The answer to one of the sea's deadliest hazards for dolphins might be sitting in a recycling bin. Researchers have found that empty plastic bottles, strapped to fishing nets, can keep dolphins from drowning in them - for almost no money.
In southern Brazil, scientists attached upcycled bottles to bottom-set gillnets to act as passive acoustic reflectors: simple sound mirrors that make a net far more detectable to a dolphin's sonar. Over 318 fishing trips logged from 2020 to 2025, the modified nets recorded 88 per cent fewer dolphin deaths. Only a single franciscana - a dolphin the IUCN rates as Vulnerable - turned up in the bottle-rigged nets, compared with eight franciscana and two bottlenose dolphins in the standard nets. Catches of target fish held steady.
The study, published in Marine Mammal Science in May, extends a long-running project led by Per Berggren, professor of marine megafauna conservation at Newcastle University.
"This is a good news story and something that we strive for - a simple solution which benefits both dolphins and the fishers who use the gear," Berggren said. "Attaching plastic bottles to fishing nets can reduce dolphin bycatch globally and is something that every fisher can afford." He summed it up: "This is genuinely recycling that rescues dolphins."
The mechanism is simple. Sound-tank experiments showed a lone bottle could make a net 100 to 1,000 times easier to detect. "It's also rewarding to know that we are using some of the plastic waste that spoils our oceans," Berggren said, noting that "the bottles are securely attached to the nets and we did not lose any plastic bottle during the trials."
It is not a cure-all, as the team readily admits. A parallel study in Fisheries Research trialled bottles across three fisheries - bottom-set nets in Brazil and surface driftnets in Peru and Zanzibar - over more than 1,600 deployments. In the surface fisheries the bottles failed to cut bycatch of dolphins, porpoises or turtles, although they did seem to lift catches of target fish and tuna.
"The difference we saw in the success of the bottom set nets compared to the nets near the surface, may be that the surface water is a noisier environment reducing the efficacy of the plastic bottle reflectors," Berggren said.
Gillnet entanglement is the biggest human-caused threat to many small dolphins and porpoises worldwide, and the method's strength is how little it demands of fishers. "Gillnets are cheap and easy to use," said Michael Osmond, a senior program officer at WWF-US. "But when turtles and cetaceans get entangled, it becomes a problem for the fishers because they have to cut them out of the net and repair it. This can take up to a day, and they lose a lot of time fishing. Also, the quality of the catch is decreased, which means they get less money."
WWF is now running its own trials in Peru, where gillnets can run for kilometres. "There are really big offshore gillnets, like three-kilometres long, and the fishers leave the gillnets for about 10 hours at a time," said Piero Uceda, a marine biologist with WWF-Peru. "We started testing plastic bottles in offshore gillnets, and while we don't have results yet, the good thing is that fishers accept being part of the project. The experimental design has been really participatory."
The takeaway is hard to ignore: in the right kind of net, a throwaway bottle may rank among the cheapest ways to keep dolphins alive.