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Monterey Bay: dolphins attack and kill porpoises

Researchers Barbara Scoles and Mark P. Cotter have taken a video of bottle-nosed dolphins attacking a porpoise in Monterey Bay. The work was performed under NOAA Permit #877-1831.

For five years, there have been a growing number of dead harbor porpoises washing up on California shores. Until now, the deaths have been a mystery. In September, marine biologists with Okeanis, a nonprofit conservation group in Moss Landing, captured the only video every taken in Monterey Bay of bottlenose dolphins attacking and killing a porpoise. The video shows a group of male bottlenose dolphin attacking a porpoise by ramming it with their beaks, raking it with their teeth, and drowning it by jumping on top of it. The dolphin then brought the porpoise carcass up to the Okeanis researchers, watched the researchers take the carcass onboard, and then swam away! The attack took place off Capitola.

Okeanis has been studying the same group of about 600 dolphins since 1990. Daniela Maldini, president of the Okeanis board, described research in the United Kingdom in which male bottlenose dolphin kill infants because female dolphin with infants become unavailable to mate. Porpoises are the same size as infant dolphins. However, in Monterey Bay, no infant dolphins are washing up, just porpoises. Some British scientists attribute the attacks in the UK to competition for food.

Okeanis is working together with the Marine Mammal Center, the Long Marine Lab, the Moss Landing Marine Mammal Center <sic? Moss Landing Marine Laboratories?>, and the Santa Barbara Museum of Natural History to investigate possible causes. The group plans to publish two papers at the beginning of 2010 on the subject. Typically, there have been one to two porpoise strandings each year. Last year there were 16 in the region served by the Santa Barbara Museum of Natural History, with 74 strandings statewide. Most of the porpoises had internal injuries, but others had tooth marks.

-Bill at Cheshire Cat Photo™

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