This paper synthesizes 30 years of literature documenting the potential effects specific overwater structures pose to important estuarine and nearshore marine habitats for juvenile salmon and other fishes in the Pacific Northwest. While the fish and shellfish species discussed are known to specifically use nearshore habitats, potential impacts at the broader scale of the nearshore ecosystem are also examined.
Overwater structures have been proved to pose potential mortality and fitness risks to these animals and their ecosystems. Mechanisms of impact are characterized as changes in light, wave energy, and substrate regimes. Modifications to these regimes by the construction of, presence of and operations around overwater structures have been found to produce significantly different distributions of invertebrates, fishes, and plants in under-dock environments than in adjacent non-shaded vegetated habitats.
Effects of light limitation (shading) from overwater structures on migratory organisms such as juvenile salmon have been characterized as 1) behavioral barriers that can deflect or delay migration; 2)reduced prey resource production and availability (i.e., "carrying capacity"); and 3) altered predator-prey relationships associated with high intensity night lighting alterations to the nighttime ambient light regime. This paper identifies known visual thresholds associated with light limitation for salmonids and other juvenile fishes.
Empirical findings indicate that the cumulative impacts of overwater structures can have significant impacts on ambient wave energy patterns and substrate types. Given what is known concerning biota and substrate relationships and shoreline geomorphology (drift cell) processes determining those substrates, the basic unit of measurement for establishing change thresholds to identify overwater structure effects is likely related to drift cell characteristics and scale. At this time, drift cell thresholds are not established; however, it is concluded that thresholds are needed to avoid and mitigate cumulative effects. Further studies are recommended to determine plant and animal behavioral thresholds and the nature and extent of direct and cumulative effects.