Interactions between bottlenose dolphins and harbour porpoises By Rebecca Bakker

Social interactions of bottlenose dolphins which behaved aggressively towards harbour porpoises in the Cardigan Bay SAC in 2014 By Rebecca Bakker

Rebecca used historical photo-identification data collected by researchers at CBMWC to investigate the interaction between bottlenose dolphins and harbour porpoises observed in the 2014 field season.

The Cardigan Bay Special Area of Conservation (SAC) is inhabited by a semi-resident population of bottlenose dolphins (Tursiops truncatus). In the summer of 2014 four attacks of bottlenose dolphins on harbour porpoises (Phocoena phocoena) were recorded and witnessed by staff and volunteers from the Cardigan Bay Marine Wildlife Centre (CBMWC).

During the study period from 2010 to 2014 land and vessel based surveys were carried out, during which photographs were taken for photo identification, as well as additional data collected including group size. During the four surveys in 2014 when the attacks were recorded, a total of 26 individuals were observed, of which ten were identified as dolphins already existing in the Cardigan Bay Marine Wildlife Centre catalogue.

Bottlenose dolphin and harbour porpoise during an aggressive encounter

Bottlenose dolphin and harbour porpoise during an aggressive encounter

Social network analysis was applied to three of the individuals with well-marked dorsal fins which were involved in these attacks, #007, #015 and #036; to investigate the social interactions these individuals have with each other, and other members of the population.

Data analysis showed that the three individuals (the culprits) did not occur in outstanding group sizes, with a mean group size of 5.61 (SD=5.31) for all three individuals. The association between the three individuals is regarded as being casual (HWI’s: #007 - #015= 0.24, #007 - #036= 0.20, #015 - #036= 0.29). However, comparing these associations to associations between the three individuals and other members of the populations, the associations among the three individuals are relatively strong. It cannot be proven that the behaviour of attacking harbour porpoises is transmitted horizontally throughout the population however; the relatively strong association among the three individuals does support this theory.

The full report is available to download here.