Dolphins in the Arcachon basin: what the summer's sightings actually show

They are the stars of the summer on the Arcachon basin. Since July, reports of dolphin groups at the channel entrances, around the Arguin sandbank and off Arcachon have followed one another, relayed by videos from boaters and local residents. Fresh reports circulated again in the first days of August. Behind the enthusiasm, specialists are putting a less spectacular but more interesting question: are there really more dolphins this year, or simply more cameras to film them?

What was seen this summer

One of the best documented sightings dates from 16 July 2026, in the middle of the morning: a group spotted between Arcachon and Cap Ferret from a cruise boat, heading towards the Arguin sandbank and then out to sea, on a high tidal coefficient and at the start of the ebb. Other reports followed through the summer, up to the first days of August, around the channel entrances and the Arguin sandbank.

These encounters are nothing new. The presence of small cetaceans in the waters around the basin is a long-standing and documented reality, which the summer season simply makes more visible: the sea is calm, boat trips are numerous, and every boater now has a camera in their pocket.

These images come from Au delà du Bassin, which runs observation trips at sea out of Arcachon and Lège-Cap-Ferret, from Lacanau to Biscarrosse, accompanied by a naturalist guide, Mathieu Ducret. The operator states that it records its sightings for scientific purposes and keeps to the rules set out below: no baiting, no pursuit, distance maintained. The detail matters, because filming dolphins from a boat is acceptable only on that condition.

Which species is actually being seen?

The Parc naturel marin du bassin d’Arcachon is clear: the marine mammals seen regularly are for the most part small cetaceans, and above all the common dolphin. The open data gathered by GBIF confirm this at the scale of the area: out of about 4,100 geolocated cetacean sightings around the basin, the common dolphin alone accounts for nearly half.

Next come the harbour porpoise, discreet and often mistaken for a dolphin from a distance, then the common bottlenose dolphin, the long-finned pilot whale and the striped dolphin. Which is to say that a dorsal fin glimpsed from the beach is not enough to name the animal: the outline, the size of the group and the shape of the back count at least as much. Our profiles of the odontocetes set out these criteria.

More dolphins, or more phones?

This is the heart of the debate, and the honest answer is that it is not known. No local study currently makes it possible to state that the population frequenting the basin has increased. What has changed for certain is the number of equipped observers and the speed at which a video circulates.

On a larger scale, the aerial surveys carried out in the Bay of Biscay give an order of magnitude: nearly 494,000 common and striped dolphins estimated in the summer period, against about 285,000 during a winter survey. These figures cover the whole bay, not the basin, and above all illustrate the seasonal mobility of these species, which follow their prey far more than they settle.

The approach rules, and they apply to everyone

The Parc naturel marin sets out precise instructions. The minimum distance is 100 metres, with a 300 metre vigilance zone around the group. Speed must not exceed 5 knots, steady and matched to the slowest animal. The approach is made from the side, from the rear towards the front, never ahead of the group nor directly astern, and never cutting across the animals’ track.

If the dolphins come closer than 100 metres of their own accord, the boat should be put in neutral and the engine switched off. Observation must not exceed 30 minutes, and less than that in the presence of newborns. Swimming with the animals, touching them or feeding them is prohibited. These rules apply without exception: leisure boaters, kayaks, paddleboards, jet skis, passenger boats, professionals, photographers and drones included.

Why a safe distance matters

A group surrounded by several craft stops feeding, resting or nursing. The energy spent on fleeing is not spent on hunting, and a calf separated from its mother in the confusion pays dearly for a few minutes of curiosity. The Arcachon channel entrances, narrow and very busy in August, combine heavy traffic with the risk of collision. The causes of mortality of the small cetaceans of the Bay of Biscay remain linked first of all to bycatch in fishing gear, but repeated disturbance adds to them without any need.

What to take away

That the summer of 2026 is indeed a fine season for sightings on the basin, without anyone being able to speak of a return or of a population explosion. That the most likely species behind a dorsal fin is the common dolphin. And that the best way to enjoy these encounters is to let them come: engine off, at a distance, without giving chase. The favourable period runs from May to September. Every sighting, finally, is worth reporting on the OBSenMER platform, where it becomes a piece of data useful to scientists rather than one more video.

Sources: France 3 Nouvelle-Aquitaine, Bassin d’Arcachon and Parc naturel marin du Bassin d’Arcachon. Distribution figures established from GBIF open data.