Seattle’s waterfront is a key migration route for juvenile Pacific salmon including: Chinook (Oncorhynchus tshawytscha), pink (Oncorhynchus gorbuscha), and chum (Oncorhynchus keta). The hardwired tendency of these salmon to inhabit nearshore waters results in close association with coastline urbanization, including piers and seawalls. Part of Seattle’s seawall was replaced in 2018 with enhancements intended to aid the movement and distribution of juvenile salmon. These enhancements include: light-penetrating glass blocks in the overhanging sidewalk to decrease shade in the water below, a bench along the seawall to restore shallow water depths, and textured seawall and shelves for invertebrate colonization. The objective of our research is to study the effectiveness of the seawall enhancements on improving juvenile salmon habitat. We used a high-frequency acoustic camera mounted to the hull of a kayak to quantify salmon and other fish population densities along enhanced and original seawall habitats. Acoustic surveys during April-August 2019 compared salmon distributions and densities 1) between enhanced and original seawall habitat and a reference beach, 2) during day and night, 3) by overhead structures with varying ambient light, and 4) compared to fish densities from snorkel surveys. We found a significant increase in salmon 1) during night surveys compared to day, 2) at enhanced (new) seawall/pier sites compared to old sites, and 3) navigating around old pier ends in deep water . Some implications are that the seawall enhancements are important to juvenile salmon both during the day and night, and that juvenile salmon may navigate more around pier ends along the old seawall that is not enhanced. Results from this study can be used to evaluate the cost-benefit of fish-friendly coastal armoring for the next phase of Seattle’s seawall and at similar sites throughout the world.
Presented by Kerry Accola