Small overwater structures: a review of effects on Puget Sound habitat and salmon

Small overwater structures (SOWS) like private docks are a common feature of Puget Sound shorelines. Large overwater structures like ferry terminals and commercial piers can cast substantial shadows into the benthos that reduce light penetration, impair submerged aquatic vegetation, and alter salmon behavior and migration. Because of these impacts from large overwater structures, there is concern that SOWS may also impact Puget Sound’s nearshore environments and salmon. Here, we review the available literature on the impacts of marine SOWS. We find that there has been considerably less research dedicated to the impacts of SOWS than there has been for large overwater structures. Even so, there is consistent evidence that SOWS impair eelgrass (Zostera marina) survival and growth. Beyond eelgrass, there is inconsistent evidence that SOWS impact sugar kelp (Saccharina latissima) and we found little research pertaining to other macroalgal species. Studies on associations between fish and SOWS are sparse, although one case study suggests that salmonids avoided moving beneath a single SOWS. To date, there is insufficient research on SOWS in the Puget Sound region to draw robust conclusions about these structures’ impacts or what design elements or restoration approaches might mediate any possible impacts from SOWS. However, we emphasize that an absence of evidence is not evidence for an absence of impacts by SOWS. We conclude by outlining critical research questions and the relevant study approaches that would address data gaps surrounding SOWS impacts and management.