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Why do porpoises and dolphins find it so difficult to avoid fishing nets?

When a dolphin or porpoise is caught or entangled in fishing gear it’s known as ‘bycatch’ and it’s the biggest threat they face. Hundreds of thousands die in fishing gear every year, more than 1,000 of these in UK seas. But why does it happen? Why can’t they just avoid the nets?

Harbour porpoise Pevensey Bay 3 CSIP-ZSL (one of pair)

Your donation will help us save lives like his.

Fishers are often looking for the same prey as dolphins and porpoises in the same areas at the same times of year. The accidental capture can happen when nets or ropes are cast, while fishing gear is hanging in the water catching fish and during the hauling of the gear back onto the boat.

Sometimes, it’s not just the time and place of the fishing that’s a factor. The method of fishing can also cause problems. In the eastern tropical Pacific Ocean, for decades schools of yellowfin tuna have been caught by locating, chasing and encircling groups of dolphins. This increases the chance of dolphins getting caught in the net.

In the UK, static gillnets are responsible for more deaths than any other gear type. We know that around 1,000 porpoises and hundreds of dolphins, including 250 common dolphins, die in gill nets in UK seas every year. These nets hang in the water, suspended by floats, and catch any creature that swims into them. We are calling on the UK and devolved governments to phase out these nets and to support fishers to transition to safer, alternative fishing gears. We want to see those with the worst levels of bycatch replaced by 2026.

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What is more perplexing is why dolphins and porpoises, with their sophisticated navigation and communication systems, find it so difficult to spot and avoid the fishing gear. There are several theories and it’s likely that all of these play a part.

Some accidental captures might happen when a dolphin is chasing a fish, trying to scavenge fish from a net or is just plain curious about what is going on. Dolphins are known to forage on fish caught in a net or on a hook. Perhaps they are successful most of the time and so have learnt the reward is worth the risk. However, they may not always be successful and that is when they become entangled or trapped.

It’s also possible that they are distracted when looking for food, socialising and playing. A distracted dolphin may blunder into fishing gear.  At times, dolphins might drift into nets when they are sleeping. When a dolphin is sleeping his or her echolocation ability may be reduced and this may mean that they don’t spot a net.

Sometimes, it could be because the dolphins don’t ‘see’ the nets. Dolphins and porpoises rely on echolocation to interpret the world around them. They produce high frequency clicks which create sound waves. When the sound waves hit an object and bounce back, the dolphins pick up the echoes and create acoustic pictures of their environment. Some fisheries use nets with very thin mesh which may be difficult for a dolphin to detect. Perhaps such materials don’t produce an echo and so it could be that dolphins and porpoises swim ‘blindly’ into them, simply not detecting that they are there. Or perhaps they can detect the nets but do not  perceive the danger.

Common dolphins

It’s also important to remember that fishing gear has become stronger and lasts longer than in the past. Once made of natural fibres, most gear is now made of lower cost but stronger plastic polymers. Dolphins may have bumped into fishing gear for millennia but were able to break free from weaker natural fibre ropes and nets. The newer plastic-based gear is nearly impossible for a dolphin or porpoise to break free from. We’ve a long way to go but we are hopeful that advances in technology will make fishing gear more detectable to dolphins and porpoises. Pingers are proving successful at warning porpoises away from gillnets in some areas by emitting beeping noises. Scientists are testing the impact of using thinner twine which is easier to break.

Scientists are also experimenting with infusing the twine with materials that enhance its acoustics, meaning the dolphins are more likely to detect it with their sonar. While we do not always know why they get caught, we do know that we need to ensure that appropriate political, technical and practical measures are taken to prevent these needless deaths. Every death is a tragedy that we had the power to prevent.

Join our campaign – use our simple form to send a message to the fisheries minister. We know the solutions – we CAN change this.

Please help end these tragic deaths.

Each dolphin and porpoise who dies in a net is a life we had the power to save. Your donation will help us say Goodbye Bycatch.

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About Julia Pix

Communications manager - Public Engagement

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