Yellowstone Lake in Yellowstone National Park (YNP) supports the largest inland population of cutthroat trout in the world, and is the core of the remaining undisturbed habitat for native Yellowstone cutthroat trout (Oncorhynchus clarki bouvieri) in the Yellowstone ecosystem (Gresswell and Varley 1988, Varley and Gresswell 1988, Keading 1995). Each spring, cutthroat trout migrate from the lake to its tributaries to spawn (Gresswell 1995). While in spawning streams, cutthroat trout are preyed upon by numerous predators including black bears (Ursus americanus) and grizzly bears (Ursus arctos horribilis) (Hoskins 1975, Mealey 1975, Gunther 1984, Knight et al. 1984, Reinhart 1990, Mattson et al. 1991).
Due to their high digestibility and protein and lipid content, spawning cutthroat trout are one of the highest sources of net digestible energy for grizzly bears in the Yellowstone ecosystem (Mealey 1975, Knight et al. 1984, Reinhart 1990). Cutthroat trout are an important late-spring and early-summer food source for bears and may provide bears the opportunity to regain body mass after den emergence and help females with cubs meet the energetic demands of lactation. Grizzly bears are known to prey on cutthroat trout in at least 36 different streams tributary to Yellowstone Lake (Hoskins 1975, Reinhart 1990). During a 3-year study of bear use of cutthroat trout spawning streams in YNP, Reinhart and Mattson (1990) estimated that approximately 44 different autonomous bears were making use of spawning streams around Yellowstone Lake. For a two-month period, spawning cutthroat trout are the predominant food source of some bears in the Yellowstone Lake area (Mattson et al. 1991). Bears prey on cutthroat trout from early May through mid August (Gunther 1985); however, greatest use occurs during the months of June and July (Mattson et al. 1991). Hoskins (1975) found that grizzly bears in YNP were most successful fishing small shallow streams or shallow riffle areas in larger streams. Larger streams with few or no shallow riffles showed no evidence of bear fishing activity. Reinhart (1990) found that bear fishing success correlated more closely with fish density/m3 of water than fish density/linear stream distance. Gunther (1984, 1985) and Reinhart (1990) reported that bear fishing activity peaked during the downstream migration of post-spawning cutthroat trout. Post-spawners are generally fatigued from spawning activity (upstream migration and building redds) while at the same time stream depths have usually decreased, effectively increasing trout density and stream fishability (Reinhart 1990).
In 1994, a small number of anglers reported catching lake trout (Salvelinus namaycush) in Yellowstone Lake (Keading 1995). Lake trout are not indigenous to the lake and their predatory habits are perceived to be a threat to the native Yellowstone cutthroat trout and to the many other aquatic and terrestrial species that depend on cutthroat trout as a food source (National Park Service 1995). Lake trout are thought to have been illegally introduced to the lake, because natural movement of lake trout into Yellowstone Lake from other waters in the park where they are found is highly improbable (Keading 1995). A 1994 survey of Yellowstone Lake's potential as lake trout habitat indicated that lake water temperatures and quality are ideal for lake trout, suitable substrate for lake trout spawning occur at numerous locations around the lake, and that lake trout may have been reproducing in the lake since at least 1989. In addition, the number of lake trout now inhabiting Yellowstone Lake was estimated to be as many as tens of thousands (Keading 1995).
Lake trout are capable of rapid population increase (Curtis 1990) and are likely to thrive in Yellowstone Lake unless preventive management actions are taken. The extent that the cutthroat trout population of Yellowstone Lake might be reduced by lake trout competition or predation is unknown but potentially substantial (Keading 1995). Macroinvertebrates eaten by cutthroat trout are also eaten by young lake trout (Elrod 1983, Elrod and O'Gorman 1991) and cutthroat trout themselves are preyed upon by adult lake trout (Keading 1995). Introduced lake trout have reduced, eliminated, and may have caused the extinction of some native trout species in some areas (Ellis 1914, Gerstung 1988, and Donald and Alger 1993). In Heart Lake, YNP, the native Yellowstone cutthroat trout population declined significantly after lake trout of unknown origin became established (Dean and Varley 1974).
There is only a slight chance that lake trout can be eliminated from Yellowstone Lake (National Park Service 1995). However, substantial control of the lake trout population's expansion is feasible (National Park Service 1995). If the lake trout population is not suppressed, the loss of cutthroat trout from present levels would probably equal or exceed 50% in the next 20 years and 70% or more in 100 years (National Park Service 1995). With suppression of the lake trout population through extensive, long-term gill-netting, trapping, and angling, the loss of cutthroat trout might be limited to 10 - 20% of present population levels (National Park Service 1995).
Cutthroat trout are an important, high-quality food source for grizzly bears that have home ranges adjacent to Yellowstone Lake. Unlike cutthroat trout, lake trout do not move up tributary streams to spawn, but spawn in the lake making them unavailable to many terrestrial predators such as grizzly bears. The actual impacts lake trout will have on the cutthroat trout population and the subsequent impact on the grizzly bears that feed on cutthroat trout is unknown at this time. As bear habitat on private land is lost through development, remaining high-quality food sources, such as spawning cutthroat trout in protected habitat in national parks and wilderness areas, become increasingly important to the long-term survival of grizzly bears. In the Yellowstone ecosystem where occupied bear habitat is isolated from other bear populations, bears are especially vulnerable to changes in carrying capacity because of limited options to shift or expand their ranges or benefit from the influx of bears from other nearby or contiguous populations (Mattson and Reinhart 1994).
However, bears are highly adaptable animals and currently make use of several high quality-food sources (such as winter-killed carrion, cutthroat trout, moths, and whitebark pine nuts) that have a high degree of variation in availability from year to year. Bears in the Yellowstone ecosystem are known to switch to other foods when one or more of these high-quality foods are unavailable. In northwest Montana, whitebark pine has been significantly depleted by extensive infections of white pine blister rust (Mattson and Jonkel 1990). Bear populations in northwest Montana ecosystems have apparently adapted to the loss of this high-quality food source by switching to other foods. The relationship between the use of alternate foods during years of low carrion, cutthroat trout, moth, or pine nut availability and the possible reproductive and survivorship effects resulting from the quality and distribution of alternate foods is not completely understood (Servheen 1986).
YNP is currently designing a management strategy with the objective of suppressing the lake trout population in an effort to reduce the impacts of non-native lake trout on native cutthroat trout and the subsequent impacts on species such as grizzly bears that depend on cutthroat trout as a food source.
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INFORMATION PAPER No. BMO-8
Kerry A. Gunther
Bear Management Office Wildlife Biologist
Yellowstone National Park June 1995