Acoustical and sedimentological characterization of substrates in and around sheltered and open-ocean mussel aquaculture sites and its bearing on the dispersal of mussel debris

Side-scan sonar, camera observations, and sediment sampling are used to determine the seabed dispersal of mussel debris at three aquaculture farm sites in sheltered to exposed environments in the Marlborough Sounds, New Zealand. At the two sheltered farm sites, side-scan sonagraphs reveal high acoustic backscatter beneath culture sites and low backscatter in the immediate surrounds. This suggests a sharp contrast in seabed material around the edges of the farm. Photography and sediment samples reveal farms are underlain by mounds of shells with biodeposits infilling intershell voids and forming a veneer over entire mounds. In contrast, the surrounding seabed is naturally sedimented soft mud. Sediment from beneath the farms had total organic contents of 8%-19% decreasing sharply to natural levels of 4%-7%, 30 m from the farm's boundaries. The third site is exposed to energetic waves and currents, and exhibits low acoustic backscatter zones beneath the farm. Photographs show the seabed is predominantly clean sand with only scattered clumps of shells and no biodeposits visible. Sediments are typical of the natural setting. This lack of mussel debris indicates that there is sufficient energy to transport and dissipate the shell and biodeposits over a wide area with little impact on the natural sediment.