Although large over-water structures alter nearshore habitat in a number of ways, little work has been done to study how docks affect nearshore fauna. In Puget Sound, juvenile chum, pink, and oceantype chinook salmon migrate along the shorelines and feed extensively on shallow water epibenthic invertebrates. As part of an ongoing project on the effects of ferry terminals on juvenile salmon, this study looked at the effects of large overwater structures on juvenile salmon and their prey. The epibenthic assemblage was sampled for juvenile salmon prey with four sampling regimes: monthly-stratified sampling of epibenthic invertebrates at three terminals, one-time eelgrass patch at a single terminal, onetime high-resolution cross-terminal at a single terminal, and one-time terminal structure sampling at two terminals. The response variables tested included taxa richness and densities of (1) total epibenthos, (2) total juvenile salmon prey, (3) common or abundant salmon prey taxa and (4) common or abundant nonsalmon prey taxa.
Both the stratified-monthly and eelgrass sampling indicated that terminals negatively affected all summary response variables and many individual taxa. High-resolution cross-terminal sampling results were less clear, but the negative impacts of the terminal were evident for some taxa. Finally, terminal structure sampling results showed some differences in assemblages on different structure-types and elevations, and an overall smaller abundance of epibenthos on terminal structures than on intertidal sediment and benthic vegetation. In general, these results agreed with impact predictions based on vessel disturbance (propeller wash) and shading of benthic vegetation, and with assessments of these attributes completed during the sampling season. The researchers concluded that decreases or changes in the epibenthos density, diversity, and assemblage at these large overwater structures were probably caused by four interacting factors: direct disturbance or removal by vessel traffic, reduced or compromised benthic vegetation, physical habitat alterations, and biological habitat alterations.
Washington State Transportation Center (TRAC)
Docks, Ferries, Habitat (Ecology), Invertebrates, Marine terminals, Salmon.