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The brown bears of Alaska’s Katmai National Park and Preserve are huge, so much so that they are the subject of a popular tournament called Fat Bear Week. Each year, an online audience votes on which bear—gorged on salmon from the Brooks River—is the chonkiest. Last summer, the 450-kilogram giant Otis was crowned the year’s fattest bear. But new research conducted on a lonely stretch of the park’s coast, far from the gaze of Fat Bear Week’s cameras, shows that some of Alaska’s bears have shifted their diets away from salmon, one of their richest foods, with unknown consequences for their future.
Working with National Park Service biologists, Joy Erlenbach led the research, which began in 2015 while she was a graduate student at the University of Washington. The work shows that the proportion of salmon in female brown bears’ diets in Hallo Bay has decreased more than 50 percent over the past three decades, possibly in response to declining salmon runs. Salmon made up 62 percent of the bears’ diets in 1989, but by 2017 it was just 28 percent, says Erlenbach.
The bears replaced salmon with increased consumption of berries, sedges, and leafy vegetation—despite the availability of clams and other fish. The finding shows that bears can get by on a largely herbivorous diet, suggesting plants are a buffer resource when favored foods like salmon decline.
The work highlights the famous dietary adaptability of bears. But it also raises questions about how leaner diets could affect them in the future.
Previous research on grizzly bears suggests that a grizzly living on a berry-heavy diet would be about half as big as one living a predominately carnivorous life. Though Erlenbach’s research didn’t explore the long-term potential of the dietary shift of female brown bears in the Hallo Bay region, she does note that a diet rich in salmon helps bears grow large, amass body fat, and attain the conditions necessary to produce cubs. She says that while her study did not show evidence that cub production has suffered from the bears’ changing diets, it is a risk, and more research is necessary to determine at what point that might occur.
Erlenbach and her colleagues made the discoveries by fitting GPS collars on 31 female brown bears to track their movement between habitats. To estimate diets, they analyzed blood and hair samples taken at key times throughout the summer, including before and after the arrival of salmon. They also spent long hours on the coast counting bears and observing their behaviors.
The GPS collars showed that even as the proportion of salmon in the bears’ diets declined, the bears still maintained a strong fidelity to the coast, staying within an average of 10 kilometers of the water throughout the summer. And although they visited a variety of habitats, the bears showed particular interest in salt marshes, where researchers believe sedges and a plant called goose tongue offer a combination of protein and digestible carbohydrates. When salmon arrived, however, the bears spent more time in salmon streams.
Erlenbach says that while declining salmon abundance is likely contributing to the shrinking proportion of salmon in the bears’ diets, other factors could also be at play, including an increasing number of human visitors to the coast and the disruption caused to the North Pacific marine ecosystem by an intense marine heatwave known as the Blob.
Wildlife biologist Christina Service, the science coordinator for the Kitasoo/Xai’xais Stewardship Authority, says the research addresses important questions.
“This is really a dynamic time on the coast,” says Service, who works more than 1,500 kilometers south of the Katmai National Park and Preserve in Klemtu, British Columbia. She says coastal bears have endured much change in recent years, including droughts, flooding, and shifts in both plant phenology and salmon availability. “The changes have a huge impact on bears and the portfolio of foods available to them,” she says.
But Service notes that bears are opportunistic and take advantage of a broad array of resources. In British Columbia, she says, they will feed on sea urchins, a wide variety of plant materials, and even salmon eggs, which they can dig from streambeds at certain water levels. Still, Service says, a better understanding of bear diets is useful.
“If we think about best providing for bears today, we need to have our finger on the pulse of what their most current needs are,” she says.