Document: Article

Abstract: The Commerson’s dolphin (Cephalorhynchus commersonii) includes two subspecies, one of which, C. c. kerguelensis, is endemic to the Kerguelen Archipelago (southern Indian Ocean). Despite high conservation stakes associated with its geographical isolation (the other subspecies is only found in southern South America), the Kerguelen Commerson’s dolphin remains poorly studied. Herein, we used observation and photo-identification data from a long-term monitoring program implemented since 2012 to assess the spatial distribution and demographic parameters of this subspecies. Using geostatistical models, we found high probabilities of encounters of dolphins in fjord-type habitats in the Gulf of Morbihan and along the southern and northern coasts of the archipelago. From capture-mark-recapture models (Robust Design and POPAN), annual survival of individuals in the Gulf of Morbihan was estimated at 0.82 ± 0.23, with an abundance in this area ranging from 40 to 100 dolphins per year between 2012 and 2020. However, high temporary migration rates in and out of the Gulf of Morbihan were detected and supported by photo-identification data, with individuals confirmed traveling up to 130 km outside this area, generating uncertainty in demographic estimates. Our findings, while providing baseline insights, highlight the need for improved knowledge on the feeding ecology and movements of dolphins to be able to produce spatial and demographic information that can be used to assess the conservation status of the Kerguelen Commerson’s dolphin.

Key Words: abundance, Commerson’s dolphin, Cephalorhynchus commersonii, photo-identification, mark-recapture, Poisson Kriging, Robust Design

DOI: https://doi.org/10.1578/AM.51.3.2025.218

Page Numbers: 218-232