Spawning cutthroat trout were a major source of energy and nutrients for grizzly bears living within range of Yellowstone Lake between the mid-1970s and late 1990s. The estimated percentage of the bear population that exploited this resource varied from 15-25%, depending on the time period, although few grizzlies still consume trout. Almost all of the exploitation of trout by grizzly bears occurred during the May-July spawning season along streams tributary to Yellowstone Lake; the bears involved in this use comprised the majority of those active within 12 km of the Lake year-round. At least within recent historical times, trends in use of trout by Yellowstone's grizzlies were driven by variations in the size of the Yellowstone Lake cutthroat trout population. Trout numbers were low during the 1940s-1960s because of heavy sport fishing, and at rock-bottom lows during the last 15 years or so because of the combined effects of climate change and predation by a non-native predator--lake trout--which was unintentionally introduced into Yellowstone Lake during the early 1990s.
The buttons to the right take you to pages that elaborate on the basics summarized here related to spatial arrangements, ecological relations, and trends. The text below provides a bit more background on the importance of trout to grizzlies during the 1970s-1990s, the seasonality of consumption, and some spatial and historical context.
The big picture
Cutthroat trout (Oncorhynchus clarkii) are widespread in the interior western United States. This extensive distribution, coupled with the complex effects of volcanism and glaciation, has resulted in varying degrees of genetic isolation sustained over varying periods of time. The upshot is a number of different clades and subspecies. The subspecies found within the bulk of Yellowstone's grizzly bear range is the Yellowstone cutthroat trout (O. c. bouvieri). The map to the right shows the historical distribution of this subspecies (in blue), together with the boundary of Yellowstone National Park and the Primary Recovery Area for Yellowstone's grizzly bears. Despite extensive overlap between the distributions of trout and grizzly bears in this region, almost all use of trout by bears has been concentrated around Yellowstone Lake. The reasons for this are not altogether clear.
Cutthroat trout are part of the salmon family (Salmonidae), which includes a number of other species popularly known as "salmon" (as opposed to "trout"). Although most people are aware of the fact that brown bears (of the same species as grizzly bears) heavily consume spawning salmon in coastal streams of Alaska and British Columbia, few probably know that grizzly bears also heavily consumed salmon in what is now the lower 48 states. Almost all of this historical consumption occurred along tributaries of rivers that drained Washington, Oregon, Idaho, and California, most notably tributaries of the Columbia River, some of which hosted spawning salmon as far inland as central Idaho and southeastern Oregon. Historically, the species most heavily used by bears were chinook salmon (Oncorhychus tshawytscha) and steelhead (O. mykiss).
The map to the immeidate right shows the approximate pre-extirpation distributions of grizzly bears and salmon in the western United States. The overlap between salmon and grizzly bears is show in dusky green; the current main distributions of grizzlies by red lines. The map also shows (as white numbers) the estimated percent contribution of salmon to the total energy ingested by grizzlies within the area of overlap. These estimates are based on the analysis of diagnostic isotopes by Grant Hilderbrand and his colleagues using either hair or bone collagen from the remains of grizzlies that were killed during the late 1800s and early 1900s. As you can see, salmon were a major, if not dominant, source of energy and nutrients for grizzlies in a considerable portion of this species' historical range.
How did cutthroat trout get into Yellowstone Lake?
One final nagging question deserves being addressed in this introductory section: How did cutthroat trout make it to Yellowstone Lake, and when? This question arises because the area currently occupied by Yellowstone Lake was covered by an ice sheet over 3000' thick during the Ice Ages--an ice sheet that didn't begin to melt until around 15,000 years ago. Moreover, the point of entry for trout after ice melt is not obvious. The Falls on the Yellowstone River below Yellowstone Lake present an insurmountable obstacle to any trout swimming upstream. Answering this question is relevant to defensibly speculating on whether, and for how long, grizzlies might have converged on Yellowstone Lake to fish for spawning cutthroat trout.
One thing is clear. The cutthroat trout in Yellowstone Lake are genetically most closely related to cutthroat trout in the Snake River drainage, especially those trout living in the Bear Lake area of Idaho. This suggests that cutthroat trout somehow made it over the Continental Divide from the Snake River, draining to the Pacific, to the Yellowstone River, draining to the Atlantic. One fish ecologist, Bob Behnke, speculates that cutthroat trout might have jumped--and continue to jump--from one basin to the other in the vicinity of Two Ocean Pass, where the headwaters of each drainage are within a short distance connected by a series of wet meadows. Another not unrelated possibility is that cutthroat trout made it into the Yellowstone Lake basin when the lake flowed south and west over what is now the Continental Divide into the Snake River drainage.
Thanks to the pioneering geologic work of Gerry Richmond, we have some idea of the timing and direction of this flow, as indicated by the arrows in the map to the left. During the post-Ice Age melt of Yellowstone's ice cap, stagnant ice periodically dammed Yellowstone Lake, blocking its outlet to the north. This raised lake levels and diverted flows into the Snake River. The extent of elevated lake levels is indicated by the extent of ancient lake deposits denoted by shades of brown in the map to the left. Under such circumstances it is not too difficult to imagine cutthroat trout swimming upstream along the Snake River and colonizing Yellowstone Lake from the south--many thousands of years ago.
Energetics, nutrition, and seasonality
The figures to the left summarize the estimated contribution of cutthroat trout to the diet of the Yellowstone grizzly bears between the late 1970s and early 1990s. These estimates are based on analysis of fecal material ("scats") deposited by grizzly bears throughout the Yellowstone ecosystem during 1977-1993. The resulting sample of diet is amongst the largest and most comprehensive ever compiled for a bear population anywhere in the world: over 6000 scats spanning 16 years for most of the bears' active period between March and October.
The topmost bar graph to the left summarizes the year-round contribution of three main grizzly bear foods--including trout (in red)--to total volume, fat (i.e., lipids), and protein ingested by Yellowstone grizzlies, as well as to total digested energy. The three graphs below, left, show these contributions by month, broken down by total digested energy, ingested protein, and ingested fat. The monthly variation in total amounts varies substantially because of seasonal differencies in overall levels of feeding activity (low in the spring, high in the fall and late summer), as well as differences in numbers of bears out of their dens early and late in the active seasonl.
As you can see, trout were an especially important source of energy and protein, provided in pulses that peaked during June and July. There is little doubt that rhe functional extirpation of trout in a number of spawning streams (see Trends) has left a major nutritional hole during these months that bears have needed to fill, apparently by preying more heavily on elk calves (which are particularly vulnerable this time of year) and by more heavily consuming lower-quality vegetal foods.
The graph to the immediate right provides a more refined picture of the seasonal use of spawning trout and associated streamside habitats by Yellowstone grizzlies. Although peak use generally occurred between roughly June 10th (Julian date [JD] 161) and August 8th (JD 220), the attenuation and amplitude of use varied from stream to stream, as illustrated by the differences between Flat Mountain Cr and Cub Cr. In general, use of streamsides and consumption of trout began earlier and built more gradually along streams draining the plateaus to the west of Yellowstone Lake (exemplified by Flat Mountain Cr) compared to streams draining the Absaroka Mountains to the east (exemplified by Cub Cr). These differences suggest that streamside vegetation--aside from trout--may have been more important to westside bears compared to eastside bears, which is not surprising given the surrounding habitats.