The winter-scale dynamics of the short-beaked common dolphin spatial distribution in the Bay of Biscay

Dates: Feb–Dec 2021

Research team: Observatoire Pelagis UMR 3462 La Rochelle Université – CNRS.

Collaborators: Dr Jérôme Spitz, Dr Matthieu Authier, Dr Sophie Laran.

The Bay of Biscay short-beaked common dolphin population (Delphinus delphis) is under strong anthropogenic threat due to increased mortality in fisheries devices during the winter season (Peltier et al. 2021, Gilbert et al., 2021), but we lack a fine understanding of what drives the observed mortality peaks, and why these peaks intensified in the last years. Thanks to an unprecedented census effort within the Bay of Biscay throughout the winters 2020 and 2021 (January–March), we described the fine-scale spatio-temporal dynamics of dolphin distribution in the area.

We confirmed the changes in dolphin distribution are driven by the spatio-temporal variability of environmental conditions. The population preferentially targeted the oceanographic front separating river plumes from shelf waters, and tracked its movements across the study area. The density of common dolphin steadily increased from January to March, reaching a maximum during this month in both study years. The common dolphin thus seems to aggregate over one of the main winter frontal area in Western European shelf seas.

The results are the first fine-scale evaluation of habitat preference changes within the winter for the common dolphin. They also provide invaluable elements that will help better understanding why dolphins bycatches increased in the last few years and inform management decisions.

This work is published in Royal Society Open Science, and the Supplementary Information can be found on GitHub.