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Hood Canal bridge has been a floating trap for young steelhead, but minor alterations could reduce predation for the migrating fish. // Photo: Bob Cortright, Bridge Ink

Minor bridge modifications could help young steelhead escape from Hood Canal

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Help could be on the way for migrating steelhead and salmon in Hood Canal, where many young fish are killed each year by seals and other predators that lie in wait at the Hood Canal floating bridge.

As many as 50 percent of the steelhead migrants perish as they arrive at the bridge, where predators pick them off one by one. The bridge is supported by floating concrete pontoons, forming a nearly solid barrier across the waterway. Young steelhead generally swim near the surface, making them especially vulnerable to predation, although some fish will dive under the bridge to get to the other side.

Concrete pontoons can obstruct and slow the migration of steelhead.
Image: Jenny Paulo for Long Live the Kings

Engineers are currently designing minor modifications to a few bridge pontoons to help the fish find their way through existing gaps in the structure — one at each end of the line of floating pontoons. Those are the places where the bridge connects to the shore, and a small truss bridge carries traffic over the span.

I first learned about problems caused by the Hood Canal bridge in 2013, when researchers with the National Oceanic and Atmospheric Administration were able to track steelhead using tiny implanted transmitters (Kitsap Sun, Oct. 13, 2014).

The researchers could track the movements of the implanted fish using receivers placed throughout Hood Canal. For many fish, their journey ended when they reached the bridge, as indicated by following their tracks recorded on receivers at the bridge. When a fish was eaten, the transmitter — now in the belly of a seal, sea lion or bird — would suddenly show a completely different travel pattern. After the fish was digested by a seal, for example, researchers might determine that an immobile transmitter was among the excrement at the bottom of Hood Canal. (See the research article led by Megan Moore in the journal PLOS One.)

Further studies showed that the movement of young steelhead — and perhaps some species of salmon — were influenced by shifting currents around the bridge. Fish would often move laterally along the bridge structure until caught in eddies formed by tidal currents through the bridge openings. Larger numbers of fish were found in those eddies.

For a more complete understanding of these findings, please check out the recently released technical report compiled by Long Live the Kings, which coordinated the recent studies, or read an illustrated summary of that report. (Full disclosure: I wrote the summary text under contract with LLTC.)

As you will see in those reports, short-term solutions involve small structures that could be attached to the bridge to reduce the eddies and hopefully move the fish more quickly along the bridge and through the existing openings in the pontoons.

According to Lucas Hall of Long Live the Kings, the goal now is to get people to understand the problem and recognize the need for a temporary solution. Designs for the bridge modifications are already under way using existing grants.

An additional $2.5 million is needed to fabricate the attachments to the bridge and begin a monitoring effort as early as 2022. Full monitoring and further modifications to improve performance could cost another $1.5 million, according to current estimates.

While that might seem like a lot of money, the benefits to salmon and steelhead could be enormous, especially when one considers all the efforts that have gone into saving Puget Sound Chinook, Hood Canal summer chum and Puget Sound steelhead — all listed as threatened in Hood Canal and likely affected by the bridge in one way or another.

Harbor seals commonly haul out on Sisters Rock, about half a mile southwest of the bridge.
Photo: Hans Daubenberger, Port Gamble S’Klallam Tribe

Millions of dollars have been spent to replace culverts to improve salmon migration in the streams, not to mention improvements to freshwater rearing habitat. But those improvements are not fully realized when the young fish, after surviving many threats in freshwater, don’t live long enough to get out of Hood Canal.

“In comparison to the investments we are making in the fish-passage arena, this (bridge mitigation) would be immensely efficient and would be a huge win,” Lucas told me.

Eventually, the Hood Canal bridge will need to be replaced, and it now seems obvious that a different configuration for the floating pontoons would allow for increased fish passage with less risk. Gaps between each pontoon would be a major improvement, even if the pontoons extended well beyond the bridge. Perhaps before full bridge replacement, that kind of alteration could be considered for one or more sections of the bridge.

Photo: Hans Daubenberger, Port Gamble S’Klallam Tribe

3 comments

  1. Paul Johanson says:

    My immediate reaction is: why are the pontoons oriented against the current/fish-passage? If there is a strength/structural issue, couldn’t at least alternate pontoon sections be in line with the current? I see that follow-up material includes documentation of that alternate pontoon configuration in other locales. It seems counterintuitive to me that the original design would have included so much needless flow blockage and thus as it turns out, fish blockage.

  2. Christopher Dunagan says:

    Paul,
    I’m not sure I could track down a complete answer, because the Hood Canal bridge was designed more than 60 years ago. At the time, apparently nobody was thinking about fish, so engineers probably looked for the greatest stability at the least cost. I’m told the design is similar to that of the Mercer Island floating bridge on Interstate 90, completed in 1940. When the west half of the Hood Canal bridge was rebuilt after the 1979 storm, the same design was followed. Similarly, the east half of the bridge was replaced from 2003 to 2009. The Norwegian bridge mentioned in the summary report apparently had some issues with pontoons twisting in the currents and needed to be reinforced to maintain the gaps between the pontoons.

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