FURROWED FLOOR: Bottom trawling--a fishing technique that drags large nets across the seafloor--has leveled the ocean bottom in the Mediterranean, and likely elsewhere, as shown in this picture of La Fonera Canyon from a remotely-operated submarine. Image: CRG Marine Geosciences, University of Barcelona
Trawlers are smoothing the floor of the Mediterranean Sea, much as farmers flattened fields across Europe centuries ago. And it's likely that similar smoothing is occurring wherever bottom trawlers operate across the Seven Seas.
New research published online by Nature on September 5 reveals that bottom trawling—dragging massive nets across the seafloor to catch food such as deep-sea shrimp—is pushing sediment to fill in gaps on a daily basis, resulting in smooth undersea plains. (Scientific American is part of Nature Publishing Group.) "Fishermen are not doing anything different than farmers many decades ago," explains marine scientist Pere Puig of La Agencia Estatal Consejo Superior de Investigaciones Cientificas (CSIC) in Barcelona, who led the research. "Fishing grounds could be seen as farm fields, but there has to be some limitations to avoid the extension of trawling impacts."
Puig and his colleagues started analyzing the impact of bottom trawling after noticing in 2002 that the fishing technique was stirring up and transporting more sediment than natural processes in an area off the coast of Spain. The La Fonera undersea canyon, near the port of Palamos, sees fishing from some 180 bottom trawlers.
The scientists moored an instrument 980 meters deep in the canyon to measure the flow of sediment from both natural processes and bottom trawling. The daily mud slides and their attendant currents, which moved as fast as 38 centimeters per second, transported more than a metric ton of sediment per square meter. All told some 5,378 metric tons of sediment moved this way over the course of 136 days of monitoring—and the fishing technique has been in place since the 1970s.
The researchers also mapped the canyon using sonar pings, revealing "a noticeable smoothing of bottom topography," they write. This smoothing occurred at depths shallower than 800 meters, which corresponds to the deepest trawls undertaken by the fishing fleet. In addition, plotting the navigation tracks of the fleet via global positioning satellite data from 2007 to 2010 showed a match between the fleet's course and the smoothing.
The effects of this smoothing on the creatures that make a home on the seafloor remain unknown, though the activity could conceivably have impacts ranging from a loss of species diversity due to the loss of unique habitats, to changes in how the ecosystem as a whole functions, Puig notes. He adds that the researchers have "no idea" how ecosystem impacts from seafloor smoothing compare with ecosystem impacts from other bottom trawling-related changes, such as the destruction of deep-water corals. But, unlike farmers, fishing fleets tend to "plough" the deep sea almost every day.
Leveling of the seafloor by bottom trawlers is unlikely to be confined to the Mediterranean coast of Spain. Trawling is an important fishing technology in most of the world's most productive fisheries, from the Gulf of Mexico off the coast of Louisiana to the southwestern Pacific Ocean near Australia and New Zealand. "Intensive bottom trawling has also occurred during several decades in continental slopes elsewhere," Puig says. In effect, human reshaping of the bottom of the sea is another unforeseen sedimentary side effect of the dawning geologic era some scientists have dubbed the Anthropocene.