There's plenty of fish to go around during summer salmon runs at Brooks Falls in Katmai National Park and Preserve.
Bear 32 Chunk was a Fat Bear Week finalist in both 2023 and 2024.
Bear 128 Grazer is considered one of the best anglers on the Brooks River at Katmai National Park and Preserve.
A cub reaches up toward a sow at Katmai National Park and Preserve.

The chonkiest challenge of the year is here.

Fat Bear Week begins Sept. 23.

Each year, the burly bears of Katmai National Park and Preserve capture global attention in a March Madness-style bracket challenge as fans pick which bear advances in each round.

“The bears have been working so hard all season long to get fat,” Sarah Bruce, a park ranger at Katmai, told USA TODAY.

For good reason. For them, bulking up is a matter of life and death.

Here's why and what else you should know about Fat Bear Week 2025.

When is Fat Bear Week?

Fat Bear Week runs Sept. 23 through Sept. 30.

Each day, from noon until 9 p.m. ET, fans can vote for their picks on the event’s website.

More than 1 million votes were cast last year.

What kinds of bears are at Katmai?

Katmai is known for its iconic brown bears, but their health is intrinsically tied to another species: sockeye salmon.

“We’re really lucky, in this part of the country, to have the most sustainable sockeye salmon run in the world, and we still have these intact ecosystems where humans have developed with the ecosystem instead of humans manipulating the ecosystem to fit our needs,” Bruce said, crediting tribal partners and Indigenous knowledge for sustaining the run for thousands of years.

“There's so much to learn about Katmai, and bears are just the tip of the iceberg.”

Which bears are in Fat Bear Week?

This year’s contestants will be a mix of returning favorites and new contenders.

“It’s so great on the Brooks River, we do have the same bears return year after year. It's part of their home range, and there have been some big players that have been in the competition before that are looking really good to be in the competition this year,” Bruce said.

Without confirming who will make the cut, she acknowledged fans will be looking for bears like 128, nicknamed Grazer, and 32, nicknamed Chunk.

“He sustained a jaw injury this year, and so seeing him still get fat even though he's been injured is like, ‘Good for you’” she said. “Then we have a couple of new bears that we haven't seen their whole story play out on the river before but that were on the river like almost all season this year. ... They're looking very fat. All of them are.”

Who won Fat Bear Week 2024?

Grazer won Fat Bear Week 2024 and 2023.

Bruce noted that 128 was the first female bear to be raising a cub when she won.

Why do bears get so fat?

Bears pack on pounds to prepare for hibernation. They need the fat reserves to survive the long winter.

“Right now they're in a phase called hyperphagia, and it's where the hormones in their body have turned off to tell them that they are full,” Bruce said. “They cannot feel fullness no matter how much they eat, so they just keep eating.”

That fat is especially important for females or sows.

“A female bear will only have a successful pregnancy if she's fat enough,” Bruce said, explaining that a fertilized egg won’t even implant unless there's enough fat to sustain the pregnancy. “We have a lot of really fat, heavy single females right now, so we're hoping for a little bit of a cub boom in the spring.”

Why do the bears at Brooks River get fatter than other bears in the Katmai National Park?

During the summer, Brooks River’s waterfalls serve up a veritable buffet.

“The salmon school up there, and it just makes it really easy pickings for the bears,” Bruce said.

Because there are so many fish, it’s not uncommon to see otherwise solitary bears chowing down relatively near one another near the falls.

What has been unusual this year, however, is the abundance of salmon, and amid that bounty, “we've seen so much play, even from older adult boars,” or male bears, Bruce said

She noted most of the salmon run is over, but there are still some live fish in the river along with plenty of dead or dying fish, berries and other things for the bears to scavenge.

Fat Bear Week 2025 livestream

Fans can see some of the bears on Explore.org, which offers a mix of live video and prerecorded highlights from Brooks Falls.

Fat Bear Week is a partnership between the national parks, Explore.org and the Katmai Conservancy.

This article originally appeared on USA TODAY: Survival of the fattest: Your guide to Fat Bear Week 2025 at Katmai National Park

Reporting by Eve Chen, USA TODAY / USA TODAY

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