Spawning Salmon

To the extent that people are even aware of bears, most would probably consider salmon to be the iconic bear food. Bears are at their most photogenic while fishing for spawning salmon, especially in areas where they are concentrated and highly visible such as Brooks River, Knight Inlet, and McNeil River Falls (for more on fishing behavior click on this link or the link immediately above). As a result, photos featuring bears eating salmon are disproportionately numerous when compared to the full diversity of bear feeding activities. Nonetheless, salmon are a primary source of energy and nutrients for the vast majority of brown bears occupying coastal regions of the Bering Sea and adjacent North Pacific. (Parenthetically, brown bears are Ursus arctos, the same species as grizzlies)

 

The graph to the right is illustrative. The height of each bar is proportional to the percent of total dietary energy contributed by vegetal foods in each of 13 North American grizzly bear populations (data are from Grant Hilderbrand and his colleagues). The remainder of each is attributable to meat of some sort. So, the shorter the bar, the greater the contribution of meat. I've highlighted the diets where meat was the source of >35% of dietary energy in shades of salmon, with a lighter shade denoting those populations where spawning salmon where the primary or sole source of meat; and a darker shade denoting those populations where ungulates and ground squirrels were the dominant sources.     

 

 

As you can see, brown bears in coastal regions of Alaska such as Katmai National Park, the Kenai Peninsula, or around Prudhoe Bay get the majority of their energy and nutrients from anadromous salmon. In places, this fraction exceeds 75%. By contrast, grizzly bears in wet interior regions that currently have no access to salmon (e.g., around Glacier NP, in the North Fork of the Flathead, and in the Selkirk & Cabinet-Yaak ecosystems) obtain 80-90% of their energy from vegetal foods, much of which is berries (see vegetal foods and terrestrial meat). The current dietary dominance of salmon in coastal regions is consistent with the estimated contributions of salmon to grizzly bear diets during pre-European times in areas where grizzlies have since been extirpated (e.g., in the Columbia River Basin and in northern California and adjacent Oregon; see Pre-European diets I). 

The geography of availability & use

The map above left and the figure above right show, in different ways, the historical and current distributions of spawning anadromous salmonids in western North America. The varying hues of green in the map correspond to the number of salmon species in any given area, with the darkest green corresponding to areas where six species are (or were) present, and the lightest green to areas with only one species. Barring a small interval along the coast in Alaska, the entire coastal region from southwest Alaska, through the Alaskan panhandle and British Columbia, as far south as the Pudget Sound and Olympic Peninsual of Washington was endowed with the full diversity of anadromous salmonids native to western North America, including sockeye (Oncorhynchus nerka), chum (O. keta), pink (O. gorbuscha), coho (O. kisutch), chinook (O. tshawytscha), and steelhead (O. mykiss). In addition to this area of greatest diversity, one or more salmon species occur in inland waterways of all of Alaska except the North Slope, into the southern Yukon, and throughout all but the far northeastern and some eastern regions of British Columbia. Anadromous salmon also historically spawned throughout essentially all of the Columbia River Basin of Washington, Oregon, Idaho, and British Columbia, and in coastal rivers draining areas as far south as coastal California and western slopes of the Sierra Nevada. Unfortunately for bears (and those who care about salmon) salmon runs in these southernmost region have been substantially diminished or even extirpated by the effects of dams and irrigation projects.

 

The map above also shows areas in North America (delineated in white) where densities of brown and grizzly bears are highest--in excess of 40 bear per 1000 square kilometers (see Bear densities). Along some parts of interior British Columbia, the densest populations of brown bears coincide with near-coastal areas that have the greatest abundance and diversity of spawning salmon; where bears can consume this nutritious food to the near exclusion of anything else from July through November. Certainly this holds true for all areas where brown bear densities are currently highest of all: greater than 100/1000 km2. Not coincidentally, these same densely-occupied areas also support physically the largest bears in North American (see Variation in bear size), no doubt because a proteinaceous and energy-rich diet of salmon promotes rapid and sustained accumulation of lean body mass (see Nutrition and Variation in bear size).

 

As you might expect, not all salmon are equally abundant in all areas, even where all six species spawn. As a consequence, the importance of different species to bears varies from region to region. The figure above right attempts to quantify these differences along a north (top) to south (bottom) Pacific coastal gradient. The relative local abundance and related intensity of consumption by bears is indicated by the vertical width of each colored bar, one for each species. Chinook salmon and steelhead are most abundant (and were most heavily consumed by bears) in south-coastal regions, including as far inland as central Idaho; sockeye is comparatively most heavily used farthest north and west, in southwestern Alaska; and chum, pink, and coho salmon are most abundant and heavily used by bears in between, in the Alaska archipelago and coastal British Columbia.  

Then there are the berries...

Then there are the abundant berries in coastal regions, which augment salmon in supporting the high densities of large-bodied brown bears. The most notable of the berry-producing species, at least in terms of prominence in bear diets, are salmonberry (Rubus spectabilis), devil's club (Oplopanax horridus), huckleberries (Vaccinium ovalifolium and ovatum), Sitka mountain-ash (Sorbus sitchensis), red elderberry (Sambucus racemosa), and stink currant (Ribes bracteosum).

 

The map at right shows the joint distribution of these species (minus V. ovatum), with darkest brown corresponding to areas where all six are present, and lightest brown to where only one is present. This shading can be viewed as a proxy for the magnitude of Pacific maritime influence on the local climate. Interpreted this way, you can pick up the interior climates of eastern British Columbia and adjacent Idaho and northwestern Montana where a maritime influence intrudes far inland, and where, not coincidently, you find the highest densities of grizzly bears in the interior--beyond the range of abundant spawning salmon. This is also where thinleaf huckleberry (V. membranaceum) is abundant and a staple of local grizzly bear populations (see Vegetal foods).        

 

A final word about berries along the Pacific coast: Farthest north, where alpine conditions can be found at or near sea level, and at progressively higher elevations farther south, the berries most prominent in diets of far-northern arctic grizzlies again show up. Most notably, these include crowberry (Emeptrum nigrum), bilberry (V. uliginosum), and lingonberry (V. vitis-idaea; for more details on these species see Vegetal foods).