Why Aren’t Steelhead Making It Past The Hood Canal Bridge?

The Port Gamble S’Klallam Tribe has been trying to figure out why juvenile steelhead are not getting past the Hood Canal Bridge.

Traffic noise? Light pollution? Water quality? Wave action? Dissolved oxygen issues?

Nope. Mostly, it’s the bridge itself. 

The bridge’s floating pontoons, which span 80 percent of the width of the canal and extend about 13 feet underwater, appear to be impeding steelhead, chinook and chum out-migration, said Hans Daubenberger, the tribe’s senior research scientist. In addition, predators, such as seals, are feasting on the fish blocked by the bridge.

From 2017-2019, the tribe and partners in the Hood Canal Bridge Ecosystem Assessment Project studied how and why juvenile steelhead and other species were not migrating past the bridge. 

Partners include the National Oceanic and Atmospheric Administration’s (NOAA) Northwest Fisheries Science Center, Long Live The Kings, Pacific Northwest National Laboratory, Hood Canal Coordinating Council, and the state departments of Fish and Wildlife, Transportation, and Ecology.

“Some of the hypotheses we had – such as sound impacts that we thought were problems – weren’t problematic,” Daubenberger said. “The driving issue is that the bridge acts like a dam. Fish within the several hundred meters of the bridge weren’t surviving.”

Fish coming out of Hood Canal had been tagged with tiny acoustic transmitters, including smolts from Big Beef Creek and from the Skokomish River; when they reached the bridge, acoustic tag receivers on the pontoons would pick up the swimming patterns of the individual steelhead, noting if the fish had made it past the bridge or not.

Scientists from NOAA discovered there was up to 50 percent mortality of juvenile steelhead at the bridge, primarily by predation, Daubenberger said.

In addition to using NOAA’s acoustic tracking data, the tribe collected water quality and zooplankton samples, conducted hydroacoustic surveys for fish density and visual acoustic surveys for predator activity, and recorded underwater video for presence and identification of species. The tribe also observed fish activity under the bridge’s light sources.

Fish in the top 6 feet of the water column are the most affected by the bridge, Daubenberger said. 

Steelhead seemed to follow the currents and visual cues along the bridge’s pontoons as if they were a dam, rather than diving under the pontoons to pass it, he said. Fish also were observed feeding on plankton in the pontoons’ corners where eddies form, creating easy access for predators. 

Juvenile chinook and chum also were observed being preyed upon by seals and seabirds at heavy rates.

The next steps are to design and implement structural changes to the bridge that would guide the fish around the pontoons. The only long-term solution is to replace the bridge, Daubenberger said.

The southern side of the Hood Canal Bridge, which is making it hard for juvenile salmon to continue their out-migration journey to the ocean. Photo: Port Gamble S’Klallam Tribe