Offshore wind and marine mammals: the sensitivity map for France

Where would the siting of offshore wind turbines cut across the interests of cetaceans most? Cerema and the Cohabys team have published a quantified answer, grid cell by grid cell, for all French waters: the Atlantic-Channel seaboard and the Mediterranean. This page offers an interactive reading of it, which can be overlaid on the areas under consideration for offshore wind.

How to read this index

The sensitivity index is not read as a density of animals. It combines three things: the estimated presence of the species in the grid cell, their vulnerability to the nuisances of a wind farm, in particular construction noise, and their conservation status. A rare and fragile species therefore weighs more heavily than an abundant and tolerant one, at equal presence.

The presence indices, for their part, range from 0 to 1 and are read more simply: they express the relative probability of encountering the species group concerned in the grid cell, in the season indicated. The harbour porpoise, the common bottlenose dolphin, the common dolphin, the long-finned pilot whale and the rorquals are treated separately, because they frequent neither the same depths nor the same seasons.

Three limits of this data layer

Three limits are worth knowing. First, the official metadata themselves report methodological biases that lead to under-weighting certain species. Next, the summer layer for the small delphinines covers only the English Channel, between 50.1 and 51.65 degrees of latitude, on a different scale from its winter counterpart. It is offered in the map, but its label says as much: elsewhere, the absence of colour signals an absence of measurement, not an absence of animals.

Two seasons are missing, and the absence comes from the data, not from the site. In the Atlantic-Channel area, rorquals are supplied for summer only: this is the only group on that seaboard with a single season, all the others exist in summer as well as in winter. In the Mediterranean, the reverse holds for the globicephalines, absent in summer. Cerema does not document these gaps; the most likely explanation is a number of contacts too small to feed a model, winter aerial surveys collecting few sightings of rorquals in the Bay of Biscay. This remains a hypothesis on our part.

Finally, the scales differ from one season to another: the annual index rises to 42.3 while the winter index peaks at 15.6. Comparing two seasons visually would make no sense, and each layer is therefore displayed with its own colour scale.

Ecological importance or sensitivity, two different indices

The map offers both, and they do not say the same thing. Ecological importance measures what is present: how much the area matters for the species concerned. Sensitivity adds to that the specific vulnerability of those species to a wind farm. An area can therefore carry a high ecological importance without being the most sensitive, if the species that frequent it tolerate the nuisances better.

Mediterranean, seabirds and distributions

The seaboard selector gives access to the Mediterranean, with its own species: no porpoise, but the common bottlenose dolphin, the striped dolphins, the globicephalines and the rorquals, including the fin whale of the Pelagos Sanctuary. Two further layers give the modelled distribution of the porpoise in the Atlantic and of the common bottlenose dolphin in the Mediterranean, calculated separately by Cohabys with their own grid.

The group selector finally makes it possible to switch to the seabirds, present in the same publication. Only the aggregated layers are offered: the source data contain some twenty species groups designated by abbreviated codes that are documented neither in the geodatabase nor in the public metadata. Expanding them would amount to guessing bird names, which we prefer to avoid.

Where the figures come from

The layer aggregates observation campaigns conducted between 2007 and 2024, among them the SAMM aerial surveys and the international SCANS 3 campaigns, supplemented by CAPECET, MiniSAMM, DUNKRISK and PELGAS. The grid is 3 arc minutes, about 5.5 kilometres on a side. These same campaigns feed the cetacean abundance estimates for the Bay of Biscay that we discuss in our pages on the odontocetes.