Submarine vehicle Liropus 2000 and trawl marks on the seabed. (Photo: Geociencias Marinas, UB)

Bottom trawling levels the seabed

WORLDWIDE
Friday, September 07, 2012, 22:40 (GMT + 9)

Deep-sea trawling smoothes out canyons on the continental slope and thus changes the habitat of deep-sea creatures, according to work published this week in Nature.

Scientists have determined that bottom trawling moves sediment lying on the seabed, displaces or harms some marine species, causes pollutants to combine with plankton and enter the food chain and breeds harmful algae blooms, Reuters reports.

In 2006, while studying canyons off Spain's Mediterranean coast, geoscientists discovered smooth slopes which occurred in a trawling zone. Pere Puig, a marine geologist at the Institute of Marine Sciences in Barcelona, Spain, and his team hypothesized that trawlers were sloughing silt off ridge tops and letting them fall into canyon bottoms.

For six months, the researchers measured silt flow in the canyons and gathered core samples from the sea floor plus video footage of a canyon, positioned the silt disturbances on a high-resolution map and compared them with four years of fishing records.

The team found higher silt flow when the trawling fleet operated and smoother canyon walls where the greatest trawling activity occurred, plus different sediments in trawled and untrawled regions.

The scientists believe that trawling has doubled the amount of sediment flowing down into the canyons since the 1970s.

"Bottom trawling has been compared to forest clear-cutting, although our results suggest that a better comparison might be intensive agricultural activities," the team wrote.

This effect lowers the number of species that can live there, said Elliott Norse, chief scientist at the Marine Conservation Institute in Bellevue, Washington. Further, it will change the make-up of species, said marine biologist Callum Roberts of the University of York, UK.

Bottom trawling might have impacts ranging from a loss of species diversity due to the eradication of unique habitats, to changes in how the ecosystem as a whole works, Puig noted.

Puig’s team now wants to survey the biodiversity of trawled and untrawled slopes.

In July, the European Commission (EC) said it wanted to ban the activity throughout the waters of the European Union (EU). Industrial fishing groups, however, will lobby to modify the proposal.

Puig thinks bans should be considered on a case-by-case basis because sustainable fisheries could materialize where the geological and ecosystem damage is "already done".

By Natalia Real
editorial@fis.com
www.fis.com

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