2023 a ‘Good Food Year’ for Yellowstone Grizzlies

Grizzly 566 weighed nearly 400 pounds when biologists in Yellowstone National Park captured him in 2013. The now-19-year-old bruin wasn’t caught again until October 2023. He weighed in at a monstrous 712 pounds, just three pounds shy of the known record for Greater Yellowstone grizzlies, recorded in 1977.

2023 was a “good food year” for grizzlies in and around Yellowstone National Park, according to Frank T. van Manen, leader of the interdisciplinary group of biologists and scientists known as the Interagency Grizzly Bear Study Team.

“If there was a year where we expected to capture a bear like this—at this end of the weight scale—2023 was certainly the year for that to happen,” van Manen told Mountain Journal. “We generally saw bears with higher percent body fat and in better condition.”

High-calorie food sources have become scarcer for Greater Yellowstone grizzlies, according to a June 2023 study co-authored by van Manen, which reported an overall decrease in food availability from 2010-2020 compared with the previous decade. But 2023 was an anomaly: last year’s above-average precipitation provided abundant forage opportunities for grizzlies, allowing for healthy body-fat percentages throughout the population.

Notably, the calorie-rich seeds from whitebark pine trees have grown fewer over the last decade. Last year’s “decent crop,” van Manen said, likely played a role in providing bears their good food year.

Wildlife biologists have been capturing bears in Yellowstone for the better part of a century to monitor health, population, distribution and survival—van Manen himself has been at it for more than a decade. He emphasized that biologists can only sample a small portion of the population annually, meaning other bears similar in weight may very well exist in the region. Of the estimated 965 grizzlies in Greater Yellowstone, 50 were captured in 2023.

The overall findings from 2023 monitoring were presented in this year’s recently published Interagency Grizzly Bear Study Team Research and Monitoring Update. Van Manen noted that grizzlies in certain areas of Greater Yellowstone are nearing the ecosystem’s natural holding capacity, indicating that bears are self-regulating their population growth due to limited area and resources. Indeed, run-ins with humans led to five Greater Yellowstone bears being killed by recreationists last year from August to October alone.

Paired with higher bear density, researchers have found that the lean body mass—the combined weight of muscle and bone—of Yellowstone grizzlies is decreasing. But despite the higher population density, body fat percentages have remained relatively stable across the population, indicating that resourceful and opportunistic grizzlies are still able to pack on their winter weight.

The increased density is also encouraging Greater Yellowstone grizzlies to expanding their range. Although researchers have yet to record genetic mixing, the gap between the Greater Yellowstone and Northern Continental Divide populations is narrowing. Dispersal movements between the two distinct populations have been recorded in four males.

“From a genetic standpoint, [dispersal] is certainly a desirable outcome for long-term genetic viability,” van Manen said, noting, however, that genetic diversity itself doesn’t pose an immediate threat to Yellowstone grizzlies.

Grizzly bears were listed under the Endangered Species Act in 1975, but following successful recovery efforts, GYE and NCDE grizzlies face the potential for delisting, which would transfer their management from the federal government to state agencies. According to van Manen, if the U.S. Fish and Wildlife Service were to delist grizzlies, an existing conservation strategy outlines specific management guidelines required to sustain a healthy population.

“[Delisting may include] a consideration for hunting in the future, which is of course very controversial,” van Manen said. “But overall, mortality levels would be very similar to what we're experiencing right now … Currently, the population is very healthy.”