In one of the most polluted rivers in Central America, a vulnerable crocodile species is thriving despite living in waters that have become a sewer for Costa Rica's capital, experts say.

​Every day, trash and wastewater from San Jose households and factories flood into the Tarcoles River, which vomits tires and plastic into the surrounding mangroves.

​Nevertheless, some 2,000 American Crocodiles have adapted to life in the toxic river that bears witness to the country's decades-long battle with waste management.

"It is a super-contaminated area, but this has not affected the crocodile population," said Ivan Sandoval, a biologist with the National University of Costa Rica.

​"The Tarcoles River is the most polluted river in Costa Rica, and one of the most contaminated in Central America. Heavy metals, nitrites, nitrates, and a large amount of human waste can be found," added the crocodile expert.

​According to the International Union for Conservation of Nature (IUCN), there are only about 5,000 of the crocodile species – found in 18 countries – left in the world after decades of hunting and habitat loss.

​The organization lists the Crocodylus acutus as "vulnerable," but says its numbers have increased in recent years. The Costa Rica population is "healthy and robust".

​Indeed, the large reptiles – basking in the sun and occasionally feeding on fish that come up the channel from the sea – appear unfazed by some 150 types of bacteria that Sandoval says have been detected in the river.

​He describes the carnivores as "living fossils" with the capacity to survive very tough conditions.

​"They haven't had to change anything in millions of years, they are perfectly designed."

​Laws not applied

Sandoval said that since 1980, Costa Rica's population of the crocodiles "are recovering," and warns of the threat of tourist activities.

​The river's crocodiles are a major draw for foreign visitors, who take boat tours to see the creatures up close.

​Some feed the animals, which is prohibited, and Sandoval worries about them getting too used to being close to people.

​Juan Carlos Buitrago, 48, who captains one of the tour boats, says he and other locals regularly pull hundreds of tires and plastic waste from the water.

​He delights in the fauna of the river, with macaws flying over ahead at sunset, but wishes his countrymen would stop polluting his "office".

​"We cannot hide the pollution," he tells AFP.

​Costa Rica has impressive environmental credentials, with a third of its territory marked for protection, 98 percent renewable energy, and 53 percent forest cover, according to the UN's environmental agency.

​However, the law is not always strictly applied, as in the case of the Tarcoles River.

​Lawyer and environmentalist Walter Brenes, 34, said that all of Costa Rica's rules and regulations "do not solve the problem".

​He said the country needs "real public policy that is completely aimed at protecting wildlife".

© Agence France-Presse