Immediate and longer-term impacts of hydraulic clam dredging on an offshore sandy seabed: effects on physical habitat and processes of recovery

A hydraulic clam dredging experiment was conducted on a deep (70–80 m) offshore sandy bank on the Scotian Shelf in order to examine the immediate impacts of hydraulic dredging on physical habitat and to follow processes of recovery over a 3-year period. Seabed structural complexity in this low-relief habitat consists of small-scale sedimentary features including pits and bivalve burrow openings as well as polychaete tubes and empty mollusc shells. The most obvious effect of dredging was a dramatic change in seabed topography due to the numerous deep (20 cm), wide (4 m) curvilinear furrows that were cut by the dredges. The loss of burrows, tubes, and shells through destruction or burial, and local sedimentation created a smooth surface. Both spatial scale and sampling resolution were critical in identifying longer-term impacts. Dredge furrows were no longer visible in video 1 year after dredging due to their low relief; however, they persisted, while undergoing changes, as evidenced in sidescan sonograms. The margins of furrows were gradually degraded, likely through the combined actions of slumping, sediment transport and bioturbation. Over time, dredge furrows act as traps for empty shells. Differences in patterns of acoustic reflectance between dredge furrows and the surrounding seabed indicate long-lasting effects on sediment structure. Densities of large burrows were reduced by up to 90% after dredging with no signs of recovery after 3 years due to the high mortalities of their architect, the propellerclam, Cyrtodaria siliqua. Dredging effects were detectable against a background of natural temporal (annual) and large-scale spatial variability in physical habitat.