Bear hair sheds light on their diverse diets

Two bears fishing along a stream in Alaska
Brown bears fishing along a creek in Alaska. Dennis Wise

“You are what you eat.” Turns out, the same can be said for bears.

Researchers from the University of Washington recently assessed the contribution of salmon to the diet of brown bears in Southwest Alaska. Their findings confirmed that while the bears are reliant on large seasonal salmon runs, they also eat a variety of other foods, including both vegetation and fauna. The research results were published November 5 in the online issue of the Journal of Fish and Wildlife Management.

Hyejoo Ro conducing a preliminary assessment of the hair (i.e. getting measurements and separating out debris).
Hyejoo Ro conducting a preliminary assessment of the hair (i.e., getting measurements and separating out debris). Hyejoo Ro

“Individual brown bears vary greatly in foraging patterns,” said lead author Hyejoo Ro, a recent BS graduate from the UW School of Aquatic and Fishery Sciences (SAFS). “A majority do tend to eat a lot of salmon, but there are also bears that seem to consume more terrestrial food items, such as moose, vegetation, and berries.” Ro’s published work is the culmination of her undergraduate capstone project.

Bristol Bay is home to one of the world’s largest sockeye salmon runs, which provides a vital nutrient source for the watershed and the region’s large brown bear population and also supports a thriving commercial fishery.

Researchers with the Alaska Salmon Program have routinely collected bear hair along the tributaries of Lake Aleknagik in Southwest Alaska. Previous research, as well as field observations of bear behavior and droppings, shows bears are selective in their consumption of salmon (prioritizing feeding on certain nutrient-rich parts of the fish), while also eating other food items. However, researchers wanted to know if individual diets could be indicative of whether some bears in the area were dominant (the assumption being that dominant bears would be more competitive and have a higher ratio of salmon in their diets).

Bear hair snagged on barbed wire over a stream
Barbed wires stretched over streams are used to painlessly snag a few hairs from unsuspecting bears. Tom Quinn

An initial DNA analysis of the hair samples distinguished between individual bears over a four-year span. Next, individual bear samples underwent an isotope analysis to identify certain dietary signatures. Samples were then matched with available food items collected from the local environment, including sockeye salmon and various berries.

One of the most challenging aspects of this process was determining how long it takes for a bear’s diet to work its way into the animal’s individual hairs and become detectable.

Co-author Jennifer Stern, a graduate student at SAFS, studies the diet of wild polar bears using hair samples and faces the same problem. To address this question, she uses observations and analysis of zoo polar bear hair to provide an estimation of how long it takes for both the hair growth rate and for isotopic signatures to show up in hair. However, assessing samples from wild brown bear populations remains challenging because of the possibility of sampling hair from the current and previous year.

Samples of bear hair in vials
The hairs are placed in vials with a chloroform and methanol mixture which “cleans” the hairs by removing any excess oils and debris. Hyejoo Ro

“The timing of hair sample collection makes it difficult to fully determine the seasonality of brown bear diets,” said Ro. “Since collection occurred during the summer when the bears are molting old fur and growing new fur, the isotopic signature is very different, depending on whether we analyzed old or new hairs.”

The team’s results illustrated that bears in the area opportunistically fed at different trophic levels (position on the food chain), despite the abundance of salmon and the possibility of dominant bears being a limiting factor in food choice.

Studying the feeding habits of Alaska’s brown bears can help researchers to understand the ecosystem as a whole and learn how seemingly distinct components are related. From a management perspective, the wide-ranging impacts that salmon have on this ecosystem are tied to their sustainability as a resource.

“There are a lot of bears in the area, and like the fishery, they need the salmon to sustain their livelihood,” said Ro. “Getting a better idea of how reliant bears are on sockeye salmon can give a better perspective on how to share this fishery.”

This research was funded in part by the Richard and Lois Worthington Endowment and the Mary Gates Endowment Research Scholarship.

For more information, contact Ro at hr27@uw.edu.

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