Improving the use of stable isotopes to estimate diets of free-ranging polar bears
Karyn Rode, Research Wildlife Biologist, US Geological Survey, Alaska Science Center
Craig Stricker, US Geological Survey, Director of Denver Stable Isotopes Lab
Charles Robbins, Professor, Washington State University
Joy Erlenbach, MS student, Washington State University
Seth Cherry, Ecosystem Ecologist, Parks Canada
Gregory Thiemann, Assistant Professor, York University
Our ability to estimate polar bear diets is currently limited by an inability to test assumptions of biochemical approaches available for diet estimation. Stable isotope techniques hold promise in providing a tool for estimating diets using blood and hair. Recent experiments with captive grizzly bears have provided an improved understanding of bear physiology relative to incorporation of dietary carbon and nitrogen isotopes into polar bear tissues. However, to accurately apply stable isotopes to estimate diets of free-ranging polar bears, further validation is needed of the degree to which dietary carbon and nitrogen isotopes are incorporated into various tissues. These measures allow us to calibrate the tissue carbon and nitrogen isotope values and estimate the proportional contribution of different food items. This study involves placing two bears at the Alaska and Oregon Zoos on a high-fat, marine diet typical of polar bears in the wild and sampling blood, hair, and adipose tissue to determine the degree to which components of the diet, including stable isotopes and fatty acids, are incorporated into polar bear tissues. Ultimately, the goal of this effort is to be able to then examine long-term trends in the diet of two polar bear populations that occur in Alaska – the Southern Beaufort Sea and Alaska-Chukotka polar bear populations. Blood and hair from bears in these populations are available as far back as the early 1980s offering an opportunity to look at relationships between diet, blubber content (as a proxy for food availability), and sea ice conditions.