“Porpicide” in California: Killing of harbor porpoises (Phocoena phocoena) by coastal bottlenose dolphins (Tursiops truncatus)

Abstract

Between 2007 and 2009, we witnessed three aggressive interactions between harbor porpoises and bottlenose dolphins in Monterey Bay, California. This is the first time such aggression has been documented in the Pacific, and the first time a harbor porpoise was collected immediately after witnessing its death, inflicted by bottlenose dolphins. Of the bottlenose dolphins present, 92% were males either confirmed (61%) or putative (31%). Since 2005, 44 harbor porpoise deaths inflicted by bottlenose dolphins were documented in California. Aberrant behavior was rejected as a cause of aggression, based on widespread documentation of similar behaviors in other populations of free-ranging bottlenose dolphins. The evidence for interspecies territoriality as a form of competition for prey was weak: there is little dietary overlap and there are differences in bottlenose dolphin and harbor porpoise distribution patterns in California. Object-oriented play was plausible as a form of practice to maintain intraspecific infanticidal skills or a form of play to maintain fighting skills between male associates. Contributing factors could be high-testosterone levels, as attacks occurred at the height of the breeding season, and/or a skewed operational sex ratio. Ultimately, we need more information about bottlenose dolphin social structure at the time of the aggression.

Armed Escorts: Dusky Dolphin (Lagenorhynchus obscurus) Interactions with Octopuses (Pinnoctopus cordiformis) in New Zealand

“…The mortalities of several cetacean species are attributed to aggressive encounters with bottlenose dolphins (Ross & Wilson, 1996;Barnett et al, 2009;Cotter et al, 2012). Indo-Pacific bottlenose and rough-toothed dolphins carry, push, or toss pufferfish they may kill but not consume (Steiner, 1995;Whitehead & Mann, 2000).…”

Short Note: Rake Marks on a Harbor Porpoise (Phocoena phocoena) Calf Suggestive of a Fatal Interaction with an Atlantic White-Sided Dolphin (Lagenorhynchus acutus)

“…The seasonality of the violent interactions also suggests that high testosterone levels in male dolphins may increase aggressive behavior (Cotter et al, 2011). Other proposed hypotheses include defensive aggression, playful behavior, practice-fighting, sexual frustration, and aberrant behavior from an isolated individual (Ross & Wilson, 1996;Cotter et al, 2011). With no evidence of blunt trauma in the present case, the hypothesis that this harbor porpoise calf was intentionally killed is not supported.…”

“…Little is known regarding other causes of mortality and population trends of harbor porpoises from the Estuary and Gulf of St. Lawrence. Harbor porpoises have been reported to be occasional victims of attacks from bottlenose dolphins (Tursiops truncatus) in Great Britain and California, and from Pacific white-sided dolphins (L. obliquidens) in Washington State (Ross & Wilson, 1996;Baird, 1998;Cotter et al, 2011). Such interspecific violent interactions have never been reported from the Estuary and Gulf of St. Lawrence.…”

“…Well-documented attacks in California involved mainly male bottlenose dolphins at the height of the breeding season, which further supports this hypothesis. The seasonality of the violent interactions also suggests that high testosterone levels in male dolphins may increase aggressive behavior (Cotter et al, 2011). Other proposed hypotheses include defensive aggression, playful behavior, practice-fighting, sexual frustration, and aberrant behavior from an isolated individual (Ross & Wilson, 1996;Cotter et al, 2011).…”

An Unusual Mortality Event of Harbor Porpoises (Phocoena phocoena) Off Central California: Increase in Blunt Trauma Rather Than an Epizootic

“…The harbor porpoises observed with blunt force trauma during the summer and fall of 2008, however, were not all small animals: some ranged from 140 to 150 cm, with one female measuring 163 cm in length. During the increased stranding of porpoises with lesions typical of blunt force trauma in 2008, direct evidence of bottlenose dolphins attacking porpoises was obtained (Cotter et al, 2011); therefore, interspecies aggression from bottlenose dolphins was considered the most likely cause of trauma in these cases.…”