It has been widely reported that plastic bottles and straws are poisoning the world's oceans, but the real threat to large marine mammals is more likely to be fishing nets.

Whales, dolphins, porpoises, seals, fish, turtles and sharks are all vulnerable to fishing gear, and they are getting into trouble up and down the British coastline. When animals get unintentionally captured in a fishing haul, it's called bycatch. Larger animals such as whales can also get lines or discarded nets wrapped around them, which is known as entanglement. The Museum is part of the UK Cetacean Strandings Investigation Programme (CSIP), and keeps data on whales, dolphins and porpoises that wash up along our coastline. Partners in the programme conduct postmortems on some of the animals to find out why they died. Between 1990 and 2014, the project carried out 3,380 postmortems, and 699 animals were found to have died through becoming bycatch. Kate Swindells, a Cetacean Strandings Research Assistant, says, 'The single most common cause of death for cetaceans in the UK is bycatch, and the worst-affected species are harbour porpoises and common dolphins. 'It's horrible to see. You can often tell what kind of fishing gear killed the animal from its injuries. Porpoises sometimes have circular rope marks around their mouths, and in the tangled whales you see evidence of bigger ropes. 'Entanglement is a major welfare issue causing a horrible death. It often results in animals starving or drowning. Big whales are strong and can sometimes break free of ropes, but they can also end up towing nets behind them for months. Smaller animals just drown. 'It's one of the biggest issues in the waters around Britain at the moment.'

It isn't just a problem in Britain. A paper from Conservation Biology in 2012 examined the cases of 1,762 dead or injured whales from between 1970 and 2009 in the northwest Atlantic Ocean. Entanglement was the primary cause of death. Of the eight species examined, six are endangered or of special concern. Another paper, published in January 2019, reports that numbers of the North Atlantic right whale have been in decline since 2011, largely because of vessel collisions and increasingly fatal and serious entanglement in fixed fishing gear. An entanglement case study: the Scottish Hebrides The summer of 2018 was a record season for whale, dolphin and porpoise sightings in Britain. More and more marine mammals are willing to swim in our waters, whether staying permanently or just passing through. Porpoises can be seen all along the British coastline, but larger mammals are usually seen in Scotland, most commonly in the Hebrides. The Hebrides are an extraordinarily diverse area of the world, thanks in part to a long, complex coastline and a nearby deep-water basin that attracts large whales. More than a quarter of all cetacean species can be found off the west coast of Scotland.

The Hebridean Whale and Dolphin Trust (HWDT) have been monitoring the cetaceans found there for 15 years. In that time, they've recorded 30,000 individuals of 15 marine animal species. The rich dataset provides evidence to policymakers to show the most important areas off the west coast of Scotland for cetaceans and basking sharks. Scientific officers at HWDT say that although the increase in sightings of marine mammals in Britain is a cause for celebration, we need to remember that these creatures are facing significant man-made threats, including entanglement. Minke whales, humpbacks, porpoises and dolphins have all been found snared in fishing gear in the last few years. In the UK, porpoises are most threatened by bottom-set trawling - dragging nets along the seafloor. This occurs more in southern parts of the country. Common dolphins are more affected by mid-water trawling. Baleen whales are more at risk from lines or lobster pots (or creels) - cages made of woven netting that are set on the seabed to catch lobsters and other crustaceans. In Scotland, minke whales, the smallest of the baleen whales, represent 87% of reported entanglements. More humpbacks have also been reported tangled up and dying over the last five years, although sightings around Scotland have been on the rise since 2000. Fishing nets and lines can restrict the movements of these gentle giants, or pin their mouths shut. Even if a whale survives being tangled, heavy ropes can leave lacerations in its skin or slice off its flukes.

Dr Lauren Hartny-Mills, Science and Policy Manager at HWDT, says, 'The west coast of Scotland is an important feeding area for minke whales, and our research has shown that some individuals have been returning to the Hebrides year after year for over a decade. 'Because this is such an important area for these animals, we have partnered with other organisations to form the Scottish Entanglement Alliance to better understand the impacts and risks of entanglement in Scottish waters.' The problem in Scotland shouldn't be overestimated. Many Scottish fishermen have never encountered an entanglement, while others refer to these incidents as a 'once in a generation' occurrence. However, given the scarcity of humpback whales in Scottish waters, it would be unwise to be complacent. What do fishermen think?

Bycatch and entanglement have been a problem for decades. Balancing marine conservation with the activities of the fishing industry can be difficult. Kate says, 'There are very large fishing industries in Scotland and Cornwall, so we see a number of strandings, entanglements and examples of bycatch in these areas. 'It's very important to find a fair balance between conservation and industry. Fishing in these areas, particularly in small vessels, has huge cultural and economic significance. We don't want to take away anyone's livelihood.' Off England's south coast, gillnet fishing is common. Crabs, pilchards and monkfish are all caught in large numbers, and mackerel and herring have been fished in the area for hundreds of years. Scallop dredging also makes up a large percentage of the total value of the Cornish fishing industry. In Scotland, one of the most economically important species fished is the langoustine, which is caught using lobster pots. Fishermen also catch mackerel, cod, haddock and whiting, often using trawlers. Fish farms rearing salmon, trout and mussels bring in millions of pounds alongside the more traditional industries.

Of all of the fishing activity in Scotland, it is often claimed that lobster pots are the most environmentally friendly because they do not disturb the seabed like trawling does, and it results very little bycatch. But this method of fishing involves ropes that can get caught in the baleen of humpback and minke whales, or around their fins and tails. Basking sharks also struggle with them and drown particularly quickly if they get caught. Entanglements can be costly for fishermen too: if ropes get pulled away by a big whale, it can cost someone thousands of pounds in equipment and lost catch. In addition, fishermen care about the wildlife they work alongside and can find it devastating to see these animals caught in or harmed by their gear. Alistair Sinclair is the National Coordinator for the Scottish Creel Fishermen's Federation. He has been creel fishing for 35 years. He says, 'It is vital that all fishermen consider themselves to be custodians of the marine environment and that they want to leave it in good shape for generations to come. 'Creel fishing is generally a low-impact way to fish that doesn't cause too much destruction. But we would love to eradicate the problem of entanglement completely.' So what's being done about it? Entanglements are a concern for conservationists and fishermen alike. Organisations representing both are partnering in the new Scottish Entanglements Alliance ( SEA), which launched in June 2018 and aims to better understand the scale and impact of the issue. The alliance is providing a platform for fisherman-led discussions and an exchange of knowledge about past entanglements and disentanglement experiences. It is also working alongside fishermen to learn more about their practises and discuss minimising the risk of entanglements occurring without disrupting fishing activity. Mr Sinclair adds, 'One of the best ways I think we can prevent whales getting stuck in creel nets is by weighting the nets down properly. Ropes can float about in the water column if they not weighted, so there should be legislation in place to guide people on best practice. All the fishermen I know want to stop this from happening.'

Back at HWDT, the team have started using laser technology for monitoring entanglement injuries. Photogrammetry equipment will take calibrated images of large whales, which the team photograph from their research vessel, Silurian. Lauren says, 'By using the calibrated images we can work out how big the marks and scars on the animals body are, and then work out what caused them. Some marks are caused by other animals and parasites, whilst others are from interaction with man-made items in the marine environment, like plastic and fishing gear. 'This will tell us how many whales are coming into contact with fishing gear, so we can understand the risk of animals becoming entangled.’ And it's not just scientists who are on the case: the British Divers Marine Life Rescue (BDMLR) team occasionally get called out to help cetaceans in trouble. BDMLR are a network of volunteers up and down the country, operating on call - meaning they'll drop everything when an ocean animal gets stuck. They include fishermen, teachers, conservationists and police officers, and each has been rigorously trained in what to do when a whale is in need.

Noel Hawkins is among the volunteers. His day job keeps him busy as a Living Seas Community Officer with the Scottish Wildlife Trust. As part of BDMLR he has saved pilot whales, humpbacks, seals and otters. He says, 'BDMLR have disentanglement teams who come together with special kit to cut marine mammals free. We are all volunteers on call. It's amazing when you can free an animal and save its life. 'We are reliant on calls from the public, and not many people know about the team and the work that we do. The team needs to get out to a whale as soon as possible, so it is vital that people call in when they see an animal in trouble.' The International Whaling Commission is leading on expanding disentanglement teams around the world, and encouraging countries to learn from each other. Underwater noise Some fishing boats, nets and farms are required by UK law to use acoustic deterrent devices (ADDs) or pingers, as part of an effort to scare off predators and combat bycatch. They make a medium- or high-pitched sound that was originally developed to scare seals away from fish farms.

Some research has been done on the overall effectiveness of these deterrents, but so far there has been little consensus.

Kate says, 'ADDs can be difficult to implement in practice. They need to be maintained and deployed correctly by fishermen in order to be effective. 'Some research has shown that deploying ADDs that are not maintained correctly results in patchy noise, which can actually increase rates of bycatch. However, besides having closed fishing seasons or changing gear types, there are few other options currently available to reducing bycatch.'