As tourists and residents visit Seattle’s downtown waterfront, it may not be immediately apparent they are walking on arguably the largest, most ambitious urban seawall project in the world that prioritizes habitat for young fish and the invertebrates they feed on.
University of Washington researchers have published an initial analysis of the effectiveness of the new habitat features as part of an ongoing partnership with the Seattle Department of Transportation. The study found that adding steps ― which are shelf-like structures protruding from the vertical underwater wall ― helped recruit a greater diversity of organisms such as algae and small invertebrates that juvenile salmon feed upon as they migrate along the shore through Puget Sound and out to the ocean.
“The big question with urban shorelines is how to protect infrastructure while maintaining stability with sea-level rise and storms — and still try to restore natural processes,” said Jason Toft, a research scientist at the UW’s School of Aquatic and Fishery Sciences and a co-author of the new study appearing in the book “Living Shorelines: The Science and Management of Nature-Based Coastal Protection.”
“We are trying to address what other cities can learn from Seattle’s approach, and what we can add to the global discussion of how to both protect and restore our shorelines.”
The city took inspiration from these findings and installed “marine mattresses” along the seawall, which are mesh bags filled with rocks that create a shelf-like surface and make the nearshore shallower for fish. Engineers also constructed uneven, cobbled surfaces along the vertical wall to encourage more organisms to congregate by the shoreline. They designed clear glass squares in the seawall’s pedestrian sidewalk to let more natural light penetrate to the water below.
All of these modifications are designed to help young Chinook, pink and chum salmon navigate, grow and avoid predators along the engineered shoreline. Juvenile fish prefer shallow water, and the mesh shelves help raise the seafloor and offer protection from predators. The young fish also avoid intense shade, so the glass squares help funnel natural light through the cantilevered sidewalk to the water below and also facilitate more natural migration movement among fish.
As shown in this preliminary study, the underwater structures featured in the new seawall attract more sea life and invertebrates, which are important sources of food for salmon. The researchers placed large concrete panels on the existing seawall in Elliott Bay with several types of texture and relief. Over a three-year period, they found that adding shelves along the vertical panels was most important in recruiting a diversity of algae and invertebrates to the concrete, and this led to incorporating shelves into the new wall.
Beginning next year, the UW scientists will begin a formal, 10-year monitoring study along the entire seawall. That study will rigorously assess whether the light squares are effective and track how fish respond to the various new features.
“This is a big experiment,” said Jeff Cordell, lead author and a UW research scientist in aquatic and fishery sciences. “Monitoring once it’s all done is very important. It will be difficult to measure in terms of increases in salmon returns, but we can gauge success by other means such as increases in salmon feeding behavior and amount of food available around the new habitat enhancements.”
The UW research team also published a supporting paper this spring in the Journal of Applied Ecology. This study looked at how building along waterfronts affects fish habitats around the world, and how waterfronts can be designed to improve the area for fish despite their heavy industrial use. Coastal cities like Sydney, Australia, and New York City, along with Seattle, are increasingly interested in incorporating ecologically friendly features into urban thoroughfares, and researchers are currently developing and evaluating these features.
“Seattle’s downtown waterfront is an unprecedented attempt to improve fish habitat along an urban shoreline, so it’s a great opportunity to learn from and apply around the world,” said lead author Stuart Munsch, a fishery biologist at NOAA’s Northwest Fisheries Science Center and a recent UW doctoral graduate in aquatic and fishery sciences.
“There are a lot of situations where you aren’t truly going to restore a shoreline. But even working within the constraint of heavy human use along shorelines, you can still do a better job than putting concrete or boulders in the intertidal zone.”
The study also stresses the importance of looking at the bigger picture of how fish behave and use habitat ― instead of just counting total fish ― when evaluating whether new engineered habitat features work. In Seattle, for example, researchers will be watching to see if fish swim under the piers to feed, or if smaller, more vulnerable fish are attracted to the shallower, protective waters that an engineered beach provides.
“If you really want to understand habitat value, you have to go beyond catching and counting fish,” Munsch said. “You have to look at their behavior and take a dynamic perspective on how they use habitat.”
The studies were funded by the National Science Foundation, the Seattle Department of Transportation, King Conservation District, Washington Sea Grant and the Washington Department of Fish and Wildlife, partly through its the Estuary and Salmon Restoration Program.