MEXICO CITY (Reuters) - The Mexican government, tycoon Carlos Slim and U.S. actor Leonardo DiCaprio on Wednesday unveiled a joint plan to protect a tiny porpoise in the Gulf of California that has become a potent symbol of critically endangered animal species.

Populations of the snub-nosed vaquita porpoise have plummeted due to gillnet fishing for shrimp and totoaba, a popular delicacy in Asia, sparking increasing calls for action.

Mexican President Enrique Pena Nieto met Hollywood star DiCaprio and Slim in his official residence in Mexico City to sign a memorandum of understanding committing to conserve marine life in the Gulf of California, including the vaquita.

There are now fewer than 30 of the vaquita left in the wild, the foundations run by Slim and DiCaprio said in a statement.

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The accord comes less than a month after DiCaprio urged his fans on social media to petition Pena Nieto to save the vaquita, which prompted the president to take to Twitter to assure the actor that Mexico was doing all it could to protect the porpoise.

Under the memorandum, the signatories undertook to make permanent a temporary ban on using gillnets in the vaquita's waters and to step up efforts to combat the use of illegal gillnets, as well as the prosecution of illegal fishing and totoaba poaching.

Gillnet fishing, which uses mesh sizes designed to allow fish to get only their head through the netting but not their body, is blamed for trapping the vaquita porpoises and killing them.

The plan also included a commitment to prohibiting nighttime fishing in the upper Gulf of California and the vaquita reserve, and to enforce limited entry and exit points in the region for fishing, among other measures.

In the last month, 200,000 people have signed the petition to save the vaquita directed by DiCaprio at Pena Nieto, the World Wildlife Fund said.

In the statement, DiCaprio, the 42-year-old star of "Titanic," called the memorandum a "critical step" on behalf of the marine mammal.

"I am honored to work with President Pena Nieto, who has been a leader in ecosystem conservation, to ensure the future viability of marine life in the Gulf," DiCaprio said.

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Pena Nieto on Wednesday evening tweeted pictures of his meeting with DiCaprio and Slim, saying that Mexico understood its environmental responsibility to the world.

A spokesman for the project could not immediately say how much money was being dedicated to the rescue effort.

(Reporting by Dave Graham; Editing by Leslie Adler)