Social structure of bottlenose dolphins in Cardigan Bay

Project Last Updated: 12/10/2004 20:50:00 Source : Rob Lott

Project Start - 2003 / Project End - 2004

Collaborators: University of Wales, Bangor and Sea Watch Foundation

Sizeable populations of bottlenose dolphins (Tursiops truncatus) in British and Irish waters are found in the Moray Firth, Scotland, the Shannon Estuary, Ireland and Cardigan Bay, Wales.

We sought to investigate, for the first time, the social associations of bottlenose dolphins in Cardigan Bay. A total of 72 photo-identification surveys were made during 2003, resulting in 134 dolphin encounters. Fifty-two identified dolphins were selected based on their number of sightings. Individuals accompanied by calves were categorized as females and extensively scarred animals as males. Associations were studied using half-weight indices and cluster analysis.

In addition, temporal analyses of associations were made plotting changes in the standardised reassociation rate over time. The social organisation was characterised by relatively fluid association patterns, with little stability over periods longer than a few days. Some longer-term companions were evident. Permutation tests revealed the presence of preferred and/or avoided companions. Both males and females interacted with a large number of other individuals. In contrast to studies from the Moray Firth and the Shannon Estuary, we found significantly stronger associations between certain male pairs.

As in the Shannon Estuary and in the Moray Firth, we found no evidence of stable female bands. The low risk of predation in these three areas may reduce the need for stable female cohorts, such as those observed in more tropical waters, while the shallow waters of Cardigan Bay may favour the male alliance strategy in coercing reproductive females. Six loosely defined clusters of dolphins, including animals of both sexes, were identified.

These clusters could reflect the existence of preferred geographical ranges that influence the dolphins’ social networks. A similar situation has been hypothesised in the Shannon Estuary. This degree of site fidelity highlights the fact that monitoring based on photo-identification needs a good geographical spread.