How many fish do the fat bears eat?

In one sitting, the internet-famous fat bear Otis once ate a whopping 42 salmon.

That could be some 150,000 calories.

Otis, like all the fat brown bears at Katmai National Park and Preserve, must outlast the long winter famine. During hibernation the bears can lose one-third of their body weight as they slowly burn through their fat stores. That's why they eat obsessively in preparation, packing on so much weight that at times they have trouble walking. The park celebrates the success of these plump bears — which global viewers watch on the livestreamed explore.org webcams — by holding an annual Fat Bear Week contest each fall. You can vote online for the fattest bear.

In Alaska's Katmai, a region with a wild and flourishing salmon run, the bears descend upon different rivers to catch and gobble 4,500-calorie fish. The opportunistic, omnivorous animals often eat the brains and skin first, because they're especially fatty. Otis' 42-salmon outing is extreme. But just how much fish do these bears — some of which eclipse 1,400 pounds — usually eat in a day, or year?

There's no single answer. It depends on the bear (is it a large male like Otis, or a smaller cub?) and the availability of fish. But Mike Fitz, a former Katmai park ranger and currently a resident naturalist for explore.org, has a pretty good idea.

It's reasonable to say that when the fish are plentiful in the Brooks River where these fat bears feast, a large adult bear can eat over two dozen fish in a day. Fitz told Mashable, for instance, that he once watched the colossal bear 747 eat 15 salmon in just a few hours.

Over the course of the summer, this intake really adds up. Biologists have researched the salmon consumption of the brown bears on Alaska's nearby Kodiak Island, which are closely related to Katmai's brown bears. By analyzing the amount of the telltale chemical mercury in the animals' hair — because salmon accrue mercury in their bodies — researchers estimated that adult males ate some 6,146 pounds of fish each year, and adult females around 3,007 pounds.

Bear 151, "Walker." He's a large, dominant bear at Katmai's Brooks River. Credit: Top: T. Carmack / NPS // Bottom: K. Grossman / NPS

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Katmai bears have a lot in common with these Kodiak bears. Notably, both populations are provided with copious amounts of salmon over the summer. And they're constantly eating. But they're not regularly eating 42 fish in a single sitting, like Otis achieved during a particularly big spike in the salmon run. So what's the reasonable takeaway? "Based on the results from Kodiak... Katmai's bears will easily consume thousands of pounds of fish over the course of summer and fall," Fitz wrote in blog post. "In Katmai as well as on Kodiak Island, salmon are the brown bears’ most important food."

The Katmai bears transform significantly between just June and September. Yes, they're eating lots of fish. But the fish are also the most rewarding in the early to mid summer. By fall, the fish are "spawned out," meaning they've entered their final stage of life after laying or fertilizing eggs. They will soon die.

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"In early summer, the fish provide bears with a bigger caloric reward since they are fresh from the ocean and have a relatively high fat content," Fitz told Mashable over email. "After they spawn, the salmon are spent. For a bear, a spawned-out salmon contains about half the digestible calories it did in early summer."

In 2022, the fat bears feasted on a prodigious run of salmon. The fish don't have their own viral online tournament, but they are the foundation of this seasonally-thriving wild ecosystem.

"None of this would happen without the fish," Fitz recently told Mashable.