Nocturnal distributions of juvenile Pacific salmon along an eco-engineered marine shoreline

Nocturnal distributions and habitat preferences of juvenile fish along urban shorelines are understudied relative to daytime investigations. As a case study, nocturnal distributions of juvenile Pacific salmon (Oncorhynchus spp.) among ecological engineered and conventional seawall and pier habitats were characterized from May through August 2019 along the Seattle, WA, USA, waterfront. A multibeam sonar mounted beneath a kayak enabled day-night fish density comparisons to identify distribution and habitat use differences. Ecological engineering included mattresses (mesh bags filled with rocks) to create intertidal benches, a textured seawall to increase invertebrate colonization, and embedded glass blocks in an overhanging sidewalk to increase ambient light. Overall juvenile salmon night presence was twice that of daytime, and counts were 1.5 times higher during peak salmon densities. Juvenile salmon presence in eco-engineered and reference habitats was more similar at night compared to day. At night, juvenile salmon avoided traditional under-pier habitats and were more likely to navigate around piers that lack nearshore eco-engineered habitats. Increased use of eco-engineered habitats by juvenile salmon at night highlights the need to incorporate diel fish distribution differences in resource abundance estimates, and in the design and construction of coastline modifications.