Grizzly and black bears that once feasted on cutthroat trout spawning in Yellowstone Lake tributaries will readily return to the food source if the species bounces back, a study predicts.
The study, “Contrasting Past and Current Number of Bears Visiting Yellowstone Cutthroat Trout Streams,” supports a multimillion-dollar effort to reduce invasive mackinaw in Yellowstone Lake. The paper was published in the Journal of Wildlife Management in February.
“We suggest that the number of spawning trout per stream will have to reach approximately 400 fish per kilometer of stream before large numbers of grizzly and black bears once again specialize on this food,” Washington State University researcher and chief author Justin Teisberg wrote.
“If the Yellowstone Lake cutthroat trout population can be recovered to such levels,” Teisberg wrote, “grizzly and black bears that still reside within the lake basin will readily find and use this high-quality food resource, potentially returning both species to higher use of backcountry habitat.”
Teisberg, who conducted field work from 2007 to 2010, suggests that cutthroat numbers have a long way to go before the species is a dependable food source for bruins once again. A similar survey of spawning activity and grizzly bear use was conducted between 1997 and 2000, providing Teisberg with comparable data.
“The decline in cutthroat trout spawning activity was more widespread than previously documented,” Teisberg wrote. “The median peak [spawning fish] count on streams decreased by a median of 98.4 percent, or 71 fish.”
In Teisberg’s study, the median number of spawning trout counted per Yellowstone Lake tributary was a dismal 2.5 fish. Only 15 out of 35 known cutthroat spawning streams sampled had spawning activity, on average, each year. No cutthroat were ever observed over three years on 20 percent of the known spawning steams sampled.
Historically, 59 of Yellowstone Lake’s 124 feeder streams supported cutthroat trout.
Grizzlies and black bears once congregated around the streams during the spring spawn.
As recently as the late 1980s, grizzlies in the Yellowstone Lake area ate an estimated 21,000 cutthroat a year, Teisberg wrote. By the late 1990s consumption dipped to 2,300 trout a year.
During Teisberg’s study, he wrote, the estimate was just 302 cutthroat a year — seven cutthroat a year for every male grizzly, three trout a year for every female.
Lake trout, believed to have been introduced to Yellowstone Lake illegally in the late 1980s, are thought to be the primary cause in the collapse of the cutthroat population. Fatal whirling disease and habitat-degrading drought are also implicated in the decline, Teisberg wrote.
Lake trout, summer spawners that lay their eggs in Yellowstone Lake, are not an accesible food source for terrestrial wildlife.
Over the course of Teisberg’s research an increasing number of spawning cutthroat were observed.
More than 1.3 million predatory lakers have been netted and killed in Yellowstone Lake since the reduction effort began in 2000, and park biologists say cutthroat numbers are starting to respond.
Rising cutthroat numbers, however, now have no bearing on the location or number of grizzly bears near Yellowstone Lake, Teisberg wrote.
“We found no relationship between trout numbers and numbers of bears in the current study,” he wrote, “presumably because the low density of remaining trout (less than 1 percent of historical numbers) can no longer be efficiently exploited by grizzly bears.”
Grasses, sedges and forbs have replaced trout as the primary bear foods found along Yellowstone Lake stream corridors.
The number of grizzly bears using the stream corridors — for any reason — decreased by an estimated 63 percent between the turn-of-the-century study and Teisberg’s study. Black bear use of the feeder streams decreased by “64 to 84 percent,” the study said.
Grizzly distribution also shifted from backcountry streams — located on Yellowstone’s Lake’s inaccessible east and south sides — to the lake’s more developed sides.
“The relative abundance of cutthroat trout may influence bear mortality rates,” Teisberg wrote. “For other major food items (e.g., whitebark pine nuts, ungulates and army cutworm moths), conflicts between grizzly bears and humans increase in the Greater Yellowstone Ecosystem during years of low availability but decrease when these preferred foods are abundant.”
Teisberg is confident that grizzly and black bears could one day relearn how to use cutthroat as a seasonal food source.
“We considered whether the use of cutthroat trout by future generations may be in jeopardy because of the loss of traditional foraging knowledge passed from mothers to cubs,” Teisberg wrote. “However, with a well-documented natural history as an opportunistic omnivore ... we are certain that bears, if abundant, will once again return to the use of this high-quality food resource.
“For these reasons, we endorse the ongoing gillnetting efforts to control numbers of lake trout in Yellowstone Lake,” Teisberg wrote in closing.
Teisberg and Mark Haroldson, a contributing author and Interagency Grizzly Bear Study Team member, did not respond to interview requests on Tuesday.