The shadows from large over-water structures built on nearshore habitats in the Puget Sound can reduce prey abundance and disrupt juvenile Pacific salmon (Oncorhynchus spp.) migratory behavior with potential consequences on survival rates. As part of an ongoing project to reduce the effects of ferry terminals on juvenile salmon, this study looked at the effectiveness of a fiber optic lighting system at mitigating dock shading impacts on juvenile salmon behavior. We conducted intensive visual observations, snorkel surveys, and video filming surveys at the Port Townsend Ferry Terminal (dock) from March through August 2008 and 2009 to test whether migrating salmon reacted to changes in light beneath the terminal and whether evident reactions by the salmon were moderated by the fiber optic lighting system.
We found that during high tides shoals of juvenile salmon (primarily pink salmon O. gorbuscha) were reluctant to swim under the dock and also under the shaded areas. Overall, less than 15 percent of juvenile salmon shoals penetrated under the terminal, and they typically remained within a few meters from the dock. No salmon swam completely under the dock during our observations in the study period. As a consequence of this dock avoidance behavior, ferry terminals likely delay migration for some juvenile salmon (pink salmon) by several hours per dock encounter, during high tide periods, daylight hours and on sunny days.
Our results also indicated that light transmitted or installed under some old and new terminals could mitigate dock shading impacts on juvenile salmon. However, our experience testing both fiber optic-transmitted natural and in situ artificial (halogen) light suggests that such light mitigation systems will need to (1) be more powerful, (2) be regulated to light only shaded areas, (3) operate on a natural light spectrum, and (4) distribute light over a wide area.
The impacts of large over-water structures on juvenile salmon behavior likely alter juvenile salmon migration behavior in shallow nearshore waters, but with an unknown impact to growth and survival. The use of artificial light is a promising mitigation method because fish appeared to respond at a low light level. However, our results were not sufficient to determine whether artificial light could completely mitigate the effects of the dock and eliminate juvenile salmon avoidance behaviors.