer said.
For more information, contact: Eric Beamer, Research Director, Skagit River System Cooperative, 360-466-7228 or [email protected]; Kari Neumeyer, information officer, NWIFC, 360-424-8226 or [email protected].