by temperature, brook trout populations are very susceptible to damage by the introduction of exogenous species. Many lacustrine populations of brook trout have been extirpated by the introduction of other species, particularly percids, but sometimes other spiny-rayed fishes. In addition to chemical pollution and algae growth caused by runoff containing chemicals and fertilizers, air pollution has also been a significant factor in the disappearance of brook trout from their native habitats. In the U.S., acid rain caused by air pollution has resulted in pH levels too low to sustain brook trout in all but the highest headwaters of some Appalachian streams and creeks. Brook trout populations across large parts of eastern Canada have been similarly challenged; a subspecies known as the aurora trout was extirpated from the wild by the effects of acid rain. Today, in many parts of the range, efforts are underway to restore brook trout to those waters that once held native populations, stocking other trout species only in habitats that can no longer be recovered sufficiently to sustain brook trout populations. Organizations such as Trout Unlimited and Trout Unlimited Canada are partnering with other organizations such as the Southern Appalachian Brook Trout Foundation, the Eastern Brook Trout Joint Venture, and state, provincial, and federal agencies to undertake projects that restore native brook trout habitat and populations. As an invasive species Although brook trout populations are under stress in their native range, they are considered an invasive species where they have been introduced outside their historic native range. In the northern Rocky Mountains, non-native brook trout are considered a significant contributor to the decline or extirpation of native cutthroat trout (Oncorhynchus clarki) in headwater streams. Non-native brook trout populations have been subject to eradication programs in efforts to preserve native species. In Yellowstone National Park, anglers may take an unlimited number of non-native brook trout in some watersheds. In the Lamar River watershed, a mandatory kill regulation for any brook trout caught is in effect. In Europe, introduced brook trout, once established, have had negative impacts on growth rates of native brown trout (S. trutta). References Further reading brook trout Freshwater fish of North America Fish of the Eastern United States Fish of the Great Lakes Ecology of the Appalachian Mountains Symbols of Michigan Symbols of Pennsylvania Symbols of New Hampshire Symbols of New Jersey Symbols of New York (state) Symbols of North Carolina Symbols of Vermont