• 450 dolphin carcasses have been found on the beaches of our Atlantic coast since the beginning of December. A phenomenon that has risen sharply since 2016 and is linked to incidental catches in fishing vessel nets.
  • In recent years, several research projects have emerged to equip ships with repellent devices, acoustic signals that dolphins do not understand but that disturb them and then move them away from the nets.
  • The "DolphinFree" project by Montpellier biologist Bastien Mérigot is trying a new approach by trying to send dolphins an informative message. Or the image that dolphins associate with nets in their language.

How do you say "net" in dolphin? More than a word, it is an image that Bastien Mérigot, lecturer at the University of Montpellier and researcher at the UMR Marbec, a research unit on marine biodiversity, is looking for. "To find their way around and detect their prey, dolphins use echolocation," he explains. They emit acoustic signals and receive in return echoes that they are a priori able to decode in image and thus obtain information about their environment."

Bastien Mérigot then sought to identify and reproduce the echo that these dolphins would have if they had emitted an echolocation signal on a net. "In other words, we try to make their job easier," explains the biologist.



Strandings, the emerged part of the hecatomb

The project is called "DolphinFree" and was launched in 2020. The context, we know it: massive strandings of common dolphins on our beaches, every year from December to April. If there have always been, they have risen sharply since 2016, notes the Pelagis observatory. Especially on the Atlantic coast. There were 1,149 in 2019, 1,299 in 2020, 669 in 2021. "Since December, we have been at 450 and the stranding season is not over," says Cédric Marteau, director of the "Nature Protection" division at the LPO. Just the tip of the iceberg of the hecatomb, he insists: "The majority of dolphins caught in the nets sink on the spot or are swept away by currents out to sea." "Pelagis estimates between about 4,000 and 8,000 common dolphins killed by fishing each year in the Bay of Biscay in recent years," confirms Bastien Mérigot.

There is little doubt about the cause of these strandings: "about 90% of the carcasses examined bear the stigmata of incidental catches from fishing," says Bastien Mérigot. But why especially since 2016? "We don't really know," he says. This could be linked to the spatial redistribution, closer to the French coast, of fish populations on which dolphins feed. The hake, the bar, the sole, the place... The same ones that fishing boats target. Everyone ends up navigating in the same corners, increasing the risk of interactions, often fatal for dolphins.

Pingers on trawlers, nothing on netters?

The solution may come from technology. NGOs doubt this (see box). However, since 2020, the twelve pairs of pelagic trawlers active in the Bay of Biscay (from southern Finistère to the Basque Country) must be equipped with "pingers". These acoustic repellents are installed on the boat and emit unpleasant sounds to the dolphins to keep them away. "Tests during the winter of 2017-2018, on pelagic trawls, showed reductions in incidental catches of 65% on average," says Bastien Mérigot.

Better, but not perfect. Above all, pelagic trawls are not the only ones to ply the Bay of Biscay. There are also the netters, those boats that deposit their nets on the seabed before returning to raise them later. Bastien Mérigot speaks of 400 French ships in the area, "some using nets of several kilometers". The obligation to equip themselves with repellent pingers does not apply to them. "They are too heavy for their nets and would impact fishing operations," says the biologist. The risk also, by equipping them all, would be to potentially exclude dolphins from too large areas. »

Put yourself in the shoes of a dolphin

Several research programmes have been launched with a view to adapting distancing devices to spinners. Pressed by the European Union to act, the France updated in January its plan to combat incidental catches, which provides for large-scale testing of three of these projects next winter.

DolphinFree is one of them and relies on a new, bio-inspired approach. "Repellent signals are alarms: dolphins move away because they are disturbed by noise, but without having identified its meaning," explains Bastien Mérigot. Instead, we sought to produce an informative, understandable signal from common dolphins. »

Better, but not easy to develop, "if only because dolphins emit on a very large frequency band, far beyond what our ears can hear," says the biologist. A first step was to record dolphin echolocation signals. Then, Bastien Mérigot's team went to sea to retransmit these signals on a net, as a dolphin would have done. "We still had to record the echo we got in return, the acoustic signature of the net in the dolphin language. It is this echo that our beacons emit. »

Dolphins less stressed?

Initial feedback is encouraging. The beacons, in the shape of a tube, were tested at sea with about fifty groups of wild dolphins. "They communicate much more with each other and echolocate more after receiving our signal, suggesting that they have more possibilities to detect the net and the associated dangers," he said. Above all, unlike acoustic repellents, they leave the danger zone in a much more peaceful way to resume their activity a few hundred meters away, without being excluded from their habitat areas. A dozen netters have equipped their nets with these beacons between 2021 and 2022, making it possible to test them on more than a thousand fishing operations. For zero accidental capture! "Well, yes, three," corrects Bastien Mérigot, "but they were related to fishing incidents. A net, for example, remained ten days at sea, because it could not be recovered by fishermen, the day after it was launched. The beacon's batteries eventually fell flat. »

The scientist is working on a new version of his beacons, smaller and lighter, so that "fishermen can install and unhook them with as few constraints as possible". The goal is to have it ready by December, when the large-scale experiment sought by the government will begin. Sixty-three netting vessels will then test the beacons developed by Bastien Mérigot and his teams. Until the end of April 2024.

Towards spatiotemporal closures in winter?

All the better if DolphinFree and the other projects are effective, says Cédric Marteau. "But it will take time and unfailing scientific rigour to evaluate the real and long-term effectiveness of these devices," continues the director of the "Nature Protection" division at the LPO. However, with up to 8,000 individuals killed in some years and a population estimated at 180,000 common dolphins in the Bay of Biscay, "these incidental catches could quickly endanger the species," he worries. Together with others, the NGO is therefore requesting, as an emergency measure, the spatio-temporal closure of fishing in the Bay of Biscay. "In the sectors most at risk, for the fisheries most involved and during peak catches, at least from mid-January to mid-March," explains Cédric Marteau. And it would, of course, be a question of compensating the fishermen affected. »

The government has not done so to date. France Nature Environment, Sea Shepherd and Aquatic Environment Defense are hopeful of forcing it to do so. The three NGOs had appealed to the Council of State to this effect in 2021. In her opinion delivered on 24 February, the public rapporteur called for the introduction of spatio-temporal closures. "A very good sign," comments Cédric Marteau, who is now waiting for the judge's decision. "Probably by Friday."

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