Each year, at least 640,000 tonnes of nets and other fishing gear goes overboard and never comes back. But just because it’s lost to the sea doesn’t mean that derelict gear stops doing its jobs. The lobster pots, crab traps and dense thickets of nets that litter the sea bottom keep snaring fish and other animals for years or even decades after they go missing.

It’s impossible to estimate how many marine animals are killed each year by “ghost fishing,” as the problem is known. However, the mosaic of local reports suggests staggering numbers—many of them of commercially valuable or endangered species. In Puget Sound, ghost gear is thought to kill more 3.5 million animals a year, including nearly 25 seals, porpoises and other marine mammals a week.

This is obviously bad for fishermen. In the case of Puget Sound, around $335,000 worth of Dungeness crabs (pdf, p.13) are killed annually. However, because ghost fishing is largely an unseen phenomenon, only a dozen or so local governments around the world have implemented measures to stop this. To date, much of the more daring cleanup efforts have been spearheaded by teams of volunteer divers in 15 countries around the world that have formed a group called Ghost Fishing, which often works in partnership with governments.

Launched in 2009, Ghost Fishing grew out of a group of recreational shipwreck divers in the Netherlands that began collecting derelict fishing gear during dives in the North Sea. Shipwrecks, it turns out, are often ghost fishing hubs, says Chuck Kopczak, a diver with Ghost Fishing’s crew in California.

“The perversity of the whole thing is that fish and other marine life tend to be attracted to structure on the bottom—whether it’s a rocky reef or coral reef or a shipwreck. So fishermen will set their nets above the wrecks,” says Kopczak. When a net becomes untethered, it’s easy for it to snag on one of the wreck’s many jagged edges. “These manmade structures are almost like magnets for these things.”

Because more recent generation of nets are designed to be hard for fish to see, fish and other animals are easily entangled. Sharks and other predators that approach the entangled fish as bait often become ensnared themselves.

Depending on the species, and the type of net, ghost nets catch anywhere from 5% to 30% of the number of target species caught by fishermen’s nets. All told, a single net can kill hundreds of pounds of commercial species per day, along with a slew of other animals.

NOAA Marine Debris Program (this image has been cropped) Divers release a seal from marine debris entanglement.

Gillnets are particularly destructive, says Heather Hamza, the US coordinator for Ghost Fishing. These are anchored to the seabed to form a vertical face of net that can span several miles and snag the gills of fish or appendages of other animals as they struggle to escape (hence the name). Kopczak notes that since California banned the use of gillnets close to shore in the 1990s, the area has seen a resurgence in populations of great white sharks and sea lions.

The most common causes of ghost nets are bad weather or an errant boat propellor. Because nets can cost tens of thousands of dollars, many coastal fishermen now attach GPS sensors so that they can track and retrieve untethered nets.

Overburdened nets can sink ships, such as the Infidel, a 70-foot trawler that sank off the coast of California in 2006 when it tried to haul up too much squid. The Infidel settled 150 feet down with around 9,000 pounds of nylon netting (pdf, p.24) still deployed, billowing over the ship from its mast to the sand. Paid by the Infidel’s owner to retrieve certain valuables, commercial divers found a dozen dead sea lions, several sharks, and a slew of other animals that had died trying to feed on the squid. The Infidel’s nets remained in place until Ghost Fishing removed them in 2012.

Not all cleanup operations involve scuba diving—particularly in parts of the world where currents tend to wash ghost nets and other debris near shore, such as in northern Australia. Backed by government funding, the Ghost Fishing affiliate there has cleared 13,000 ghost nets since 2004. A slew of governments have launched similar programs to clean up ghost gear from shorelines. Local US governments have also paid fishermen to collect derelict lobster pots and crab traps.

Gear lost in deeper water is much trickier, though. While South Korea has tried to drag the seabed (paywall) with heavy hooks—a method that can also harm marine life—scuba diving is generally the way to go.

Dive-team retrieval is potentially dangerous, though. Cutting away nets is tricky enough that the Ghost Fishing divers are trained in military-level diving techniques. The biggest danger happens when the divers start cutting free a ghost net, which they attach in segments to ”lift bags,” which look sort of like undersea hot air balloons. Once the net is slashed free, the lift bags shoot to the surface, net in tow; if the diver making the final snips doesn’t get out of the way fast enough, she could be pulled up with it.

Shingo Ishida Ghost Fishing divers cut away net from the Infidel, a squid boat that sunk off the coast of California in 2006.

This form of retrieval is also expensive. And since the expense usually varies with how long gear has been in the water—one study found retrieval efforts ranging between $65 a tonne and $25,000 per tonne (pdf, p.44)—the cost-benefit analysis isn’t always clear, which is perhaps why most governments haven’t tackled the problem.

One notable exception, however, is the state of Washington in the US, which just budgeted $3.5 million toward the derelict fishing gear removal program the state established in 2002, in partnership with a Ghost Fishing affiliate. The initiative has removed more than 670 acres (1 square mile) of nets—most of them gillnets used to catch salmon (pdf)—and 3,200 crab pots from Puget Sound—gear that had entangled nearly 330,000 animals. So far the project has only cleaned up waters shallower than 105 feet; the group is seeking funding to retrieve the 200-plus nets it’s found in deeper waters.

Northwest Straits Foundation Where Puget Sound’s ghost gear accumulated.

Washington is also leading the way in preventive measures. Starting in 2012, the state began requiring fishermen to report lost gear within 24 hours so that it can be retrieved. This mandatory reporting approach is probably the most effective way to reduce ghost fishing, says Ghost Fishing’s Hamza.

“When a net is newly loss not only is it exponentially easier to cleanup because it’s not yet tangled, but it also hasn’t killed any animals,” she says.

Particularly in cash-strapped US states like California, lawmakers are unlikely to prioritize similar programs. Fortunately, public awareness around the issue is growing, says Hamza. Ghost Fishing hopes that its new partnership with the Discovery Channel, which is promoting the group during the channel’s famed “Shark Week” program that’s on during evenings this week, will help boost that even more.

Public awareness can also be an important force in influencing fisheries to adopt sustainable and responsible methods, say Hamza and Kopczak. By eating only seafood that has been caught without using highly destructive gear—like those nearshore gillnets that California banned—consumers can limit the number of other animals that died in the process of bringing seafood to their tables. (Monterey Bay Aquarium’s Seafood Watch is a helpful guide, and it has an app.)