Grizzly bears get stressed from salmon decline

Grizzly bears in coastal British Columbia bulk up on salmon in the fall. But they experience stress when the fish are scarce.

James Wheeler/Flickr

Salmon aren’t doing very well in most of the world. They’ve been overfished most everywhere. Their natural habitats have been destroyed in numerous ways, including by agriculture, logging and oil spills. Their freshwater rivers have been blocked up with dams. And climate change may be affecting their migration from river to sea and back.

This decline in salmon isn’t good for fans of the omega-3-rich fish, or the people who make their living off of them. That includes native tribes in the Pacific Northwest that have depended on salmon fishing for their existence. But salmon feed more than people. They play an important role in their ecosystem, and their loss can have consequences that echo throughout the food web. So here’s one consequence that’s particularly scary: Fewer salmon may equal stressed out grizzly bears.

Grizzlies are omnivores, and they can survive on a vegetarian diet. But salmon provide a bear with several advantages. In the fall, salmon swim upstream to spawn. For the bears in coastal British Columbia that have access to these spawning areas, eating salmon allows them to fill their dietary needs more efficiently than bears that only have plants to eat. And efficiency is a definite advantage in the autumn, when the bears are trying to bulk up for winter hibernation. As a result, bear populations that have access to salmon tend to have higher densities of grizzlies, bigger bears and more babies.

For thousands of years, bears in salmon-rich areas have been able to rely on the fact that the fish would come each fall. The number of fish and the timing of their arrival might vary, but some salmon would be available. That pact with nature has been severed, however, and the fish have declined in number or disappeared entirely from some streams.

Losing a source of food is generally a stressful event, whether you’re a human, fish or bear. To measure stress in the bears — admittedly a much more difficult task than measuring stress in a person — Heather Bryan of the University of Calgary in Canada and colleagues looked to cortisol levels in grizzly bear fur. Their study was published November 27 in PLOS ONE.

Cortisol is a steroid hormone released in times of stress. By measuring it in bear hair, which, in North American bears, grows for about six months from spring to fall, the researchers got a picture of an animal’s level of stress over that whole period, including during the salmon spawning. They gathered hair in 2009 through 2011 from barbed-wire hair-snagging stations in coastal and inland British Columbia and also got samples from bears that had been killed between 2004 and 2009.

Bears on the coast and inland had similar levels of cortisol. But when the scientists looked at how cortisol levels varied with the amount of salmon consumed — estimated from carbon and nitrogen isotopes in the fur of coastal bears — they saw a trend. As the amount of salmon eaten increased, cortisol levels decreased.

“If salmon returns consistently decline in the future, grizzly bears that do not obtain enough salmon might experience chronically elevated cortisol … with unknown, but probably adverse, fitness costs,” the researchers write.

As for why the inland bears aren’t more stressed than the salmon-happy coastal bears, the scientists suggest that the populations might experience similar levels of overall stress or simply have different baseline levels of cortisol. But I wonder if it might mean that the inland grizzlies are just as satisfied with their vegetarian diet as the coastal ones are with their salmon. And after a few generations, maybe coastal bears that grow up less dependent on the yearly salmon run won’t fret as much when it doesn’t happen.

Sarah Zielinski

Sarah Zielinski is managing editor of Science News for Students. She has a B.A. in biology from Cornell University and an M.A. in journalism from New York University. She writes about ecology, plants and animals.

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