Ecological Relations of Grizzly Bears & Cutthroat Trout

Zones of Attraction

 

The figure immediately above amplifies information presented in the section on spatial relations of trout and bears--more specifically, the extent to which grizzlies that consumed spawning cutthroat trout concentrated around spawning streams year-round (A), and the heightened likelihood that any bear spending any amount of time near a spawning stream would end up fishing for trout (B). Panel A simply shows the portion of all locations obtained from radio-collared grizzlies that were attributable to fish-eating bears as a function of increasing distance from a spawning stream. In other words, fish-eating bears comprised over 75% of all locations within 2 km of spawning streams and over 50% of all locations within 12 km. These concentrations add further credence to the 2-km and 2-12-km zones of influence described in the Spatial Relations section. Panel B shows three curves representing the probability that a bear would have likely consumed spawning trout as a function of the percentage of that bear's locations concentrated in the 2 km zone (black line), the 2-12 km zone (dark gray line), and beyond 12 km of a spawning stream (light gray line). In other words, if a bear had roughly 12% of its radio-telemetry locations within 2-km of a spawning stream, it was 80% likely to have fished for trout; or, if it had roughly 18% of its locations within the 2-12-km zone, it was 60% likely to have fished for trout.

Explaining Local Distributions

Research done by Dan Reinhart during the mid-1980s provides valuable insight into the factors affecting how grizzly bears distributed themselves along Yellowstone Lake spawning streams during the spawning season. Between 1985 and 1987, Dan collected information on numbers of spawners, timing of spawning, and levels of bear activity, including fishing.

 

The figure to the left immediately above shows the relationships between densities of spawning cutthroat trout and levels of bear fishing (B) and activity other than fishing (A). As one would expect, the relationship is positive in both cases. But the relationship between fishing activity and spawner densities is much stronger and tending to increase at an increasing rate, whereas the relationship between other bear activity and spawner densities is weaker, with a tendency to increase at a decreasing rate (i.e., flatten). In other words, fishing activity is much more tightly linked than is other activity to abundance of spawners--which makes sense. Another important point is that density (fish per cubic meter) is a stronger determinant of bear activity compared to rote numbers of spawners . You can have a large stream on the east shore such as Clear Creek, with thousands of spawners, but have less bear activity compared to a smaller stream such as Cub Creek, with fewer spawners concentrated in a smaller volume of water--which predictably makes the trout that are present easier to catch.

 

Another factor driving distributions seems to be other bears--especially large lone bears (most likely adult males). Such an effect would not be surprising given that adult males will kill other bears, most notably cubs; you would expect females with young to avoid these potential killers. Dan Reinhart collected information on track sizes, from which he and I could deduce the presence of a family group and the presence of large lone bears (adult males?). The figure to the right immediately above shows, in A, the relationship between numbers of tracks of family groups relative to the numbers of tracks left by large singletons. The relationship is substantially negative, but conditioned on where you are around the lake, especially relative to human facilities. Panel B shows the number of family group tracks you would expect after adjusting for the effect of large lone bears (green versus gray dots). The biggest positive boost is on the east shore, where spawner densities are not only high, but where streams are also remote from humans. Panel C shows the relationship between adjusted numbers of family group tracks and spawner densities by stream group (see Spatial Relations). These few data points suggest a positive relationship, which is what you would expect from relations presented in the figure to the left above. Bottom line: Adult male grizzlies probably affected the distribution of females with young, with implications for what might unfold as cutthroat trout populations declined (see Trends).  

Importance of Trout to Males and Females

It is not clear to what extent cutthroat trout were historically an important source of energy and nutrients for female verus male grizzlies. I use the past tense here given the dynamism of the Yellowstone Lake cutthroat trout population and the extent to which this population has declined since the 1980s. The main contradictions arise from the results of mine and Dan Reinhart's work and that of Laura Felicetti. Felcetti's results suggest that male grizzlies ate roughly five times more trout than did female grizzlies (135g/kg versus 26g/kg), whereas Dan Reinhart's and my work suggest otherwise. A critically important contextual distinction pertains to the status of trout populations when the research supporting each result occurred. Reinhart's research occurred during the peak of Yellowstone's cutthroat trout population (1985-1987); Felicetti's when the population was at low ebb, albeit during a minor resurgence (1997-2000; see Trends).

 

Some key results from the mid-1980s are summarized in the figure immediately above. Of the bears strongly suspected of consuming cutthroat trout, females spent nearly 1.6 as much time near spawning streams as did males (during the spawning season; left above, Panel A). Moreover, analysis of scats collected within 500 m of spawning streams suggests that the bears that were present had a diet comprised of 90%+ cutthroat trout (Panel C). Although scats dropped by females couldn't be distinguished from scats dropped by males, the inescapable conclusion is that any bear near a spawning stream was eating almost wholly cutthroat trout during the spawning season. Logically, then, females were eating more trout than were males, at least during the mid-1980s. The only way this could not be the case is if females were concentrating near spawning streams, but not eating trout, which seems implausible in light of everything know about what motivates the foraging behavior of animals.

 

Another interesting paradox arises from the fact that Felcetti estimated the median consumption of trout by bears during 1997-2000 to be around 0.024-1.09 kg per individual, at the same time that the Haroldson study (from which she obtained her samples) estimated that bear activity around spawning streams--including fishing--had not dramatically diminished from highs during the mid-1980s (see Trends). The claims by Felcetti and Haroldson find little support in independent observations. For one, trout populations had declined substantially--if not catastrophically--between the 1980s and late 1990s (see Trends). Given the strong relationship between trout densities and bear fish activity shown above, it seems implausible that such a decline in trout populations would have had a minor effect on bear activity. The claim by Felcetti that trout were roughly 5-times more important a source of energy for males compared to females also starkly contradicts the results described immediately above. Finally, the fact that I documented a single bear consuming roughly 6 kg of trout in a single 41 minute bout of fishing (see Fishing behavior) suggests that any bear spending any amount of time fishing streams under favorable circumstances would have consumed many kg of trout during a single season--not something less than 1 kg.

 

An explanation for contradictions between the results of the mid-1980s and late-1990s studies potentially takes two forms. One is that the differences are simply a result of starkly different numbers of spawning cutthroat trout, with related changes in relative access to streams by females versus males. There is ample evidence that adult male grizzlies can dominate concentrated food resources such as spawning trout, especially to the exclusion of security-conscious animals such as females with dependent young (the case two years out of three for Yellowstone females). Thus, if prime fishing opportunities had become increasingly spatially restricted, males would have become the primary winners, and females the losers. The result would have been a shift from disproportional use of the trout resource by females to disproportional use by males.

 

The second explanation pertains to methods. Felicetti depended upon mercury concentrations in tissue collected from trapped bears to make inferences about levels of trout consumption. Reinhart depended upon observations of tracks and contents of collected scats. One could argue about the merits of the respective approaches, but the veracity of mercury as an indicator of trout consumption depends upon the assumption that nothing in a bear's interactions with the environment would affect mercury levels other than eating fish. Although Maurice Chaffee and others make a case for the extent to which mercury signals trout consumption by bears in Yellowstone, this confidence is called into question by the fact that similar confidence in sulfur concentrations as a signature of whitebark pine seed consumption (see whitebark pine) was called into question by Charles Schwartz and others when they concluded that this chemical signal was not correlated with any other known annual measure of pine seed consumption. A similar cautionary note was struck by Grant Hilderbrand regarding the use of nitrogen concentrations to infer consumption of terrestrial meat. It is entirely possible that similar as yet unidentified biases affect mercury concentration, especially in an environment such as Yellowstone's where all sorts of chemicals circulate for reasons related to spatial variations in geological substrate and geothermal activity.

 

Perhaps, as much as anything, I harbor suspicions about any increasingly indirect measure of bear behavior that necessarily relies upon an increasing number of assumptions and increasing reliance on reductionist methods. These kinds of suspicions are supported by the considerable on-going controversies regarding diets of the extinct Eurasian cave bears, all of which organize around different interpretations of how chemical isotopes are concentrated by diets and sampled by researchers. Any one research article would lead you to conclude that the matter had been decisively settled, only to be unsettled by the next, and so on. Although over-confidence is not the exclusive domain of laboratory scientists who make broad inferences, I would argue that asserted assumptions of any form are cause for caution.