Sixteen harbor porpoises were found washed up on Santa Barbara beaches in 2008. This number is up from an average of one or two per year, said Michelle Berman, the assistant curator at the Santa Barbara Museum of Natural History.
The growing number of dead porpoises remains a mystery to marine biologists. A total of 74 porpoise carcasses were reported to have washed up on California shores last year.
Scientists with Okeanis, a nonprofit conservation group based in Moss Landing, suspect bottlenose dolphins have been attacking porpoises because the bodies are found with rake marks and internal bleeding. In September, the group captured the only known video footage of bottlenose dolphins attacking a porpoise in Monterey Bay.
The video captured by Okeanis researchers shows a group of male dolphins corralling a porpoise, then smashing its body with their beaks and scraping and raking it with their teeth. The dolphins drown the porpoise by jumping on it, and then bring the carcass to the Okeanis researchers and swim away.
According to a report by the Santa Cruz Sentinel, the Okeanis researchers are unsure why dolphins would attack harbor porpoises. They ruled out competition for food as a factor, saying sea lions would be a more likely candidates for attacks if food were the issue. Berman added that there is no significant effect on the local marine food chain since the number of porpoises killed remains low.
Berman said Okeanis is seeking answers to the mystery of whether and why bottlenose dolphins might be killing harbor porpoises.