Social structure and abundance of coastal bottlenose dolphins, Tursiops truncatus, in the Normano-Breton Gulf, English Channel
Résumé
A large, but poorly studied, bottlenose dolphin community, Tursiops truncatus, inhabits coastal waters of
Normandy (Normano-Breton Gulf, English Channel, France). In this study, the social structure and abundance of
this community were assessed using photo-identification techniques. Like other bottlenose dolphin communities
worldwide, this resident community has a fission–fusion social structure with fluid associations among individuals
(half-weight index = 0.10). Association patterns were highly variable as indicated by a high social differentiation
(S = 0.95 ± 0.03). The majority of associations were casual, lasting days to months. However, individuals exhibited
also a smaller proportion of long-term relationships. A mean group size of 26 was large compared with other
resident coastal communities, and variable, ranging from 1 to 100, which could be the results of ecological
conditions, in particular resource predictability and availability. Analyses also showed that the community was
organized in 3 social clusters that were not completely isolated from each other. Abundance was estimated at 420
dolphins (95% confidence interval: 331–521), making this coastal community one of the largest identified along
European coastlines. Because human activities in the Gulf are expected to increase in the upcoming years, longterm
demographic monitoring of this dolphin community will be critical for its management.