Spatial Arrangements of Cutthroat Trout & Bears

Despite the extensive overlap between cutthroat trout and grizzly bears in the Yellowstone region (see the Introduction), almost all of the documented consumption of trout by grizzlies has been along streams tributary to Yellowstone Lake. Incidental exploitation of trout by bears has also been documented at Trout Lake in northeastern Yellowstone Park, and at several streams to the north.  The map to the left shows tributary streams about which we know something regarding bear activity and trout spawning. These streams are grouped according to proximity, overlap in use by individual bears, and physical characteristics--each group denoted by a Roman numeral.

 

Several different studies have estimataed the numbers of bears using different streams. The large bold numbers denote the number of grizzly bears estimated to have used each stream group by Dan Reinhart during the mid-1980s. Instances of disproportional use by different classes of bears (e.g., lone adults, subadults, family groups) are also described for the same study period. The small black bold numbers by the mouth of individual streams is the number of grizzlies estimated to have been using the stream by Mark Haroldson and other during the late 1990s. Finally, the small bold gray numbers at stream mouths is the number estimated by Jennifer Fortin and Justin Teisberg for 2007-2009, a period when bear fishing activity had declined to essentially nil (see Trends). It is clear from this information that large lone bears (e.g., adult males) comprised a disproportional percentage of grizzlies using east shore streams, and that family groups comprised a disproportional percentage of bears using Flat Mountain Arm streams. It is also likely that use of streamside areas by bears shifted from streams along the South and Southeast Arms to front country streams (West Thumb and Lake areas) in response to declines of trout during the 2000s.

One final note about the map above: Witch Creek, tributary to Heart Lake, is identified in the lower left. Grizzlies were documented here fishing for spawning Utah suckers (Catostomus ardens) during May. Grizzlies are also suspected of fishing for spawning mountain whitefish (Prosopim williamsoni) upstream along the Snake River from Jackson Lake.

Areas of Attraction and Concentration

The figures and map to the right provide information on the extent to which cutthroat trout spawning streams served as an attractant for Yellowstone grizzly bears prior to the demise of cutthroat trout (see Trends). Figures A and B show the proportion of telemetry locations from radio-collared females (A) and males (B) known to have used spawning streams as a function of distance--for the period when spawning occurred (1 May-31 July, in red), and for the remainder of the year (in green). The gray area in each figure represents the total area available to these fish-eating bears within their home ranges, also as a function of distance.

 

Two zones of concentration are evident, one <2 km in extent, in which females concentrated during the spawning season, and another 2-12 km out, in which both males and females concentrated for the remainder of the year. The extent of these two zones is shown in black and gray, respectively, on the map in Panel C. The extent of the combined home ranges of fish-consuming grizzlies (Minimum Convex Polygon [MCP]) is also shown in orange (females) and brown (males). The main point here is that fish-eating bears were highly concentrated near Yellowstone Lake year-round, but potentially ranged over an area that comprised a substantial portion of the grizzly bear Primary Conservation Area (PCA).