Birds & Mammals that Consume Yellowstone Cutthroat Trout in Yellowstone Lake and Its Tributaries
by Daniel J. Bergum, Kerry A. Gunther, & Lisa M. Baril
Yellowstone Lake in Yellowstone National Park (YNP) contains the largest inland population of Yellowstone cutthroat trout (Oncorhynchus clarkii bouvieri) in North America (Behnke and Tomelleri 2002). Historically, these native trout provided food for a variety of wildlife. In 1994, however, park managers discovered non-native lake trout (Salvelinus namaycush) had been surreptitiously introduced into the lake. Their numbers increased rapidly and, in turn, their piscivorous (i.e., fish eating) food habits became a significant threat to the Yellowstone cutthroat trout population (Koel et al. 2005). In addition, the non-native parasite (Myxobolus cerebralis) that causes whirling disease, a significant cause of juvenile mortality, was discovered in tributaries to Yellowstone Lake during 1998 (Koel et al. 2006). Plus, a recent period of drought reduced the survival and recruitment of juvenile cutthroat trout (Koel et al. 2005). These combined effects caused a 90% decline in the cutthroat trout population in Yellowstone Lake (Koel et al. 2005). Lake trout are not a suitable ecological substitute for cutthroat trout in the Yellowstone Lake system.
Lake trout inhabit deeper waters and, unlike cutthroat trout, do not move into tributary streams to spawn; therefore, lake trout are inaccessible to many avian and terrestrial predators (Koel et al. 2005). Although lake trout do move into shallow waters of the lake to spawn, they often spawn at night which makes them unavailable to shallow water avian predators with diurnal habits. Spawning cutthroat trout migrate throughout the day and night (Gresswell 2011), which makes them vulnerable to nocturnal, crepuscular, and diurnal shallow water predators.
The decline in cutthroat trout could have negative consequences on the reproduction and survival of birds and mammals that consume them in the Yellowstone Lake watershed. We reviewed available literature to determine which predator and scavenger species are known to prey on or scavenge cutthroat trout in Yellowstone Lake or its tributaries and, as a result, might be adversely affected by the decline in cutthroat trout. Our literature review included all 221 bird and 67 mammal species known to inhabit YNP (see www.nps. gov/yell/learn/nature/ to view species checklists for the park).
We identified 20 species, including 16 birds and 4 mammals, which prey on or scavenge cutthroat trout in Yellowstone Lake or its tributaries (table 1). The bird species known to prey on cutthroat trout include osprey, American white pelican, Caspian tern, double-crested cormorant, belted kingfisher, common merganser, eared grebe, great blue heron, California gull, bald eagle, common loon, American dipper, Barrow’s goldeneye, bufflehead, common raven, and great horned owl. In addition, four species of mammals—American black bear, grizzly bear, mink, and river otter—have been documented preying on cutthroat trout in Yellowstone Lake or its tributaries. The cutthroat trout decline has likely affected each of these species differently, due to their varied feeding habits and lifestyles.
Although the decline in cutthroat trout in Yellowstone Lake probably has caused some disruption in the food supply for all 20 of these species, the greatest impacts have been incurred by five species: osprey, American white pelican, Caspian tern, double-crested cormorant, and river otter. Nesting success for ospreys, cormorants, American white pelicans, and Caspian terns has decreased over the last 20 years, coinciding with the period of cutthroat trout decline (Smith et al. 2012, 2013, Baril et al. 2013). The only known nesting area for Caspian terns, American white pelicans, and dou ble-crested cormorants in YNP is on the Molly Islands in the Southeast Arm of Yellowstone Lake. The reduced availability of cutthroat trout and lack of an alternative prey source means these species have to travel farther to find food, which in turn likely reduces nesting success and increases the energetic costs of foraging with possible consequences to survival.
The river otter has also incurred significant impacts. Crait and Ben-David (2006) found little evidence otters could switch to feeding on introduced lake trout as an alternative prey to cutthroat trout. Lake trout are abundant and lipid rich, but are generally found in deep water beyond the foraging depth range of otters (Ben-David et al. 2000). Although longnose suckers (Catostomus catostomus) are a slow moving species, grow to a large size, and are obligate stream spawners (Brown and Graham 1954), they are less abundant (Stapp and Hayward 2002) and of lower energetic value (Ruzycki et al. 2003) than cutthroat trout. Therefore, longnose suckers are not an equivalent alternative prey for otters (Crait and Ben-David 2006). The decline in the cutthroat trout population in combination with the inaccessibility of lake trout and lower abundance and energetic return of alternative prey is expected to reduce the population density of otters in Yellowstone Lake (Crait and Ben-David 2006).
Grizzly bears, black bears, and bald eagles with home ranges around Yellowstone Lake have exhibited dietary flexibility and switched to other foods, limiting the potential nutritional stress caused by the loss of cutthroat trout as a food item. Bears are opportunistic omnivore generalists that feed on many species of plants, fungi, mammals, insects, birds, and fish. Fortin et al. (2013) found evidence that bears with home ranges adjacent to Yellowstone Lake may now be preying more on elk calves during the time period when they used to fish for cutthroat trout. Bald eagles appear to have compensated for the decrease in cutthroat trout by preying more on waterfowl (Baril et al. 2013). Like bears, the California gull and common raven are also opportunistic omnivore generalists (Davenport 1974, McEneaney 2002) capable of eating many different foods, thereby minimizing the impacts of the cutthroat trout decline.
In the past, cutthroat trout comprised a large portion of the diets of common loons, common mergansers, eared grebes, great blue herons, and belted kingfishers around Yellowstone Lake (Davenport 1974). However, except for the common loon, the reproduction and nesting success of these species has not been monitored. As a result, the impacts of cutthroat trout decline on these species are not known. The number of adult common loons observed in YNP appears stable, although nesting pairs and fledglings have decreased since 1987 (Smith et al. 2012). As of 2014, one-third of the park’s 29 loons nested and foraged on Yellowstone Lake. Loons in YNP are of special concern since they represent nearly 64% of all loons in Wyoming and are isolated from other populations by more than 200 miles (Evers et al. 2013). Cutthroat trout comprise a minor component of the diets of mink (Lariviere 2003, Melquist et al. 1981), American dippers, Barrow’s goldeneyes, buffleheads (Davenport 1974), common ravens, and great horned owls (McEneaney 2002). Therefore, the impact of the cutthroat trout decline on the reproduction and survival of these species is likely minimal.
The cutthroat trout is an iconic and important species due to its place in the food web in Yellowstone and the Greater Yellowstone Ecosystem. Park managers are attempting to restore cutthroat trout in Yellowstone Lake and its tributaries through an aggressive lake trout removal program. If this program results in a significant long-term reduction in predatory lake trout, native cutthroat trout may reestablish at higher numbers in Yellowstone Lake and its tributary streams, and once again become an important dietary item for the birds and mammals that feed on this resource in the Yellowstone Lake watershed.
For an expanded version of table 1 which includes all known, suspected, and possible species that consume Yellowstone cutthroat trout, please visit go.nps. gov/who_eats_YCT
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Dan Bergum first visited Yellowstone during the 1988 fires as a boy, leading him to pursue a BS in Ecology at the University of Minnesota at Mankato. He has worked as a biologist for the past 12 years and a Bear Management Technician in Yellowstone National Park for the past five. Dan, his wife Karin, and two sons Colter and Theodore, spend their time outdoors exploring the mountains of Wyoming.