Why do common dolphins venture so close to our shores?

Why do common dolphins venture so close to our shores?

Violet Field inPlanetJan 20, 2021 4 min read3 views

The species is rather accustomed to evolving in great depths and stranding is increasing due to the tides

Common dolphins swim off the French coast. An increasingly common phenomenon that questions scientists. - Rex / SIPA
  • Dolphin watchers are surprised to see the common dolphin species approaching closer and closer to the French coast.
  • In Brittany, two of them were saved from certain death while they were prisoners of the mud in the bay of Mont Saint-Michel.
  • Scientists have trouble explaining the displacement of this population, which is more used to the deep seabed and which suffers from accidental fishing captures.

They were both able to return to sea after experiencing an episode of intense stress which could have been fatal to them. On Sunday, January 10, two common dolphins were saved from certain death by the Cancale firefighters and Gaël Gautier, coordinator of the Al-Lark association. Not all are so lucky. On Monday, one of their fellows, dead, ran aground on the coast in Saint-Malo, more used to welcoming bottlenose dolphins, a species used to the coast. “Common dolphins tend to live offshore, in the Bay of Biscay. When they approach our coasts, they find themselves trapped by the tides, ”explains Gaël Gautier.

For five years, the number of strandings on the Breton coasts has tended to increase, according to all observers of the species. The reasons for this new phenomenon are, however, difficult to establish. “The common dolphin is an oceanic species, which lives on the continental slope (a zone of transition between shallow depths and the abyss) but which we see getting closer and closer to the coasts. We know that its movements are linked to food prospecting. It is undoubtedly their prey which leads them towards these danger zones ”, explains Sami Hassani, coordinator of the stranding network which manages the care center of Océanopolis, in Brest.

Fishermen are seeing more and more ...

These “dangers” mentioned by the scientist are multiple. If they are sometimes trapped in the tides by following schools of mackerel and sardines, common dolphins are especially caught accidentally by fishing boats, numerous in these areas full of fish. A phenomenon strongly denounced by the NGO Sea Sheperd, whose positions often piss off fishermen. The French state has even been condemned for these facts. “Fishermen tell us that they are seeing more and more dolphins. But that doesn't mean there are more of them. We cannot have a spontaneous increase in populations because it is a slow-growing species, ”said Willy Dabin, a member of the Pelagis observatory.

From January 1, 2021, trawlers will have to be equipped with pingers in order to keep cetaceans away in the Bay of Biscay. We will not stop there. The players in the fishing industry are willing to go further. Together, we will make fishing more virtuous. pic.twitter.com/kQxF9Gr3iO

- Annick Girardin (@AnnickGirardin) December 3, 2020

Based in La Rochelle, this engineer also has no answer as to the increased presence of cetaceans near the coast. At the beginning of January, 32 common dolphins stranded on the island of Ré (Charente Maritime), and three died. “The most pessimistic hypothesis would be that the resource is over-exploited offshore. The most optimistic would be that the resource would be more and more abundant near the coast ”, continues the engineer.

The warming of the waters in question

The warming of the waters, under the effect of climate change, is also an advanced avenue even if it cannot be confirmed today. However, population displacement phenomena have been observed in recent years among cousins of the dolphin. “We have seen porpoises migrating south probably because of the warming of the North Sea, which is shallow and more sensitive to warming. Cetaceans are a very mobile species. Maybe she goes through the same thing, ”suggests Sami Hassani.

To assess the extent of the phenomenon, aerial overflights of the French Atlantic coast should take place in the coming months. Scientists hope to take the opportunity to get a better idea of the cetacean populations living near our coast.