Fish migrating to and from the upper Columbia River region pass between seven to nine major dams on the Columbia River depending on their starting place. Fish cannot travel above Chief Joseph Dam, effectively cutting off 40 percent of the historic habitat. Efforts are underway to explore reintroduction of anadromous fish into this blocked habitat. At passable dams downstream, immense efforts have been made during the past 50 years to improve passage at dams and through reservoirs. For example, changes have been made to the temperatures in the fish ladders and new ladders have been built. Generally, the rate of fish surviving dams has improved substantially during the past decade but some fish are still lost through the dams and mainstem river. Partners continue to look for ways to improve survival.