Case Study: Fish Movement Near Seattle’s Evergreen Point Floating Bridge

Located east of Seattle, Lake Washington is home to the world’s longest floating bridge, the Evergreen Point Floating Bridge. Commonly referred to as the 520 Bridge, it stretches a whopping 7,710 feet.

Before the Washington State Department of Transportation began the process of replacing the original bridge in 2011, it set out to better understand nearby fish behavior with the help of the U.S. Fish and Wildlife Service. It’s known that structures, including bridges in and near waterways, can influence the ecological dynamics of the aquatic environment, including fish behavior, habitat use and survival.

Many naturally reared Chinook salmon smolts in Lake Washington must pass under the 520 Bridge as they migrate toward Puget Sound, so the study sought to evaluate movement and habitat use of the salmon smolts and two predators—northern pikeminnow and smallmouth bass.

Researchers employed Innovasea’s fine-scale acoustic tracking system to track fish in a 42-acre area along a 1,838-foot stretch of the bridge from late May through early August. The study site was on the west end of the bridge and was believed to lie within a major migratory corridor for salmon smolts.

Tagged smolts were released 2,624 feet south of the study site (upstream) to observe behaviors as they entered the study site and encountered the bridge. Most predators were caught on-site, tagged and released near the place of capture. Though differences in timing of both migrational cues, physiological smolt status, water temperature and prey availability may have contributed to differences in behaviors observed between release groups, the study helped answer key questions about how the bridge affected fitness and survival of Chinook salmon and how the replacement bridge could designed and sited to minimize impact to the breed.

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