The environmentalist group Sea Shepherd says it found the badly decomposed body of what appeared to be a vaquita porpoise, one of perhaps only 10 that remain in the world.
The group said Thursday the remains were too badly decomposed for immediate identification, but had been turned over to authorities for further study.
Two Sea Shepherd patrol boats found the animal in a net Tuesday in the Gulf of California, the only place the critically endangered vaquita marina lives.
The vaquitas get caught in nets set illegally for totoaba, a fish whose swim bladder is considered a delicacy in China.
An international commission of experts estimated this month that only 10 Vaquita remain, and almost certainly no more than 22.