Social structure of bottlenose dolphins (Tursiops truncatus) in Cardigan Bay, Wales

Cally Barnes, BSc (Hons) Marine Biology and Oceanography, University of Plymouth

Social behaviour studies on bottlenose dolphins in the Northeast Atlantic are limited. This paper presents the analysis of a five year data set to determine the social structure of the semi resident population of bottlenose dolphins, (Tursiops truncatus) inhabiting Cardigan Bay, Wales.  Photo-identification of uniquely marked dolphins encountered during non standardised boat surveys provided a data set where association of individuals could be analysed, in relation to school membership. Two non consecutive years, 2006 and 2008 were used as independent data sets, and compared with the full five years, to see if any long lasting associations were evident.  Between May 2005 and October 2009 there were a total of 784 recorded dolphin encounters, with 61 dolphins during 2006 and 42 in 2008. The mean association index between pairs was low for 2006, 2008 and the long term (0.07, 0.08 and 0.04), respectively. The mean maximum association value for all dolphins seen during the five year period was 0.42.  The association matrix for dolphins seen during 2008 differed significantly from a random structure, using an iterative permutation test. Hierarchical cluster analysis and principle co-ordinate analysis were used to identify three loosely aggregated groups. These groups were examined to see if they were gender based, or if different home ranges were occupied. However all groups were mixed, and home range overlapped.  As a result of this study, the population of bottlenose dolphins using Cardigan Bay were found to exhibit fission-fusion behaviour; consisting of loosely associated individuals. 
Key words: Social structure, photo-identification, fission fusion, Cardigan Bay, Tursiops truncatus

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