This project is assessing whether ferry terminals affect juvenile salmon migrating along estuarine and nearshore marine environments and, if so, how design of and modifications to both ferry terminals and operations can mitigate those impacts. This project has three phases. This report summarizes results from Phase I (status of knowledge, initial terminal surveys) and implications for Phase II and III research.
We addressed three issues about over-water structures impacts on juvenile salmon: (1) alteration in migratory behavior, (2) reduction in prey production and availability, and (3) increased predation. An assessment of over 60 direct sources of information found evidence that juvenile salmon react to shadows and other artifacts in the shoreline environment created by shoreline structures. While changes in light have been shown to affect salmon migration behavior and thus potentially place them at increased mortality risk, no quantitative information on the significance of these behavioral responses to juvenile salmon survival were found. Juvenile salmon also encounter limited prey resources under shoreline structures when important habitats such as eelgrass (Zostera marina) are disturbed. Modifications to light, temperature, salinity, nutrient levels, and wave action beneath an over-water structure influence the rate of photosynthesis, plant distribution, and survival of specific plant species that directly or indirectly support prey resource composition and production. Despite considerable speculation about increased predation around docks, quantitative evidence for significant increases in predation on salmon associated with docks is lacking.
We also conducted short-term underwater diving and video surveys at five ferry terminals (Clinton, Kingston, Port Townsend, Seattle, Vashon) to gather preliminary information on the relationships among variations in over-water structures, fish occurrence and relative abundance, light conditions, biological communities, and potential predators. Juvenile salmon were observed migrating under several structures.
Existing information indicates that the effects of shoreline structures on migrating juvenile salmon may vary, depending on the design and orientation of the shoreline structure, extent of alteration of the underwater light field and presence of artificial light. However, our understanding of the significance of the short-term delays in the salmons' migration and cumulative or synergistic effects is insufficient to provide the quantitative relationships that would be necessary as the basis for developing retrofitting or design modifications. Field studies are recommended.