Fish Distribution, Abundance, and Behavior along City Shoreline Types in Puget Sound

Abstract

AbstractShoreline modifications, such as bulkheads, riprap, and overwater structures, have altered many of the natural habitats in nearshore urbanized areas surrounding coastal cities, including those in Puget Sound, Washington. The effects of such structures on ecological processes are poorly known, especially those impacting juvenile salmonids Oncorhynchus spp. The goal of our study was to compare the relative abundance and behavior of juvenile salmonids and other fishes along various modified and undeveloped shoreline types. We used enclosure nets and snorkel surveys to sample fishes during high tides in areas adjacent to shore at five main habitat types: cobble beach, sand beach, riprap extending into the upper intertidal zone, deep riprap extending into the subtidal zone, and the edge of overwater structures. Bottom‐dwelling fishes exhibited the only significant differences in density among cobble beach, sand beach, and riprap that extended into the upper intertidal zone. This suggests that substrate type and slope are important influences on fish densities when shoreline modifications only extend into the upper intertidal zone. Differences in pelagic fish density and behavior were more evident when shoreline modifications extended into shallow subtidal waters, truncating the shallow‐water zone and creating deep water at the shoreline. We typically found higher fish densities, larger schools of salmon, and fewer terrestrial riparian insects in salmon diets at these sites. Juvenile salmonids avoided swimming beneath overwater structures, whereas surfperch (family Embiotocidae), crabs (infraorder Brachyura), and sculpins (family Cottidae) were observed beneath or adjacent to pilings. Overall, our results indicate that shoreline modifications have the greatest effect on nearshore fish assemblages when the alterations extend from the supratidal zone into the subtidal zone. Our data suggest that the differences in fish behavior and usage between modified and unmodified shorelines were caused by physical and biological effects of the modifications, such as changes in water depth, slope, substrate, and shoreline vegetation.