This paper synthesizes scientific information on the effects of dredging activities on marine habitats. Direct and long-term effects, dredge methods, regulatory framework, contaminated sediment issues, and a separate bibliography of contaminated sediment-related reports are also presented.
Direct behavioral effects include entrainment, increased turbidity, fish injury due to suspended sediment exposure, decreased dissolved oxygen levels, and the effects of noise. A turbidity threshold of 200 mg/L could reduce dredge-induced salmonid prey-predator reaction changes. High sediment load related fish injury deserves further analysis. Gill injury thresholds specific to marine environments have not been identified. Suspended sediment size, shape, and exposure duration are likely important risk assessment factors for salmonids and other fishes. The most relevant issue is likely the fish ability to avoid plumes and dredge areas. Benthic infauna, epibenthic and demersal organisms, such as burrowing shrimp, crabs, and fish, are subject to entrainment risks. A clearer understanding of dredging effects to biota requires further synthesis of physiology, life-history strategies, water column use, and timing.
In Washington State, maintenance dredging conversion of shallower subtidal to deeper subtidal habitats is much more frequent than new construction dredging conversion of intertidal to subtidal habitats, which is rarely allowed. Loss of intertidal habitats represents potential reductions in coastal habitat carrying capacity. The fish effects of channel deepening alteration to estuarine-freshwater mixing are most evident during early life history stages. Productivity recovery rates range from three months to many years, depending on the species. Lack of long-term and dredge pre- and post-project monitoring makes it difficult to conclusively identify effects.