Yellowstone Lake's cutthroat trout population has exhibited huge ups and downs since the 1940s, when the Park Service first began to track trout numbers. Consumption of trout by bears has correspondingly varied. The composite figure above attempts to summarize these key trends.
Panel A shows trends in numbers of spawning trout censused in Clear Creek, a major spawning stream on the east side of Yellowstone Lake (the blue line); the average length of trout captured in trawl nets (gray line); and numbers of fish killed by human anglers (red line). Key trends in numbers of spawning trout have been a major increase during the 1960s, a substantial dip during the early 1980s, a resurgence during the late 80s, and a subsequent sustained decline to the present. Average size of trout has consistently varied inversely to population trends, indicating lack of recruitment of fish into smaller size classes during periods of decline.
Panel C shows levels of bear activity along spawning streams since 1989 (transects are located on streams concentrated on the west and northwest shores of the Lake). During 1989-1998 bear activity was substantial but varied. However, beginning in 1999, activity declined to the point where, now, bears rarely fish for spawning trout. This decline in bear activity is self-evidently related to the decline in numbers of spawning trout shown in blue in Panel D. Panel D also shows, in brown, numbers of lake trout captured in gill nets, standardized to reflect level of effort. Non-native lake trout were first documented in Yellowstone Lake in 1994, after which their numbers increased, then declined, and then increased again to the present. Lake trout prey on cutthroat trout and are implicated in the decline of cutthroat numbers.
One important feature of Panels A, C, and D is the denotation, as gray vertical bars, of different grizzly bear studies of relevance to documenting consumption of trout around Yellowstone Lake; each study is labeled with the name of the lead investigator(s). The Hoskins, Reinhart, Haroldson, and Fortin studies surveyed tributary streams specifically to document bear activity along with size and duration of spawning runs. Panel B at the very top summarizes the results of each of these studies in terms of: number of streams with spawning runs (SSs, dark blue bar), with bear activity of any sort (BA, dark gray bars), and with sign of bear fishing (BF, light gray bars).
Several important conclusions can be drawn from these trends. First, a lack of bear fishing during the Craighead brothers' 1959-1970 study is plausibly related to a dearth of spawning trout, in turn attributable to high levels of angler-caused mortality. Second, the Hoskins and Reinhart studies more-or-less coincided with peak in spawner numbers. Lesser levels of bear activity during the Hoskins study compared to the Reinhart study can perhaps be attributed to fewer bears and a lag in discovery by bears of the trout resource. Third, the Fortin study occurred during a nadir in trout numbers and related bear activity, whereas the Haroldson study was conducted during a temporary resurgence of trout numbers after substantial declines from the peak in the late 1980s. Differences between results of the Reinhart and Haroldson studies suggest that the large-scale decline in cutthroat trout numbers was manifest more in the duration of spawning runs rather than in the numbers of spawners at the peak of the run. Although bear fishing and other indicators of bear activity declined between the mid-1980s and late 1990s, these declines were not as dramatic as might have been expected. One plausible explanation might be lags in the behavioral and population-level responses of bears to declines in a food resource that was not only high-quality, but also part of a learned tradition--a mirror image of the lags that might have affected the comparatively low levels of bear use documented by Hoskins during the mid 1970s.
But, why did cutthroat trout numbers declined so catastrophically in the 1990s and 2000s? No doubt predation by non-native lake trout was a major factor, especially during the 2000s. However, declines in cutthroat numbers predated any conclusive evidence of lake trout. Lynn Kaeding, who wrote his dissertation on the topic, implicates climate change as a major driver, especially in producing lesser stream flows. This climate-induced phenomenon also probably got a kick-start from the wildfires of 1988, which had significant negative effects on streamflows and the physical condition of numerous spawning streams.
Finally, and perhaps most importantly, there is no doubt that a food resource that was once of major seasonal importance to a large number of Yellowstone's grizzly bears (see Introduction) has essentially been lost. Given climate warming, this loss will be irreversible.
The figure to the left focuses on differences in results between the Reinhart and Haroldson studies, the former during a period of peak spawner numbers and the latter during a brief resurgence following a period of dramatic decline. Panel A shows differences in numbers of spawners at the peak of the run (light pink) as well as duration of the run (dark pink) for streams along different quadrants of Yellowstone Lake, with each bar denoting the magnitude of difference between the mid-1980s and late-1990s. The duration of runs had declined substantially everywhere except along the various arms of the Lake to the south and west; number of spawners at peak had declined less so, and seemed to have actually increased in streams tributary to the Arms.
But bear activity, however reckoned, declined in all areas, although levels of fishing activity were least impacted, especially in the Lake area and along the Arms, where trout continued to fare relatively well. Overall, though, these differences are less than might have been anticipated given the massive declines in cutthroat trout populations documented during more comprehensive longer-duration surveys, as described above.