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Pozo said the local people — who he knows through his work with the Biodiversity Conservation of The Usumacinta group — have tried to help the monkeys they see around their farms.

For the study, Pease traveled to the states of Tabasco and Chiapas, where she investigated the snook's almost 400-mile migration up into the rainforest habitat of the Usumacinta River.

A man who helped board the migrants for the five-minute boat ride Sunday from Guatemala across the Usumacinta River knew the count because each one received a ticket.

No, this is the Usumacinta River, which forms the border between Mexico and Guatemala in the Lacandon Jungle of Mexico’s southern Chiapas state.

The Usumacinta — where howler monkeys screech from overhanging trees, crocodiles lounge on sandbanks and jaguars prowl adjacent rainforest — is more than 1,000 miles from the mostly arid environs of the Rio Grande.