Once out of the hands of a fisherman, abandoned, lost or discarded gear often ends up floating on the surface of the water with ocean currents, or sunken toward the ocean floor, where it can smother coral reefs and other delicate habitats.
"Any object that floats in the ocean will start to grow organisms on its surface in a process known as bioaccumulation, or fouling," Stelfox said. "It is believed that this acts like a food source for different species and the (fishing gear) also acts as a form of shelter - essentially it forms a mini ecosystem. However, fish will become entangled in the nets during feeding or hiding and this attracts more species and larger predators such as sharks, turtles, etc."
This domino effect of trapping and killing perpetuates the ghost fishing cycle.
Stelfox and his colleagues, Jillian Hudgins and Michael Sweet, also found that the same cycle applies to lost traps and pots. Stelfox explained that "trapped animals inside the traps and pots act like bait for more animals," thereby continuing the sinister ghost fishing.
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The researchers emphasize that the majority of fishermen do not want to lose or discard their gear. Stelfox said that often gear is lost during times of stormy weather, operational damage, by mistake or due to other factors.
Some fishermen, however, do intentionally dump their gear because they lack convenient port-side disposal facilities, or for other reasons. The dumping is particularly common among those who were already engaged in illegal, unreported and unregulated fishing.
The scientists estimate that 640,000 tons of such equipment wind up in oceans each year, equating to 10 percent of the world's marine debris. Those figures are likely on the low side, given how difficult it is to track the lost fishing gear.
For the new study, the scientists carried out an extensive review of journals and other reports to compile a list of losses.
Humpback whales were the most commonly caught animals, followed closely by the North Atlantic right whale. Many of the whales showed signs of being entangled before. Antarctic fur seals, California sea lions, manatees, sea turtles and sharks were also frequently entangled, according to the study. Ghost fishing nets and monofilament lines posed the most frequent threats, the researchers found.
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Stelfox said that "small scale fishers are now moving away from traditional nets made from natural materials to nets made of synthetic materials" like nylon. Additionally, monofilament superfine nets are favored now in many areas. They can be damaged easily, increasing the chance they'll become ghost gear.
Oona Lönnstedt, a postdoctoral researcher at Uppsala University's Evolutionary Biology Center, told Seeker the gear poses other dangers. Fish and other marine life sometimes eat plastic debris, especially if it exists in smaller pieces. She said that such long-lasting pollution "highlights the need for replacing plastic products with biodegradable options instead."
Stelfox and his team propose that, in addition to removing as much of the harmful gear as possible, authorities need to work with fishermen.
"For instance," the scientists wrote, "a fisher incentive program to deposit old or damaged nets at designated collection points was met with resistance in South Korea; but, after educational workshops, the program quickly became a success with Incheon City collecting 18,000 tons of derelict gear in only 4 years." Financial support helped this and other programs, rewarding fishermen who safely disposed of their nets.
Marking all fishing equipment to identify its source can help reduce the problem, as can technology. Prior research has shown that video, thermal imaging and radar used in manned or unmanned aircraft can be effective ways to locate the ghost gear.
Photo: A shark is entangled in a fishing net. Credit: Prodivers Maldives, Oliver Ridley Project
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