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.

The first segment of the along Seattle’s Elliott Bay is nearly complete, stretching north from the Colman Dock to the Seattle Aquarium. The new seawall includes habitat features that protect and encourage young salmon to migrate along the shore, while still allowing for normal waterfront commerce and activity.

91̽»¨ 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 , 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 “.”

“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 “” 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 91̽»¨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 , lead author and a 91̽»¨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 91̽»¨research team also published a 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 , a fishery biologist at NOAA’s Northwest Fisheries Science Center and a recent 91̽»¨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.

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For more information, contact Cordell at jcordell@uw.edu or 206-543-7532; Toft at tofty@uw.edu or 206-221-5460; and Munsch at stuart.munsch@noaa.gov or 206-302-1748.

The fish hatch in the rivers and streams that feed into Puget Sound and almost immediately rely on eating small organisms near the shore, including in the heart of Seattle’s commerce-filled waterfront.

Though salmon share the busy Elliott Bay waters with boats and barges, scientists suspect built-up, “armored” shorelines and large piers may be the main culprits disrupting fish habitat. These artificial structures block light and confuse the fish as they make their way to the ocean.

But are concrete seawalls actually affecting what the salmon eat, and by how much? A 91̽»¨ , with small chum salmon seeming to be most affected.

The study looked at the diets of young salmon passing through Elliott Bay. Researchers measured the types of prey in the water along armored shorelines and along restored beaches. Scientists then collected young salmon in nets — corralled by boats or divers — and flushed out their stomachs to look at what they ate recently.

The stomach contents showed that young pink and Chinook salmon that feed on organisms either floating in the water or on the water’s surface were able to eat the same amount of food, whether they were feeding near a concrete shoreline such as Seattle’s ferry terminal at Coleman Dock or along shoreline that has been restored to look like a natural beach, including along Seattle Art Museum’s Olympic Sculpture Park.

However, young chum salmon that munch on critters found mainly in bottom habitats had a noticeable change in their eating patterns depending on the type of shoreline. These small chum salmon ate more invertebrates floating in the water when swimming by armored sites, and more bottom-dwelling crustaceans — which they prefer — when feeding near beaches. Larger juvenile chum behaved more like their pink and Chinook counterparts.

“Our study shows that armoring affects what species of prey are available,” said lead author Stuart Munsch, a 91̽»¨doctoral student in aquatic and fishery sciences. “Fish that normally eat those missing prey will feed on alternative species at armored sites, but we don’t know what the costs of that change are to the fish.”

The were published Sept. 15 in the journal .

The article details the latest in a series of recent studies along Seattle’s waterfront that is trying to better understand how fish behave in urban, industrial waterways. The shores of Elliott Bay are almost fully walled-in with concrete and , a layer of large stones designed to keep soil from eroding. The most natural shorelines are along several manmade sandy beaches, restored recently for public recreation and natural beauty.

The study confirmed that areas converted to look like beaches attract more diverse organisms, including small crustaceans known as . These weren’t seen much along armored shorelines, which instead had more barnacles — not an appetizing choice for young salmon.

“Engineered shorelines like these manmade beaches are going to have more components of a natural ecosystem than a featureless wall,” said co-author , lead investigator on the project and a 91̽»¨research scientist with aquatic and fishery sciences. “Manmade beaches will produce more diversity and numbers of the kinds of food juvenile salmon utilize.”

The researchers found that while the types of organisms in the water did indeed change depending on shoreline, only the small chum salmon actually adjusted what they ate.

Maybe the other fish, the pink, Chinook and larger chum salmon, ate prey that wasn’t directly affected by the type of shorelines present — such as plankton, which was in the water at both locations — or were large and strong enough to swim through both areas, eating along the way, before their stomach contents were measured.

But small chum salmon are especially dependent on the tiny crustaceans more common along restored beach sites. And while none of the fish studied were starving, the fish whose diets changed may have used up considerable energy trying to keep a balanced diet.

“The [type of] copepods that chum salmon usually feed on are brightly colored and they’re found near the bottom,” Munsch said. In other words, the chum’s typical diet is easy prey. “We think that chum salmon along armored shorelines might have to spend more energy searching for prey that are harder to see, or chasing prey that are more evasive, when that energy should be allocated to growth or migration.”

This study and other recent papers by Cordell’s research team are informing Seattle’s , which is replacing the current waterfront wall with a structure that intends to be friendlier to fish while protecting city infrastructure.

The research was funded by the Seattle Department of Transportation and a National Science Foundation Graduate Research Fellowship.   Jason Toft, a research scientist in 91̽»¨fisheries, is another co-author on this paper.

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For more information, contact Cordell at jcordell@uw.edu or 206-543-7532 and Munsch at smunsch@uw.edu.