Attack of the killer dolphins

Ah, dolphins. So sweet, so playful, so… vicious?

Yes, bottlenose dolphins have been known to kill harbor porpoises when the two species’ paths cross. Off the coast of California, warmer waters triggered by El Nino have allowed the dolphins to creep farther north into the porpoises’ territory. From 2007-2009, researchers saw 23 bottlenose dolphins attacking porpoises in Monterey Bay. And examinations of 216 porpoises stranded in California from 1998-2010 revealed that about a quarter of the animals had likely been killed by dolphins. Cause of death: blunt force trauma.

Why such aggression? The species probably aren’t battling over food, researchers say in Marine Mammal Science. Harbor porpoises eat market squid, anchovies, rockfish, and sardines, while bottlenose dolphins prefer surfperches and croakers. Instead, male dolphins — which sometimes kill young dolphins — may be “practicing” their infanticidal behavior on the porpoises or simply mistaking porpoises for dolphin calves.

To find out how the porpoises were reacting to their attackers, a research team studied data from devices that recorded echolocation clicking sounds in Monterey Bay from October to December 2011. The detectors were installed about 15 meters below the surface and could pick up both the dolphins’ mid-frequency clicks and the porpoises’ high-frequency clicks.

The study authors found that when dolphins weren’t around, porpoise sounds were detected during an average of about 11 minutes per hour. But when dolphins were present, the frequency of porpoise noises dropped to an average of about 4 minutes per hour.

The researchers don’t know if the porpoises are fleeing from the dolphins or just staying quiet to avoid detection. But they worry that dolphins could be pushing porpoises out of their habitat. At least 7 percent of the bottlenose dolphins off the coast of California have attacked porpoises, the team notes, so “[i]f this behavior continues to spread… direct mortality and habitat exclusion could negatively impact harbor porpoise populations.” Roberta Kwok | 31 July 2014

Source: Jacobson, E.K., K.A. Forney, and J.T. Harvey. 2014. Acoustic evidence that harbor porpoises (Phocoena phocoena) avoid bottlenose dolphins (Tursiops truncatus). Marine Mammal Science doi: 10.1111/mms.12154.

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