FILE PHOTO: A California Gulf porpoise, also known as a vaquita, monument (R) is seen in San Felipe in Mexico's state of Baja California March 7, 2008, The vaquita, a tiny stubby-nosed porpoise found only in Mexico's Sea of Cortez, is on the brink of extinction as more die each year in fishing nets than are being born, biologists say. Picture taken March 7, 2008. REUTERS/Henry Romero/File Photo

MEXICO CITY (Reuters) - Fishing skiffs chased and shot at a vessel carrying environmental activists monitoring a near-extinct porpoise species in a protected zone of Mexico’s Gulf of California at the weekend, conservation group Sea Shepherd said.

There are thought to be only between six and 20 of the vaquita porpoises left. Poachers have been flouting an international ban on entering the species’ last sanctuary off the coast of Mexico.

The small, stub-nosed porpoise could become the first species subject to maximum protection under the Convention on International Trade in Endangered Species (CITES) to become extinct, raising questions about the effectiveness of the convention.

The attack on Sea Shepherd activists on Saturday morning happened in what the group described as a “critical zone” where several vaquita porpoises were recently sighted. It said at least two bullets landed in the ocean alongside the vessel. There were no injuries.

“This just shows how aggressive the poachers are here,” said Sea Shepherd captain Jacqueline Le Duc in a statement, adding that the area was protected by federal law and recognized by the United Nations Educational, Scientific and Cultural Organization (UNESCO).

Officials from Mexico’s police, navy and environmental prosecutor were on board the vessel, Sea Shepherd said. Mexico’s environment ministry did not immediately respond to a request for comment.

The main threat to the vaquita porpoise are gillnets, set up by poachers in an effort to catch totoaba, an endangered species of marine fish sought by Chinese buyers on the black market for its prized swim bladders.

Mexico is suffering record levels of violence, often linked to organized crime. Environmental activists are routinely threatened, harmed or even killed as a result of their work.