MOSS LANDING, Calif. — Marine biologists think they've figured out why a growing number of dead harbour porpoises have been found on California beaches in recent years: dolphin attacks.

The conservation group Okeanis shot video of bottlenose dolphins fatally attacking a porpoise. It shows male dolphins coralling the porpoise, ramming it and drowning it, then bringing the carcass to researchers on a boat and swimming away, according to the Santa Cruz Sentinel.

The video helps explain why carcasses were found with internal bleeding, rake marks and broken bones.

Scientists say about 74 dead porpoises washed up last year in the state.

Biologists at the Moss Landing-based group are working with the Santa Barbara Museum of Natural History, Marine Mammal Center and other groups to find more evidence.