A small porpoise, known as a vaquita, can only be found in the wild at the northern end of the Gulf of California.
Because there are so few left, it's been called the world's "most endangered cetacean".
Supposedly, these marine mammals are being protected in the Vaquita Refuge.
But the area has been targeted by gillnetters. And according to the Sea Shepherd Society, one of these extremely rare creatures was likely found dead in one of these nets in the Upper Gulf of California.
The corpse was discovered by crew members on the society's M/V Farley Mowat as they were removing the gillnet from the water.
"After sending preliminary photographs to marine mammal experts and Sea Shepherd scientific advisers, it was determined that morphology and length matched the body of a vaquita porpoise," Sea Shepherd stated. "Genetic analysis is needed to confirm identity of species."
The corpse has been turned over to government authorities in San Felipe.
Since 2015, the Sea Shepherd Conservation Society has been in the Upper Gulf of California investigating the trapping of marine mammals by gillnetters.
Of 36 animals that have been discovered so far in the nets, nine were cetaceans. The only survivor was a juvenile humpback whale.