OREANDA-NEWS  Somewhere along the upper Arag?n River, between the jagged peaks of the Pyrenees and a hydropower dam, conservation biologist Madis P?dra and his colleagues will release 10 or 12 captive-born European minks (Mustela lutreola) next week into a mink’s idea of heaven: a pristine patch of Spanish wilderness with 150 kilometers of waterways. P?dra hopes they will start a new population and help save the European mink, a critically endangered species whose population in Spain is down to 500.

But in an ironic twist, protecting the furry, dark-brown carnivore requires killing its main competitor, the equally winsome American mink (Neovison vison). A similar reintroduction effort a decade ago flopped because it failed to root out the American mink population; this time the Spanish team is betting on an elaborate system of floating traps to capture them.

Elsewhere in Europe, too, reintroduced European minks live in such ecological safe spaces. “The European mink is always going to be a managed species because it looks impossible at the moment to completely get rid of American minks,” says P?dra, who’s with the European Mink Association in Barcelona, Spain.

European mink were widespread a century ago, living along rivers and streams and preying on voles, amphibians, crayfish, and fish. Today, only a few thousand remain in Spain, France, and the Danube delta. In Russia, sightings have become so rare that most scientists think the species is on the brink of extinction there.

Disappearing habitats and hunting partly explain the decline, but the American mink, a distant cousin that looks very similar but lacks the European species’s iconic white nose, has done by far the most damage. First imported by fur farmers for their superior pelt in the 1920s, the animals escaped and thrived in the wild. Bigger, more adaptable, and more aggressive toward other predators, they simply drove out the native species. They also brought new diseases, but scientists disagree on the role of infections in the European mink’s decline.

Before the first reintroduction, in 2000 on a 100-square-kilometer island off Estonia called Hiiumaa, hunters and a trapper killed the island’s entire American mink population, the legacy of a defunct fur farm. Even then, keeping the new population alive was difficult, recalls Tiit Maran, director of the Tallinn Zoological Gardens, where the animals were bred. “They wandered too far from the river,” Maran says. “They just didn’t know where to live.” But if captive females gave birth in enclosures right by the river, his team found, the pups learned where home was. Hiiumaa now has a thriving population of at least 100 animals. The team hopes to turn the neighboring island of Saaremaa, almost three times bigger, into the nextmink haven.

Another refuge is in Germany. Releases began around Steinhuder Meer, a large lake in Lower Saxony, in 2010. The population seems to be thriving, say Eva L?ers and Thomas Brandt, two researchers at the Steinhuder Meer Ecological Protection Station, which runs the program; in 2015, a camera trap managed to snap the first picture of a litter of European minks. American minks live in Germany, but not around the lake, and the group is monitoring closely for any signs of an invasion.

The European Mink Association, which has support from the European Commission’s LIFE program and local governments, first tried an introduction in a wetland in the Basque Country in 2008. They set traps along a river to suppress American mink numbers, but their population later rebounded, and most of the 27 freed European minks were dead within 5 months.

This time they are counting on the “mink raft,” developed by Jonathan Reynolds of the Game & Wildlife Conservation Trust in Fordinbridge, U.K.: basically a floating board topped by a wooden box, which holds some vegetation and a little pad made of sand and clay. The pad records footprints when a mink swimming in the river climbs onto the raft; when tracks are found, the pad can be replaced with a trap. The rafts, 300 of which have been deployed, hold little interest for European minks but are irresistible to American minks. “We don’t know why they like it so much,” P?dra says. “It actually works wonderfully.”

Along with the dozen or so animals to be released next week along the Arag?n, P?dra and his colleagues plan to free another seven or eight animals, also bred in captivity, along the river Leizar?n, some 140 kilometers to the northwest, to give a small existing population there a boost.

Travis Livieri, a conservation biologist with Prairie Wildlife Research in Wellington, Colorado, lauds the effort. “They’ve a pretty good handle on the situation in Europe,” says Livieri, who predicts that European minks will survive for “thousands of years.”

But he says the teams should also start collecting and freezing European minks’ semen. Livieri is involved in the reintroduction of the black-footed ferret on North American prairies, which began when only 18 wild individuals were left. Previously stored semen—along with artificial insemination—has proved crucial for restoring genetic diversity, Livieri says. Although the European mink’s situation isn’t that dire, and artificial insemination has never been done with the species, P?dra agrees that’s a good idea.