NOAA Fisheries biologists say that threatened Puget Sound steelhead smolts are facing challenges on their migration route, with the Hood Canal floating bridge being a major source of mortality for about half of the smolts while trying to get past the bridge, or soon after.
Puget Sound steelhead are listed as threatened under the Endangered Species Act, and most populations are shown to have declined since 2007.
Research biologist Megan Moore of the Northwest Fisheries Science Center said researchers had no idea the bridge was causing such an impact on the migrating steelheads. The research report, led by Moore, was published recently in the peer-reviewed journal Ecosphere by the NOAA Fisheries Northwest Fisheries Science Center and Pacific Northwest sustainable fishing non-profit Long Live the Kings.
Researchers using acoustic telemetry tags implanted in the fish and over 30 receivers along the migration route found the steelhead smolts slowed considerably at the bridge. They took one to two days on average to navigate past the bridge, compared to just five to seven hours to travel through open, unobstructed sections of the canal, data show.
The Hood Canal Bridge, which spans the northern outlet of Hood Canal in Washington’s Puget Sound, is called a floating bridge, but construction actually extends 15 feet below the surface. Migrating steelhead tend to swim near the surface, so the submerged section of bridge forms a partial barrier for these smolt migrating to the Pacific Ocean.
Moore said that physical barrier is only part of the problem, as the smolt can figure out how to eventually get past it. The real challenge is predators that congregate around the bridge, she said.
Steelhead typically swim rapidly through Hood Canal toward the ocean, but the bridge acts as a speed bump, causing then to pause and making them more vulnerable to predators like harbor seals, which are plentiful in the era. For their population to grow, more smolt need to make it from rivers to the ocean and back again.
Reducing mortality along the way, including passage by the Hood Canal Bridge, could possibly double their survival and help add individuals who could grow the population, researchers said.
Moore and her team are looking at a range of solutions, including a change in bridge design and devices to deter predators when these smolt are at their most vulnerable, and plan to do testing in 2023.