The Mexican navy has begun a controversial plan to drop concrete blocks onto the bottom of the Gulf of California to snag illegal nets that drown critically endangered vaquita marina porpoises.

As few as eight of the tiny, elusive porpoises remain in the Gulf, also known as the Sea of Cortez. It is the only place they live, and they cannot be captured and bred in captivity.

Vaquitas become trapped and drown in gill nets fishermen set illegally for totoaba, a fish whose swim bladder is a delicacy in China and sells for thousands of dollars per pound.

The Mexican government has largely abandoned efforts to keep small fishing boats out of a 110-square-mile “zero tolerance” area near San Felipe, Baja California, where the few remaining vaquita have been seen.

Environmentalists said the plan to sink 193 concrete blocks was approved with no public consultation and expressed concern that the metal hooks attached to the blocks may accumulate remnants of nets that could continue to entangle and drown sea life.

“This is a total surprise, because the environmental impact statement was approved in record time, in six weeks. It wasn’t opened to public comment,” said Alex Olivera, the Mexico representative for the Centre for Biological Diversity.

Mexico’s environment department acknowledged there had been no public consultation because nobody had requested one. The department has become known for quickly signing off on government projects.

Doubts about the plan abound. It would scatter one block, with a metal hook attacked, every kilometre over the zero tolerance area. It is not clear how, or whether, any snagged nets would be recovered from underwater.

“A net can be snagged on these hooks, and we don’t know, we’re talking about nets that are hundreds of yards long, so we don’t know if a net snagged down there might be a double-edged sword, and trap vaquitas,” said Mr Olivera.

Abandoned nets, known as “ghost nets”, can continue killing marine life for years.

Another expert said the plan might discourage the illegal fishermen by causing them to lose nets to the snags.

But he added that it would crucial for the navy to regularly clear out any snagged nets, “or other species could be killed down there”.

In a statement announcing the plan, the navy made a vague mention of “recovering detained nets”. In practice, it would probably require divers to descend and manually cut nets off each of the 193 blocks every few days.

Given the defiance of the fishermen and the lucrative nature of the illegal trade in dried totoaba bladders, there is also no guarantee that fishermen might not mark – either physically or with GPS – the location of the blocks and fish around them.

Last year, the Mexican government abandoned the policy of keeping fishing boats out of the “zero tolerance” zone in the upper Gulf.

It then introduced a sliding scale of punishments if more than 60 fishing boats are seen in the area on multiple occasions.

Mr Olivera expressed doubts. “They can’t be checking these blocks every day,” he said.

Earlier this year, the United States filed the first trade-based environmental complaint under the US-Mexico-Canada trade pact, arguing that Mexico is failing to protect the species.

Mexico has agreed to an investigation. Under the treaty, which took effect in 2020, the complaint could lead to trade sanctions.