Meet the world's hottest freshwater fish

The Julimes pupfish lives in water that reaches 114 degrees


The Julimes pupfish sports a big head and likes it hot.

Known locally as El Cabezón de Julimes, several thousand pupfish take up exclusive residency in El Pandeño de los Pandos hot springs in this small municipality about 80 minutes southeast of City of Chihuahua, Mexico. Rarely more than two inches long, pupfish live in water that reaches 114 degrees Fahrenheit, earning it the title of “hottest fish in the world.” 

The pupfish is found solely in these hot springs—along with a tiny snail that serves as dinner—in an area of about 8,000 square feet, which includes the original spring and a man-made canal. Other than humans, they have no natural prey because of the water’s heat. Now, thanks to the efforts of the farmers who use the springs to irrigate their farms, and a number of conservation groups including WWF, the pupfish’s habitat is protected.

“The farmers used to scrub the bottom (of the spring), disturbing the fish’s habitat,” Dr. J. Alfredo Rodríguez-Pineda, hydrologist for WWF-Mexico, said. “But they stopped doing it and made this a sanctuary. They feel very proud of this little fish, because it is now internationally known as the fish that lives in the hottest water in the world.” 

looping pupfish

With the farmers, WWF has set up three water-monitoring stations in the springs to measure seasonal changes in temperature and flow. The site is now protected by a chain-link fence and kept locked, but tours are welcome.

Eduardo Pando owns the land where the pupfish lives. He is also president of the irrigation unit of 15 farmers that receives water from the springs and helped create a conservation group to get resources from the government, called Amigos del Pandeño.

“We have a different attitude toward the fish,” Pando said. Not disturbing the sediment at the bottom of the shallow spring (about knee deep) preserves the fish but reduces the water flow for irrigation. Rodríguez has been working for several years to cement the irrigation channels, which improves flow and balances out what is lost by not touching the springs.

“It is not easy for other farmers to understand the importance of the little fish,” Pando said. He added that he must work with the other farmers to come to a consensus.

The springs serve three purposes for the area. Besides serving as a now-protected and expanded habitat for the pupfish, the water feeds a popular park, called Los Manantiales, which attracts many visitors, especially in the autumn months, who believe the hot springs have healing qualities. After that, the water passes through to irrigate 70 hectares of alfalfa, chili peppers, oat, cantaloupes, watermelon and pecans.

The locals used to call the pupfish charalito, a generic name for all small fish. Now they know better and El Cabezón is a point of civic pride.

“We want to protect the area for the fish,” Pando said.

For Rodríguez, the conservation success and pride that the pupfish has brought to the area is important, but just one piece in the larger puzzle of protecting the Rio Grande/Rio Bravo basin. WWF works in both the US and Mexico to secure fresh water for the people and nature that thrive throughout the Chihuahuan Desert.