Solutions sought for salmon’s fear of the dark in Puget Sound, where shadows slow migration

Humans aren’t the only species whose kids are afraid of the dark. Juvenile salmon depend on light for a number of functions as they migrate from river to sea — so much, in fact, that they often refuse to cross heavily shadowed regions in the water. As people install more docks and piers along river banks, shadows criss-crossing popular migratory routes are slowing passage for the young salmon.

Two researchers from the University of Washington’s School of Aquatic and Fishery Sciences experimented with ways to increase light levels near docks and other shoreline constructs around Puget Sound. Their findings were published online ahead of print in Ecological Engineering.

“We’ve got a number of species of salmon [in Washington],” said Charles Simenstad, University of Washington School of Aquatic and Fishery Sciences research professor and co-author of the study. “Juvenile salmon, when they’re migrating to the oceans from the rivers, stay very shallow along the shoreline.”

The salmon’s instinctive behavior to ride the shoreline might protect them unseen threats lurking in deeper waters, but that same instinct is what makes the fish wary about swimming around the shadows. This behavior seems to be dependent on the time of day, Simenstad explained. At night the fish appear less concerned about water depth than when the sun is shining.

“This is speculative, but juvenile salmon are likely — like a lot of fish — to perceive the dark areas as a potential area for predation,” Simenstad said. “They’re reluctant to enter vegetation, for instance.”

In the wilderness, overhanging structures aren’t naturally prevalent along the salmon’s migratory paths. But where rivers cross through civilization, docks, piers and ferry terminals can stretch across the water, casting strong shadows that stop juvenile salmon in their tracks. While conducting the study, the Simenstad and co-author Kotaro Ono found that less than 15 percent of juvenile salmon shoals swam under the Port Townsend ferry terminal.

The University of Washington team working on the fiber-optic system at the Port Townsend ferry terminal (Credit: Charles Simenstad)

To mitigate this effect, the researchers worked with the Washington Department of Transportation to implement a fiber-optic lighting system under the terminal. In theory, the system would track the sun and emit light at the appropriate times to illuminate shadowy areas beneath the dock. But projected outcome and actual outcome can differ greatly, as the researchers discovered.

“This seemed like a potentially feasible mitigation approach to getting enough light to diffuse these shadows,” Simenstad said. “Unfortunately the system, technically, didn’t turn out to work very well.”

The problems were numerous. While the fiber optic light had a positive effect on salmon passage when reducing dock shading, it actually deterred salmon when projected in a bright environment. Within a year, the system stopped tracking the sun and eventually broke down altogether. A replacement system using halogen floodlights put out more light than the fiber optics, but with a less natural hue (Ono and Simenstad weren’t able to observe this system in action, but in the paper Ono suggested that it showed less promise than the fiber optics). Furthermore, the water level in Puget Sound proved quite variable.

“We have a 3-plus meter tide range,” Simenstad said. “At one hour, the fish could be right under the lights, and the lights could be effective. But an hour later they could be a meter down.”

Despite the experiment’s shortcomings, Simenstad asserts that any failure “wasn’t a fault of concept,” but of application. In addition to improved lighting systems, the Department of Transportation has tested other means of reducing shadow. One Puget Sound ferry terminal was renovated so that the portion of the dock nearest shore was thinner, and therefore cast a slimmer shadow, than the wider end in deeper waters. Other docks have used gridded designs or incorporated glass blocks to allow light to pass through.