Steelhead could be running into a trap at the Hood Canal Bridge

Fishermen fish for salmon north of the Hood Canal bridge, but researchers say the bridge may be an obstacle to the migration of young steelhead. Kitsap Sun photo by Larry Steagall
Fishermen fish for salmon north of the Hood Canal bridge last week, while researchers say the bridge could be an obstacle to the migration of young steelhead. // Kitsap Sun photo by Larry Steagall

I’ve often wondered if the Hood Canal bridge might be an obstruction for killer whales, which could simply choose to back away from the wall of floating pontoons, which are anchored to the seabed by a confusing array of crisscrossing cables. Old-timers have told me that orcas used to come into Hood Canal more frequently before the bridge was built.

What I never considered seriously, however, was that the bridge could be an obstacle for fish as well. In Sunday’s Kitsap Sun, I wrote about recent findings from a study tracking juvenile steelhead by means of implanted acoustic transmitters. The study was conducted by researchers at NOAA’s Northwest Fisheries Science Center.

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The bottom line is that something is happening at the bridge, where many of the transmitters either disappeared or winded up staying in one place near the bridge, continuing to send out their signals for weeks. The leading hypothesis is that seals or other predators are eating the young steelhead, and some of the acoustic tags are being digested and excreted near the bridge.

Why the bridge serves as an obstacle to steelhead remains unclear. But other studies have suggested that steelhead swim near the surface. As they move out of the canal, the fish may encounter the bridge pontoons as a physical barrier, since the concrete structures go down 12 feet underwater. Also, currents around the pontoons could be a strange condition for the fish. If a young steelhead slows down in the process, a harbor seal or other predator could be waiting to take advantage of the situation.

We’ve all heard about sea lions capturing adult salmon by hanging out at fish ladders at Seattle’s Ballard Locks in Seattle or at Bonneville Dam on the Columbia River. Maybe the same thing is happening at the Hood Canal bridge with smaller prey as the target of the marine mammals.

I was also intrigued by an analysis conducted by Tarang Khangaonkar, a researcher at Pacific Northwest National Laboratory in Seattle. He told me that in all the models of circulation in Puget Sound and Hood Canal, the bridge tended to be ignored. Since the pontoons go down 12 feet, the bridge disrupts the relatively thin low-salinity surface layer moving out of Hood Canal.

Tarang calculates that the bridge could reduce the circulation by 10 percent or more, which has serious implications, not just for steelhead at the bridge but for the ecological health of all of Hood Canal.

“We have to examine what the bridge is doing,” Tarang told me. “It slows the entire system down. Water quality is maintained in Puget Sound by the flushing effect, which flushes the system out and maintains a balance. Our preliminary finding is that it could slow down by about 10 percent. That effect is cumulative.”

The bridge, he said, could effectively create a more stagnant body of water, where oxygen can become depleted. More study is needed, he said.

Most of the folks I interviewed for this story agreed that the first priority for further research was to see what is happening to the steelhead — and possibly chinook and chum salmon — at the bridge. Studies could focus on the fish, predators and currents at the bridge.

The project is gaining support, but it could require a special legislative appropriation of about $2 million.

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