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A group of poachers have attacked two vessels carrying marine conservationists in protected waters in Mexico's Gulf of California, the Sea Shepherd conservation group said on Wednesday.

The protected waters—a UNESCO World Heritage Site—are home to the threatened vaquita porpoise, of which only 20 specimens are thought to remain worldwide, and the totoaba, another critically endangered species of marine fish.

Both animals have been hunted to near extinction for their swim bladders, an organ that is considered a status symbol in China and can fetch thousands of dollars on the black market.

Conservationists and apprehended poachers while they were retrieving their , Sea Shepherd said in a statement. The poachers refused to cease their activities.

Another 20 to 25 subsequently arrived and joined the poachers in attacking the two conservationist vessels with Molotov cocktails and rocks.

"Today's events show the unfortunate lows that the poachers will (go to) to achieve their goals of pillaging our oceans," Sea Shepherd Captain Octavio Carranza said in a statement.

The crew managed to repel the attack using water cannon. The poachers finally dispersed when military personnel on board one of the boats fired a warning shot. No one was injured in the incident.

In 2017, Mexican conservationists launched a breeding program for vaquita porpoises in captivity but it was suspended after a female specimen died in their care.