Part of a series of articles titled Alaska Park Science - Volume 19, Issue 1 - Below the Surface: Fish and Our Changing Underwater World.
Grizzly bears (Ursus arctos) are brown bears living away from coastal areas. Grizzly bears are food generalists (omnivores), which allows them to take advantage of a wide variety of food resources. Yet, their simple digestive system and seasonal hibernation when food is scarce, requires them to locate large quantities of seasonally abundant, energy-rich foods to satisfy yearly energetic requirements (Pritchard and Robbins 1990). While coastal-dwelling brown bears typically rely heavily on salmon and other marine resources, interior grizzly bears are thought to rely on terrestrial meat sources (Hilderbrand et al. 1999). The need for efficient energy acquisition is pressing for grizzly bears above the Arctic Circle, since they hibernate up to seven months of the year, limiting their time to satisfy annual food requirements.
The Arctic is extremely limited in terms of energy-rich plant resources (Swanson 2017). The combination of winters that can endure eight months and cool summer temperatures leads to lower plant productivity than in other biomes. Poor soil conditions and permafrost also limit vegetative growth. This low primary productivity leads to low densities of herbivores such as moose (Alces alces), Dall’s sheep (Ovis dalli), muskox (Ovibos moschatus), and caribou (Rangifer tarandus) that grizzly bears could potentially use as food. Owing to these environmental factors, grizzly bears in Arctic Alaska are living at the species’ northern-most extent and are noticeably smaller in comparison to the southern, salmon-fed coastal bears (Hilderbrand et al. 2018).
Northwest Alaska is one of the least developed regions in the world. Within this region, a 220-mile (354 km) industrial road (the Ambler Road) is proposed that would parallel the southern flank of the Brooks Range. A road here could potentially affect fish and wildlife in and around Gates of the Arctic National Park and Preserve (NP&Pres), including bears. Grizzly bears are considered good indicators of ecosystem health because they are apex predators and long-lived (some live more than 20 years). We wanted to better understand the ecosystem prior to industrial development because past studies show that human development and activity can alter bear behavior and habitat use (Gibeau et al. 2002, Fortin et al. 2016). With this in mind, we designed a study to improve our understanding of grizzly bear habitat use and ecology in Gates of the Arctic NP&Pres to help inform how bears might be affected by road development and increased human activity in the region.
We began tracking movement patterns of grizzly bears across the landscape using Global Positioning System (GPS) collars in 2014. During the next three years, we collected movement and morphological data (e.g., body size and lean mass; Hilderbrand et al. 2018) from 53 bears. The GPS data allowed us to identify habitats most frequented by bears and provided a better understanding of their ecological importance. Not surprisingly, collared bears denned in the mountains and generally remained there throughout the spring. What was surprising was that beginning in mid-July, many grizzly bears began concentrating movement near streams at the headwaters of the Kobuk and Koyukuk rivers. This behavior is typical for bears when streams contain a food resource, such as salmon, however many of the streams visited by bears were not known to support salmon (Figure 1). We took to the field to investigate why bears were frequenting these streams.
Our field investigations of streams in and around Gates of the Arctic NP&Pres used by grizzly bears revealed not only the presence of chum salmon (Oncorhynchus keta), but also that bears were taking advantage of this energy-rich resource; a fact highlighted by the piles of salmon jaw bones we found lining the banks (Figures 2 and 3; Sorum et al. 2019). The GPS data suggested that bears were fishing up to 15 different streams throughout the upper Kobuk and Koyukuk watersheds, many of which had not been previously identified as supporting salmon. A stable isotope diet analysis was conducted and results from this analysis provided additional evidence that salmon were an important food item for grizzly bears within the region (Mangipane et al. In review). In fact, salmon contributed to a significant portion (>25%) of the diet for at least half of the collared grizzly bears, which was unexpected.
Our field investigations documented chum salmon presence in three streams not yet listed in the Alaska Department of Fish and Game’s (ADF&G) Anadromous Waters Catalog (AWC); Alaska’s regulatory catalog of waters known to support salmon and other anadromous (sea-going) fish. Nominations of those streams to the AWC spurred discussion with ADFG’s Fresh Water Fish Inventory team regarding the lack of systematic surveys focused on salmon distribution within the upper Kobuk and Koyukuk drainages that might be impacted by the proposed Ambler Road. We forged a collaboration between ADF&G and NPS to begin filling this data gap by conducting fish surveys across the upper Kobuk and Koyukuk watershed during summer of 2018. This collaborative effort documented nearly 400 miles (644 km) of previously unknown salmon habitat for nomination to the AWC including 11 streams crossed by the proposed Ambler Road (see Cathcart and Giefer this issue). Designation into the AWC for these salmon streams will add additional regulatory protections to help reduce impacts to salmon habitat from road development.
Movement data from GPS collared bears provided valuable information to improve conservation efforts not only for bears, but also for salmon. We plan to further study the dynamics between bears and salmon by determining how salmon influence movement patterns, habitat selection, and survival of grizzly bears across the Arctic Interior.
Fortin, J. K, K. D. Rode, G. V. Hilderbrand, J. Wilder, S. Farley, C. Jorgensen C, and B. G. Marcot. 2016.
Impacts of human recreation on brown bears (Ursus arctos): A review and new management tool. PLOS ONE 11: e0141983.
Gibeau, M., A. Clevenger, S. Herrero, and J. Wierzchowski. 2002.
Grizzly bear response to human development and activities in the Bow River Watershed, Alberta, Canada. Biological Conservation 103:227-236.
Hilderbrand, G. V., D. D. Gustine, B. A. Mangipane, K. Joly, W. Leacock, L. S. Mangipane, J. Erlenbach, M. S. Sorum, M. D. Cameron, J. L. Belant, and T. Cambier. 2018.
The importance of meat, particularly salmon, to body size, population productivity, and conservation of North American brown bears. Canadian Journal of Zoology 77:132–138.
Hilderbrand, G.V., D.D. Gustine, B. A. Mangipane, K. Joly, W. Leacock, L. Mangipane, J. Erlenbach, M. S. Sorum, M. M. Cameron, J. L. Belant, Jerry, and T. Cambier. 2018.
Body size and lean mass of brown bears across and within four diverse ecosystems. Journal of Zoology 306: 53-62.
Mangipane, L. S., D. J. R. Lafferty, K. Joly, M. S. Sorum, M. D. Cameron, J. L. Belant, G. V. Hilderbrand, and D. D. Gustine. In Review.
Dietary plasticity and the importance of salmon to brown bear (Ursus arctos) body size and condition in a low Arctic ecosystem. Polar Biology.
Pritchard, G. T. and C. T. Robbins. 1990.
Digestive and metabolic efficiencies of grizzly and black bears. Canadian Journal of Zoology 68: 1645–1651.
Sorum, M. S., K. Joly, and M. D. Cameron. 2019.
Use of salmon (Oncorhynchus spp.) by brown bear (Ursus arctos) in an Arctic, interior, montane environment. Canadian Field Naturalist 133: 151–155.
Swanson, D. K. 2017.
Trends in greenness and snow cover in Alaska’s Arctic national parks, 2000-2016. Remote Sensing 9: 514.
Last updated: May 29, 2020