More than half a million tonnes of fishing gear is estimated to be lost or abandoned every year in the world's seas and oceans. Some of it entangles and kills wildlife at sea and on shore.
Conservationists call it "ghost gear".
It includes fishing nets, long lines, fish traps and lobster pots left drifting at sea usually after being accidentally lost from fishing grounds or boats, or discarded in an emergency such as in a storm.
"Fishing gear is designed to trap marine organisms, and it can continue to do so long after the gear is lost or discarded in the ocean," says Joel Baziuk of the Global Ghost Gear Initiative (GGGI).
"When lost fishing gear keeps catching fish after its intended lifespan, it is called ghost fishing."
He said ghost gear was the most harmful form of debris to marine life because of the risk of entanglement or entrapment.
GGGI estimates at least 640,000 tonnes of fishing gear are lost or abandoned every year.
The hotspots include the Gulf of Carpentaria in Australia and Hawaii in the Pacific.
Joel says: "Ghost gear is a problem anywhere fishing takes place, and that includes Scotland."
The risks this marine pollution poses to wildlife include entanglement, when animals get wrapped up in rope and other gear.
In Scotland, the Scottish Marine Animal Strandings Scheme (Smass), which investigates marine animal deaths, recorded 12 entanglement cases in 2019.
They included a pregnant whale found dead and tangled in a fishing net in Orkney in October. The net was jammed in the animal's baleen, the filter-feeder system inside its mouth.
In May, a humpback whale entangled in fishing gear washed up dead close to Scrabster, near Thurso on the north Caithness coast.
The previous month, another humpback whale was found to have been entangled in rope for "weeks, if not months" before it drowned off the East Lothian coast near Tyningham.
What are the other risks?
Entanglement is not the only threat posed to whales.
A sperm whale that died after stranding on the Isle of Harris in November had a 100kg "litter ball" in its stomach.
Fishing nets, rope, packing straps, bags and plastic cups were among the items discovered in a compacted mass during an investigation by Smass.
Seals have also been caught up in nets and ropes, though there have been successful rescues of these animals, including the saving of a five-week-old grey seal pup entangled in a plastic net on Lewis.
A hotline run by British Divers Marine Life Rescue (BDMLR) has received 47 reports of entangled seals this year in Britain. Some of the animals were lucky and were rescued, or managed to free themselves.
Other ghost gear victims include animals that forage on shorelines.
In 2017, stags on the Isle of Rum were found with fishing gear caught in their antlers. Two of the animals died after becoming snarled up together in discarded fishing rope, while another stag was photographed with an orange buoy and rope balled up in its antlers.
Even tiny fragments of ghost gear is a risk, say conservationists.
Noel Hawkins, of the Scottish Wildlife Trust's Living Seas project, says: "Some of the small stuff can be as devastating to wildlife as many seabirds swallow it thinking it is fish eggs or food.
"They choke on it and even use it as nest material, which endangers chicks."
What is being done?
Scotland is playing its part in a global effort to tackle ghost gear.
In a GGGI project, divers from the Ghost Fishing UK initiative have carried out underwater clean-ups in Orkney.
BDMLR, meanwhile, is part of the Scottish Entanglement Alliance (Sea), a coalition of conservation groups, rescue teams and fishermen.
The coalition is seeking to find best practices to avoid entanglements and the most effective responses to any incidents.
This year the alliance trained 20 people working in the fishing industry throughout Scotland in how to help disentangle animals.
Public agency Scottish Natural Heritage (SNH), which is involved in Sea, describes ghost gear as a "huge international problem".
Fisheries policy and advice manager David Donnan said: "Fishing gear that has been lost can continue to be a hazard, with ropes and netting frequent sources of animal entanglement.
"To combat this issue, SNH has worked with partners, including fishers, to raise awareness and help develop solutions."
And there have been success stories. In October, BDMLR helped to free a humpback from fishing ropes in Orkney.
New technology, such as prawn creels that can be lowered into the sea and returned to the surface without the need of ropes is also being trialled.
What else is happening in Scotland?
The Scottish Fishermen's Federation says the fishing industry across Europe is "actively engaged" with the issue of discarded gear.
"Very little" fishing equipment is lost at sea by the Scottish fleet, according to the federation's chief executive Elspeth Macdonald.
She says: "Trawl nets are expensive, which means that skippers try to get as much use as possible out of them, and put them ashore to be mended when required.
"The bulk of the ghost gear found in the Scottish sector is monofilament netting used by French and Spanish gill netters and longliners on the west coast."
There is also an effort to clean up ghost gear that washes up on Scotland's shores.
In the north west Highlands, Scottish Wildlife Trust's Living Seas project has been setting up beach clean stations in remote locations.
The stations are large pallet boxes with litter pickers and bags attached and members of the public walking along the beaches are encourage to use the stations to gather up any litter they find.
The project's Noel Hawkins says: "One of these just north of Ullapool at Dun Canna beach has taken in over tonne of rubbish alone."
In July, tonnes of rubbish was removed from the Summer Isles in the north west Highlands in another of the project's clean-ups.
Fishing ropes and nets were among the other items gathered in a clean-up
But Noel says: "It is worth remembering that some estimates think only 3 to 5% of rubbish actually comes ashore though.
"There is still a lot more out there."
He added that the "majority" of the plastics and other rubbish being found on beaches and shores was from Scotland's fishing industry, not material lost by other European boats.
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