Social structure and abundance of coastal bottlenose dolphins, Tursiops truncatus, in the Normano-Breton Gulf, English Channel

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, long-term demographic monitoring of this dolphin community will be critical for its management