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Ghostnets

Ghostnet refers to lost or abandoned fishing gear that drifts in the ocean continuing to catch fish and entangle marine mammals, turtles, and sea birds. The synthetic materials currently used in fishing nets decay extremely slowly, so these nets can continue to drift for years. Many of these 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. To remove these nets from reefs, divers must cut the nets off with knives and load them into inflatable boats. It is extremely laborious and dangerous work. During 2001, a multi-agency effort consisting of 3 ships and 18 divers removed nearly 70 tons of debris during 270 ship days at sea. clearing only two atolls in the 1200-mile Hawaiian Archipelago.

Imaging lidar image of an active net.
Imaging lidar image of an active net.

Satellite
image of an eddy in the Gulf of Alaska.
Satellite image of an eddy in the Gulf of Alaska.

Given the magnitude of the problem and hazards associated with cleaning the reefs, a multi-agency effort is under way to locate ghostnets in the open ocean and collect them before they reach reefs. (Visit High Seas Ghostnet homepage) The ghostnet team has shown that many of these nets pass through the Sub-Tropical Convergence Zone north of the Hawaiian Islands in the spring of each year when the convergence is particularly strong. Oceanic debris of all types is expected to accumulate here and in other convergence zones. For this reason, searching for ghostnets and other debris in convergence zones was expected to be more efficient than searching the entire ocean.

The Ghostnet team used satellite imagery to locate convergence zones in the North Pacific. An aircraft equipped with a suite of remote sensors was directed to these zones to see if there was, in fact, a higher concentration of debris in these areas. The sensors included an imaging lidar, visible imagers, and an infrared imager. The figure shows an eddy in the Gulf of Alaska that is located in the sea-surface height image from a satellite. This type of eddy indicates convergence of the surface currents. The locations of detections are presented as circles on the flight track across the eddy. It is clear that there are more detections across the eddy than on either side, so this type of search is much more efficient than a random search of the ocean. The next step is to tag nets during an aerial search, so they can be located and retrieved by surface ships.