Monterey Bay researchers say dolphins are causing porpoise deaths

By Alia Wilson
Santa Cruz Sentinel
6 November 2009

For the past five years, the growing number of dead harbor porpoises washing up on California shores has been a marine mystery — until now.

In September, marine biologists with Okeanis, a nonprofit conservation group based in Moss Landing, captured the only video footage taken in Monterey Bay of bottlenose dolphins attacking and killing a porpoise.

Scientists suspected dolphins, which have a relatively docile reputation, have been attacking the smaller porpoises because the porpoises have been washing up on the shore with rake marks and internal bleeding, but they never had any proof.

"Now we know the how, but we still are unsure of the why," said Daniela Maldini, president of the Okeanis board. "We have been observing the social structure of these animals and it has taken us 20 years to do that. This type of behavior is something new." Maldini has been monitoring the same group of about 600 dolphins since 1990. Maldini said Okeanis believes in collecting a variety of data before problems arise to better diagnose critical changes in the environment. But what they captured raises more questions than supplies answers.

The video shows a group of male dolphins using several techniques to assault a porpoise as it tries to get away. The dolphins corral the porpoise, ram it with their beaks, scrape or rake it with their teeth and drown the porpoise by jumping on top of it. The dolphins then bring the carcass up to the Okeanis researchers, watch them bring it on board and then swim away.

The attack took place in the waters off Capitola.

"It was incredible to finally fit some action to our theories," said Okeanis field director Mark Cotter, who captured the footage along with fellow researcher Thomas Jefferson. "It diminishes the image of the bottlenose dolphin in a way that wild animals are not like those you see at SeaWorld. They all have their own individual personalities." Cotter said the next step is to figure out the why the dolphins attacked. Scientists have several theories, but none has been proven.

Maldini said there have been documented cases of male dolphins killing infants in the United Kingdom because there were few females available to mate with. Once the females have their babies they become unavailable to mate.

"Harbor porpoises are the same size as baby bottlenoses. No bottlenose babies are washing up onshore though, just porpoises," Maldini said. "It would be weird to think they are doing it out of competition for food because sea lions are a more likely candidate for those attacks. We're on the fence." Okeanis is collaborating with the Santa Barbara Museum of Natural History, the Marine Mammal Center, Long Marine Lab and the Moss Landing Marine Mammal Center in researching the possible causes. The group is working on publishing two papers at the beginning of the year about their findings.

Last year, there were about 74 porpoise strandings statewide, according to Michelle Berman, the assistant curator at the Santa Barbara Museum of Natural History.

"Historically, the strandings have been due to fishery interactions but with these strandings there were no external indications right away," Berman said. "With most of these porpoises, the injuries were internal. Several had broken ribs, one had a fractured skull that had punctured the brain. But once I could see the tooth marks on others, I knew right away that it had to be a bottlenose dolphin." Berman said she began seeing the strandings in 2004. Typically, her region sees one to two a year, but last year they had 16.

"It doesn't involve just one type of harbor porpoise. We've had adult males, females and even older males too," Berman said. "It's frustrating for us to know what went on but not know why or where so having a group like Okeanis collecting this type of data, out in the ocean, is huge for us."

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