Between 1991 and 2011, 137 harbour porpoises (Phocoena phocoena) died as a result of attacks by bottlenose dolphins (Tursiops truncatus) in Cardigan Bay. The suggested reasons for these non-predatory aggressive interactions include: aberrant behaviour, geographic overlap, dietary overlap, object-oriented play for infanticide practice or for fighting practice, elevated testosterone levels during the breeding season of bottlenose dolphins or skewed operational sex ratio of the bottlenose dolphins (the number of sexually available males relative to sexually available females).
Geographic overlap and object-oriented play for infanticide practice and fighting practice were investigated using boat survey data (2005-2011) from the Cardigan Bay Marine Wildlife Centre and strandings data (1994-2011) from the Cetacean Strandings Investigation Program. Dietary overlap was examined by reviewing literature based on stomach samples taken from the two study species located in the British Isles. The remaining factors were not investigated due to time and equipment constraints, and aberrant behaviour was dismissed, as there is widespread documentation of non-predatory aggressive behaviour between bottlenose dolphins and harbour porpoises.
Results showed that there was a minor geographic overlap; under a fifth of each species’ area coverage, and thus it was unlikely to cause the attacks. There was a dietary overlap; although the extent of the overlap is uncertain, so dietary overlap remains a possible cause of the non-predatory aggression. Object-oriented play for infanticide practice seems to be unlikely, as only three bottlenose dolphin calves died (with unknown cause of death) over 18 years, indicating that either infanticide is not occurring or it is extremely rare. Object-oriented play for fighting practice was supported by literature but the results were inconclusive, therefore, it is also considered a possible reason for the attacks on harbour porpoises. Further investigation is required to determine which of these possible reasons is the definitive cause of the non-predatory aggression.