Fish trawling reshapes deep-sea canyons

5 September 2012

Source: Nature

Author: Lucas Laursen

Deep-sea trawling smooths out the wrinkles of canyons on the continental slope, making marine mountainsides look more like ploughed fields, changing the habitat of deep-sea creatures. The process rivals landslides and storms as a shaper of the deep sea, according to work published today in Nature1.

For almost a century, fishing fleets have trawled for shrimp off Spain's Mediterranean coast by dragging nets along the flat, shallow coastal sea floor. But in the 1960s, they also started to pursue shrimp farther offshore and into rugged canyons as deep as 800 metres. The impact they had on this rougher terrain was a mystery.

In 2006, geoscientists surveying canyons off Spain's coast found smooth slopes which they attributed to an underwater cascade, but one of the smoothed slopes was in the lee of the proposed cascade2. While trying to come up with reasons, Pere Puig, a marine geologist at the Institute of Marine Sciences in Barcelona, Spain, and his colleagues realized that the anomalies occurred in a trawling zone and hypothesized that trawlers were scraping silt off ridge tops and dropping it into canyon bottoms.

For six months, the researchers measured silt flow in the canyons and took core samples from the sea floor and video footage of a canyon. Then they plotted the silt disturbances on a high-resolution map of the canyons and compared them with four years of detailed fishing records. They found higher silt flow during hours when the trawling fleet operated and smoother canyon walls in areas with the greatest trawling activity, and different sediments in trawled and untrawled regions. The team estimates that trawling has doubled the amount of sediment flowing down into the canyons since the 1970s.

For more, please go to: http://www.nature.com/news/fish-trawling-reshapes-deep-sea-canyons-1.11356