The Sea Shepherd environmental group published a video Thursday showing an attack by about two dozen small fishing boats on the vessel Farley Mowat in Mexico's Gulf of California.
Fishermen in the Gulf, also known as the Sea of Cortez, have long complained about environmentalists trying to protect the vaquita marina, the world's smallest and most endangered porpoise.
Sea Shepherd said fishermen threw lead weights and tried to douse the Farley Mowat and waters around it with gasoline Wednesday.
The video shows some of the fishing boats carried gill nets, though they are banned within the reserve designed to protect the vaquita.
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The vaquita is nearing extinction due to gill nets set illegally to catch totoaba, a fish whose swim bladder commands astronomical prices because it is considered a delicacy in China.
Some fishermen threw a net in front of the Farley Mowat to foul the propellers of the Sea Shepherd vessel. Others boarded the ship and apparently carried off some items.
The Farley Mowat's crew can be seen using a hose to repel some of the smaller boats.
Sea Shepherd operates in the Gulf with the knowledge and cooperation of the Mexican government to help detect illegal nets.
Experts say as few as 15 of the marine mammals remain in the Gulf, the only place they exist, and none have ever been held in captivity.
While the Mexican government has banned the use of gill nets in the vaquita's known range, the rule has been almost impossible to enforce.
Scientists have called on Mexican environmental officials to ban possession of the nets in the whole area, as well as carry out land patrols and inspections of boats setting out to sea to enforce the ban. At present, authorities patrol the area but poachers often flee in high-powered boats and make it to shore.
A voyage by researchers in late September sighted six or seven of the creatures, including a mother with a new calf. The sightings suggested that most of the remaining vaquitas have gathered in a roughly a 12- by 25-mile (20- by 40-kilometer) rectangle, a small enough area that it could potentially by protected by floating barriers to keep out the small boats used by poachers.