DiCaprio tweeted on May 11 that "the vaquita is the most endangered marine mammal in the world" and urged his 17.5 million followers to "take action."

"I welcome @LeoDiCaprio and @World_Wildlife's concern regarding the Vaquita Marina," President Pena Nieto wrote in his first series of tweets about the issue. "@Mexico has focused all its efforts to prevent the extinction of this species."

Per Reuters, populations of the vaquita, a small snub-nosed porpoise, have dropped sharply in recent years. That's because of extensive gillnet fishing in the Gulf of California for shrimp and totoaba (a delicacy in Asia).

Although Mexico imposed a two-year ban on gillnet fishing in the vaquita's habitat in 2015, critics say the ban needs better enforcement.

According to Viva Vaquita, a group dedicated to saving the porpoise, the vaquita population was estimated to be less than 50 in January 2017. July 8 is "International Save the Vaquita Day" if you want to get involved.

DiCaprio is well known for his environmental activism. He joined the April Washington D.C. Climate Change March with a sign that read "Climate Change is Real." He also stars in and produced the documentary "Before the Flood," about the impact of global warming around the world.