Seattle seawall

SHORELINE MODIFICATIONS like armoring and pier placement can cause changes to beach slope, substrate, water depth, shading, and vegetation, alter nearshore species assemblages, and influence fish use and behavior (Toft et al. 2007). Juvenile salmon abundances and feeding behaviors are reduced under piers and armoring can reduce their access to shallow water habitat. Piers may interrupt seaward migration as juvenile salmon wait for low tides to pass under piers when more light penetrates beneath (Munsch et al. 2014). Shoreline armoring and piers can also reduce the abundances and taxa richness of epibenthic invertebrates including typical prey of out-migrating juvenile salmon (Cordell et al. 2017a). These effects have been observed along the Seattle waterfront.

 

Pier 67, original seawall
Engineered pocket beach at Olympic Sculpture Park

SHORELINE ENHANCEMENT   Rehabilitation through the removal of shoreline structures or by enhancement of these structures can improve habitat for juvenile fishes and ecological function. Construction of a shallow pocket beach following the removal of shoreline armoring (a) and construction of an eco-engineered, subtidal bench where armoring removal was not possible (b) increased juvenile fish densities and frequency of feeding behaviors relative to previously armored conditions (Toft et al. 2013). Epibenthic invertebrate densities and taxa richness also increased at the two sites (Cordell et al. 2017b).

 

ECO-ENGINEERED SEATTLE SEAWALL The seawall and piers along half of the Seattle waterfront have been enhanced with eco-engineering. Three changes to the original seawall include

An overhead glass-block sidewalk (GB) to increase light penetration to the water below in order to 1) aid visually-oriented juvenile salmon migration and feeding, and 2) increase primary production and epibenthic invertebrate grazers,

a raised intertidal habitat bench, (“marine mattress” – MM) to 1) create a shallow water corridor for migrating juvenile salmon, and 2) create substrate for epibenthic invertebrates, including juvenile salmon prey, and

a textured seawall with shelves (TS) to increase invertebrate colonization. In combination, these enhancements created an intertidal corridor, also known as the ‘salmon highway‘.

Beneath the sidewalk at Pier 57

HAS IT WORKED? Our post-construction research shows the eco-engineered seawall increases habitat value for juvenile salmon by increased light, feeding, and more homogeneous dispersion in nearshore pier habitats with eco-engineered enhancements (Sawyer et al. 2020). Using acoustics, salmon distributions were compared between restored sites (Toft et al. 2013), eco-engineered seawall sites, and original seawall sites. Juvenile salmon are 31 times less likely to be present under original pier habitats, and 11 less likely under eco-engineered habitats (Accola et al. 2022). High salmon densities along pier ends early in the year show many young salmon are avoiding pier habitats and swimming around the entirety of the pier, with unknown predation and bioenergetic costs.

Photosynthetic Active Radiation (i.e. available light) sampling
Algae and invertebrates on seawall and mattress
juvenile Chinook salmon school

Salmon prey

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Epibenthic invertebrate sampling

Salmon at night

With the exception of non-eco-engineered, under-pier habitats, juvenile salmon seawall presence and densities are twice as high at night compared to day. Increased nocturnal emigration and an attraction to the artificial light along the urban waterfront may contribute to day/night differences.

Juvenile salmon have higher presence and densities in the eco-engineered corridor at night compared to day, but have minimal day/night presence in traditional under-pier habitats. Salmon have higher presence along pier ends that have no shoreline eco-engineering, likely to avoid the dark, under-pier habitats.

Photo: Dorothy Edwards, Crosscut

Select publications

Accola KL, Horne JK, Cordell JR, Toft JD (2022) Acoustic characterization of juvenile Pacific salmon distributions along an eco-engineered seawall. Mar Ecol Prog Ser 682:207-220. https://doi.org/10.3354/meps13917

Sawyer AC, Toft JD, Cordell JR (2020) Seawall as salmon habitat: Eco-engineering improves the distribution and          foraging of juvenile Pacific salmon. Ecol. Eng. 151:105856. DOI: https://doi.org/10.1016/j.ecoleng.2020.105856

Cordell JR, Munsch SH, Shelton M and JD Toft (2017a) Effects of piers on assemblage composition, abundance, and taxa richness of small epibenthic invertebrates. Hydrobiologia 802 211-220 DOI 10.1007/s10750-017-3262-8

Cordell JR, Toft JD, Munsch SH and M Goff (2017b) Benches, beaches, and bumps: how habitat monitoring and experimental science can inform urban seawall design, in Living Shorelines: The Science and Management of Nature-Based Coastal Protection (pp. 419 – 436). CRC Press.

Munsch, SH, JR Cordell, JD Toft (2017) Effects of shoreline armoring and overwater structures on coastal and estuarine fish: opportunities for habitat improvement. Journal of Applied Ecology. DOI: 10.1111/1365-2664.12906

Munsch, SH, JR Cordell, JD Toft, E Morgan (2014) Effects of Seawalls and Piers on Fish Assemblages and Juvenile Salmon Feeding Behavior, North American Journal of Fisheries Management, 34:4, 814-827, DOI: 10.1080/02755947.2014.910579

Toft JD, Ogston AS, Heerhartz SM, Cordell JR, Flemer EE (2013) Ecological response and physical stability of habitat enhancements along an urban armored shoreline. Ecological Engineering 57:97–108.

Toft JD, Cordell JR, Simenstad CA, Stamatiou LA (2007) Fish distribution, abundance, and behavior along city shoreline types in Puget Sound. North American Journal of Fisheries Management 27:465–480.10.1007/s10750-017-3262-82017ss