Ghostnets - Invisible Predators

Thousands of miles from any human habitation, fishing nets lost or abandoned foul huge swaths of the Pacific Ocean. These "ghostnets" continue to fish, untended, entangling and killing fish stocks, marine mammals and birds. While this problem has been known to fisheries managers and fishermen alike, the sheer mass of ghostnets has come as an unpleasant surprise to NOAA scientists. NOAA researchers are developing techniques to identify areas in the open ocean where debris is concentrated and can be cost effectively retrieved.

Because the synthetic materials used in fishing nets decay extremely slowly, lost nets can continue to drift for years. Many end up trapped on the coral reefs, where entanglement rates are even higher than in the open ocean and where they damage the fragile coral. Once snagged on reefs they are extremely costly, time-consuming and dangerous to remove.

Catching Ghostnets

One strategy to prevent reef damage and protect marine resources is to predict areas of the open ocean where debris is likely to accumulate and can be more efficiently removed. NOAA scientists are using satellite and other technologies to predict areas where currents and winds combine to accumulate debris into convergence zones. Ships can then be efficiently deployed to catch the debris.

A recent field deployment to confirm the accuracy of satellite predictions gave the first substantive look into the severity of the ghostnet problem. According to James Churnside, a researcher with the NOAA Environmental Technology Laboratory in Boulder, Colorado, the bottom line is: "There is a lot more trash out there than I expected."

Using data from several satellites, scientists from the NOAA Satellite and Information Service and the NOAA Fisheries Service tracked the Pacific convergence zone through the winter.

Figure: William G. Pichel, NOAA Satellite and Information Service

Researchers used this composite image to identify potential convergence zones and direct flights to map ocean debris. The background shows satellite derived sea surface temperature. Areas where warm and cool colors meet indicate the meeting of warm and cool currents. Blue dots indicate concentrations of chlorophyll. Red lines indicate slicks identified by satellite based radar. The purple line indicates the P3 flight track from Hawaii linking many likely convergence zones.

In late March and early April, Churnside headed a field survey of areas in the Pacific from a NOAA P-3 Orion Aircraft based in Honolulu. The survey was joint project of NOAA and Airborne Technologies, Inc. of Wasilla, Alaska. Over three days, the plane overflew the convergence zone to allow scientists to make visual observations and to use an electronic imaging system with automated pattern recognition to determine how much and what kinds of debris had accumulated. Churnside said that about 2,000 individual pieces of debris were seen. These included at least 100 that were identified as nets or pieces of net. A number were balls of net up to 10 meters (30 feet) across. "One piece of driftnet that was still stretched out, and presumably still fishing, was 200-300 meters long," Churnside said.

Although surprised by just how much material was found in the convergence zone, Churnside said that analysis will need to be done to determine whether convergence zones are more efficient at trapping debris or whether there simply is much more debris than suspected. "Those are the two possibilities, and we don't have data yet to pick one or the other," he said. Meanwhile, based on the success of the Orion flights, planning is underway to develop a cost-effective removal effort. There are also plans to use unmanned aerial vehicles instead of the larger Orion to identify and track the debris in the convergence zones. A pilot project for at-sea removal could begin as early as next year.

This year, NOAA has also undertaken efforts to re-establish a centralized marine debris capability within the agency. The NOAA Office of Response and Restoration is coordinating these efforts by working with Churnside and other NOAA scientists to bring together, strengthen and increase the visibility of activities related to the prevention, reduction and mitigation of debris in the marine environment. One area of focus will be coordinating activities that identify and reduce the impacts of sea-based sources of marine debris (i.e. fishing nets and derelict gear) on endangered, threatened or protected species, and sensitive habitats in United States waters.