Casting Shadows on the Sound

How docks and piers affect marine habitat and salmon

Overwater structures are common across Puget Sound’s shorelines, and they come in all shapes and sizes. In addition to docks and piers, the term “overwater structures” includes ferry terminals, shipyards, and various other constructions built on top of the water.

A multitude of private docks along Puget Sound shoreline. Image from the National Agricultural Imagery Program.

Large overwater structures like ferry terminals and commercial piers act like big umbrellas that cast shadows over marine habitats.

Because of the important role sunlight plays in shoreline marine habitats, this shadowing effect can impact life underwater. Sunlight guides the behavior of many marine animals, and marine vegetation like eelgrass and kelp need sunlight to photosynthesize, grow, and reproduce.

Large overwater structures negatively affect marine habitats

After decades of research exploring how shadows cast by large overwater structures impact shoreline habitats in Puget Sound, scientists found that large overwater structures shade out marine vegetation and alter the makeup of species in shoreline habitats.

Salmon eat forage fish like these surf smelt. Forage fish may also be impacted by overwater structures.

Shadows from these large structures also change the behavior of migrating juvenile salmon. As juvenile salmon migrate along Puget Sound shorelines, they often stop at these shaded areas until the shadow is reduced at low tide or when the sun sets. This pause in migration may cause salmon to eat less, hindering their growth. It may also make them more vulnerable to being eaten by predators.

More research needed on how small overwater structures impact marine habitats

There is a question if smaller overwater structures — like private docks — may also impact shoreline marine habitats, salmon, and other wildlife. While Puget Sound has relatively few larger structures, there are thousands of small overwater structures, most of which are small, residential private docks.

The Washington Department of Fish and Wildlife (WDFW) is one of the agencies that permits small overwater structures. WDFW’s researchers reviewed the science surrounding possible impacts of small overwater structures and recently published the outcome of their review.

This scientific review found that considerably less research has been dedicated to smaller overwater structures. However, there is consistent evidence that small overwater structures impair eelgrass survival and growth. Because eelgrass is an important component of fish spawning and foraging habitat, this research led to changes in the mid-1990s in how small overwater structures are permitted.

Sugar kelp is a common species in Puget Sound and grows near, under, and on overwater structures like private docks.

Beyond studies on eelgrass, little research has addressed possible impacts of small overwater structures to other marine vegetation like kelp or to fish communities, behavior, health, and survival. There is not enough research to draw conclusions about the impacts of small overwater structures in Puget Sound or which design elements and restoration approaches might offset possible impacts.

An eelgrass bed along the Puget Sound shoreline at low tide. Photo: Ingrid Taylor

Unfortunately, it is challenging to apply research on large overwater structures to small structures because the dimensions are significantly smaller in length, width, and surface area.

Limited research has also been done on the effects of small overwater structures on salmon in freshwater habitats like Lake Washington. These studies often find that salmon avoid swimming under freshwater structures. However, salmon biology is intimately tied to water salinity and so the effects of small overwater structures on salmon migratory behavior may differ between freshwater and marine environments.

Our inability to draw conclusive extrapolations from freshwater structures and large marine overwater structures demonstrates the need for more research on small overwater structures in Puget Sound.

WDFW permit process for small overwater structures

WDFW uses the best available evidence to inform our regulations and decisions. When we lack robust, defensible scientific evidence, we balance taking precaution from presumed impacts and applying reasonable natural history inferences to guide our permitting.

Private dock with nearby sugar kelp looking towards a Puget Sound shoreline.

People who want to do a project in or near state waters, including a pier or dock installation, need a Hydraulic Project Approval (HPA) permit from WDFW.

All proposed overwater structures must achieve a “no net loss” of current habitat conditions through various mitigation actions. WDFW must determine if these structures will mitigate any possible impacts to fish and their habitats based on available information. However, when permitting a small overwater structure, WDFW cannot require measures to enhance fish habitat conditions that are out of proportion to the impacts of the proposed structure.

Looking ahead to research and partnership opportunities

WDFW scientists are seeking funding to study small overwater structures and better understand which design factors (like deck grating or height) could offset any possible impacts of these structures.

Our scientists are also actively seeking partners to help us with this research and encourage other researchers to study small overwater structures to help WDFW and other agencies better manage Puget Sound habitats.

If you have a private dock that you would be interested in having WDFW include in future research, please reach out to WDFW Research Scientist Max Lambert.



The Washington Department of Fish and Wildlife is dedicated to preserving, protecting and perpetuating the state’s fish and wildlife resources.

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The Washington Department of Fish and Wildlife

The Washington Department of Fish and Wildlife is dedicated to preserving, protecting and perpetuating the state’s fish and wildlife resources.