What happens under that mussel farm?
Science Centres: Fisheries
New Zealand has over 550 mussel farms totalling 4500 hectares, most of them in the Marlborough Sounds. Large farms proposed for open coastal regions could result in a 10-fold expansion in area in the near future, and increased awareness of mussel farming’s impact on the marine environment.
Side-scan sonar images of the seabed beneath mussel lines.
Photographs of mussel farm seabed corresponding to the areas marked on the sonar images.
Mussels produce faecal material which may accumulate on the seabed, depending on the amount of water movement under the farm. At present, little is known about the dispersal of these deposits in and around the farms, but it is an important factor to consider in the sustainability of mussel farming.
A side-scan sonar (which can map large tracts of seabed quickly) was used to map the dispersal of these deposits at three sites in the Marlborough Sounds. Two of the sites were in low-energy environments with currents of less than 8 cm/s and little wave energy. The third site was in a much more turbid environment with maximum currents exceeding 25 cm/s.
Side-scan sonar transmits a pulse of sound towards the seabed and then detects the reflected acoustic energy (or backscatter). Objects such as rocks and shells show up as dark features, with finer sediments, such as clays and mud, appearing as lighter shades of grey. Mussel debris consists of shells and fine, organically enriched mud. The shells increase the roughness of the surface of the seabed and generate dense backscatter on the side-scan record.
At the two low-energy sites the mussel debris predominantly occurred directly beneath the farm. The sonar traces showed a strong contrast between the area under the farm and the areas immediately around it, highlighting the distinctive nature of the mussel shell debris compared to the natural sediment deposits.
By contrast, at the high-energy site there was little sign of any mussel debris under the farm. The seafloor sediment was relatively homogeneous, indicating that there was sufficient wave and current energy at this site to transport and dissipate the mussel debris over a wide area, making it difficult to detect any changes to the natural sediment.
In summary, side-scan sonar is a useful tool for seabed mapping in mussel culture areas. Mussel debris is readily identified. Side-scan imagery can easily map large tracts of seafloor, and, in combination with grain size data and photographic records, can provide a good understanding of the sediment dispersal footprints of mussel farms.