Dredging

Over time, sediment builds up on the bottom floor of waterways around harbours, access channels, docks, and ports as a result of storms, tides, and currents. Removing bottom sediment through the direct application of mechanical force to dislodge and excavate material is known as dredging. Dredging is a common practise used to clear benthic navigational obstructions so larger ships can enter ports etc. Of the total material dredged, probably two- thirds is associated with operations to keep harbours, rivers and other waterways from silting up. The other third involves new works. If areas were not dredged there would be less trade and economic input into ports and surrounding communities. However dredging itself often has negative effects on local fishing businesses, as it has in Jamaica. Final stages of the development of the Kingston shipping port included dredging. This was bound to effect local marine life, and thus, local fishing success. The port authority did have to replenish the dredged areas and has replanted 6000 species of corals, sea urchins, and soft corals, gorgonians.

The new Hong Kong airport has caused many areas to be dredged and the loss of coral reefs and seagrasses there has been massive. Sedimentation from disturbing the benthos by dredging will have even further reaching effects on coral reefs and marine life.

Shrimp trawler, San Felipe, Baja California, Mexico. Photo taken by E.A. Norse.

Dredging, like trawling, is a method used to collect scallops. Both bottom trawling and scallop dredging dramatically affect marine ecosystems. The large weighted nets not only catch all marine life in midwaters, whether targeted or caught incidentally, but also cause severe seabed habitat destruction. Sea creatures living on the bottom are crushed, buried or exposed to predators, and clouds of sediments rise, altering seabed biochemistry.

Before and after photos of a healthy cobble-shell bottom habitat for scallops, to simulate one pass of a scallop dredge. Photos taken by Peter Auster, courtesy of the National Undersea Research Center.

About 80-90% of the material dumped at sea results from dredging and currently amounts to hundreds of millions of tons a year. The dredged material is also disposed of in designated areas of lakes, and rivers close to the dredged site itself. Caps of sand are often used to separate the dredged material from aquatic organisms. Open water dumping has the potential to create huge environmental problems depending on the physical behaviour of the dredged material.

Dredging is also practised commercially to collect gravel and sand for sale so disposal is not necessary. However the loss of seabed habitat and life is still an important issue to consider.