Shanta Barley, New Scientist 22 Oct 09;
For the first time, scientists have used a submersible to investigate the debris piling up in deep-water canyons off the coast of California. To their surprise, they found that recreational fishing gear accounted for 93 per cent of the underwater trash.
"Sometimes we had to change the path of the submersible to avoid becoming entangled with recreational fishing lines and nets," says Diana Watters of the US National Oceanic and Atmospheric Administration's Fisheries Service in Santa Cruz, California.
"This is a really surprising result," says Anthony Jensen, who studies fisheries and artificial reefs at the National Oceanographic Centre in Southampton, UK, and was not involved in the survey. "Who would have thought that recreational fishers would account for more rubbish in a deep-sea ecosystem than the commercial fishing industry?"
Trash density
Watters believes that previous attempts to quantify underwater garbage by trawling with nets have underestimated the true scale of the problem because that method doesn't pick up all of what's down there and so cannot provide good information about the density of the debris. Nor can nets be dragged over rocky sea floors as they can snag on pinnacles.
To estimate the extent of underwater debris more accurately, Watters' team spent over 400 hours in a submersible called Delta, surveying the sea floor off California. They examined 22 sites, mainly within nature reserves. They noted the density of trash, its likely source and whether or not it had been colonised by fish or corals.
Watters and her colleagues estimate that there are 7000 pieces of trash for every square kilometre of sea floor along the central California coastline. These estimates are up to 3500 times higher than those made by trawler-based studies in other parts of the US, says Watters.
Net loss
The study also found a number of commercial fishing nets snagged over rocky outcrops. "This is worrying, as fishing nets continue to catch and kill fish for years and years after they've been lost in the sea," says Tom Blasdale, a marine fisheries advisor at the UK's Joint Nature Conservation Committee.
"In fact, there's often a cycle: floating nets catch fish, sink to the sea floor, attract predators that eat the trapped fish, and then [the nets] bob back to the surface again," says Blasdale.
Abandoned fishing nets pose such a threat to wildlife that one study estimates that lost gill nets can persist for seven years in the Pacific Northwest's Columbia River, where they annually kill a third of all the white sturgeon that could otherwise be commercially harvested.
Eggs on the line
On the positive side, some of the underwater debris seems to be providing valuable housing and shelter for sea creatures, says survey team member Mary Yoklavich, who is also based at the NOAA Fisheries Service.
"To our surprise, we witnessed egg cases that had been 'hung out' like laundry by sharks along abandoned reams of monofilament fishing line," says Yoklavich. "Sharks are very selective about where they hang their eggs, so the debris may actually be helping them to breed."
Jensen is sceptical about the supposed benefits of trash: "Don't throw your coke cans in the sea just because you think it might become habitat for a sea animal."
Journal reference: Marine Pollution bulletin, DOI: 10.1016/j.marpolbul.2009.08.019
