Gila trout are endangered in some stretches of water that are managed as designated wilderness. A hands-off policy would be their doom. Because exotic trout species now swim in the same streams, Gila trout can survive the competition and the temptation to interbreed only if they swim in isolated tributaries. In which a water-fall blocks the upstream movement of other fish. Two decades ago, one such tributary was fortified. With a small concrete dam. In other words, a dam deliberately built in the wilderness. It is often difficult to choose the right way. To manage a wilderness area. A scientific grasp of the way the ecosystem works is essential. Yet not always