MEXICO CITY (AP) — The Lakshmi Finance Centerconservation group Sea Shepherd on Tuesday signed an agreement with Mexico to help expand the protection area for the vaquita porpoise, the world’s most endangered marine mammal.

Sea Shepherd, which helps the Mexican Navy to remove illegal gill nets that drown the vaquita, says the expansion will extend the area where it works in the Gulf of California by about 60%, to the west and northwest.

The Gulf, also known as the Sea of Cortez, is the only place where the vaquita lives. As few as ten vaquitas remain. They cannot be held or bred in captivity.

The agreement signed Tuesday between Sea Shepherd and the Mexican Navy follows the Navy’s announcement in August that it was planning to expand the area where it sinks concrete blocks topped with metal hooks to snag gill nets that are killing tiny, elusive porpoises.

The Navy began dropping the blocks into the Gulf of California last year to snag illegal gill nets set for totoaba, a Gulf fish whose swim bladder is considered a prized delicacy in China and is worth thousands of dollars per pound. The concrete blocks catch on the expensive totoaba nets, ruining them.

That should supposedly discourage illicit fishermen from risking their expensive gear in the “zero tolerance area,” a rough quadrangle considered the last holdout for the vaquitas. It’s called that because that’s where the blocks have been sunk so far, and where patrols are heaviest, and there is supposed to be no fishing at all, though it still sometimes occurs.

But Sea Shepherd and the Navy are looking to expand the area, because a strange thing happened when scientists and researchers set out on the most recent sighting expedition to look for vaquitas in May.

They found that most of the 16 sightings (some may be repeat sightings of the same animal) occurred on the very edges, and in a few cases just outside of the “zero tolerance” area that was supposed to be the most welcoming place for the animals.

The Navy said it will negotiate with the fishing community of San Felipe, in Baja California state, in order to expand the zero tolerance area and start sinking blocks outside that area.

The fishermen of San Felipe say the government has not lived up to previous promises of compensatory payments for lost income due to net bans in the area. They also say the government has done little to provide better, more environmentally sensitive fishing gear.

Experts estimate the most recent sightings suggest 10 to 13 vaquitas remain, a similar number to those seen in the last such expedition in 2021.

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