Yellowstone Science10 In 1994, an angler caught a lake trout

Wyo-ming. Judging by the size of the trout, and from subsequent data provided by the National Park Service and the U.S. Fish and Wildlife Service, biologists now be-lieve that someone must have illegally planted lake trout in the lake at least 20 years earlier. Although lake trout inhabit at least four other lakes in Yellowstone National Park, biologists blame humans for the introduction because natural move-ment of this non-native species into Yel-lowstone Lake is improbable. Based on catch and mortality rates, biologists esti-mate that thousands, perhaps tens of thou-sands, of lake trout of several age classes, some capable of spawning, live in Yel-lowstone Lake (Kaeding et al. 1995). Rivers and lakes are vulnerable to in-vasive fish species, and Yellowstone Lake is a prime habitat for lake trout because they thrive in cold, deep water (Yellow-stone Science 1996). But the problem is that Yellowstone Lake is the last premier inland Yellowstone cutthroat trout (Oncorhynchus clarki bouvieri) fishery in North America. After years of work to restore the native Yellowstone cutthroat trout population back to viable levels, lake trout put this cutthroat population at risk. Experts fear the lake trout popula-tion will expand and cause a serious de-cline in the cutthroat population, espe-cially juveniles (see Ruzycki and Beauchamp 1997). If left unchecked, some biologists have predicted that this voracious exotic species could reduce the catchable-size cutthroat population from 2.5 million to 250,000–500,000 within the near future (Kaeding et al. 1995). As if putting native cutthroats at risk were not enough, lake trout also place some other native species at risk. Lake trout do not replace cutthroat in the food chain. For example, grizzly bears (Ursus arctos), listed as threatened under the U.S. Endangered Species Act since 1975, feed on cutthroat when they spawn in over half of Yellowstone Lake’s 124 tribu-tary streams. Researchers there have ob-served an adult female grizzly harvest an average of 100 fish per day for 10 days (Schullery and Varley 1995). Lake trout do not replace cutthroat in the food chain because, unlike cutthroats, they spawn in the cobble and rubble in the lake, far from many predators ’ reach. As Yellowstone’