This fat bear's before and after photos are stunning

Welcome to Fat Bear Week 2021!Katmai National Park and Preserve’s brown bears spent the summer gorging on 4,500-calorie salmon, and they've transformed into rotund giants, some over 1,000 pounds. The Alaskan park is holding its annual playoff-like competition for the fattest of the fat bears (you canvote onlinebetween Sept. 29 through Oct. 5). Mashable will be following all the ursine activity.

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Otis is an aging bear, but a profoundly wise one.

The Katmai bear, numbered 480, is around a quarter-century old with missing teeth. He arrived at the salmon-rich Brooks River, home to the livestreamed explore.org webcams, a month or so late this year. He looked gaunt.

But Otis, famously successful at catching fish with a tried-and-true, unflashy fishing strategy, rapidly persevered. Over the course of around seven weeks, he transformed from a bear with a visible ribcage to one of the fattest of the fat bears.

"He's certainly made up for lost time," Mike Fitz, a former Katmai park ranger and currently a resident naturalist for explore.org, told Mashable. "He's gained an incredible amount of body mass in about seven weeks."

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All the fat bears have impressive summer transformations. But the images below show his particularly stunning 2021 change.

Otis' impressive summer transformation. Credit: n. Boak / C. Spencer / Nps / explore.org

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Otis clearly knows how to survive the long, harsh Alaskan winter hibernation, where bears subsist off their fat stores. An old brown bear is 20, and Otis has already far eclipsed that. He has a stationary approach to fishing, where he sits for hours in the same spot beneath the waterfall, as opposed to dashing in the water or standing atop the waterfall for leaping salmon like many other bears. Some might interpret this as Otis acting lethargic or disengaged.

But no. It's wise. He wastes little energy while catching bounties of calorie-rich fish over the long run.

"I like to describe Otis as a master of energy economics."

"Otis is extremely patient when fishing," Fitz previously told Mashable in 2019. "I like to describe Otis as a master of energy economics. He uses his patience and experience to make a huge profit in calories while expending little energy."

Otis' profit is often huge. He's a three-time Fat Bear Week champion, though other formidable bears have bested him over the past few years.

SEE ALSO: The female fat bear that's as dominant as the big, bad male bears

Otis' first Fat Bear Week match-up is happening Thursday, Sept. 30. You can vote on fatbearweek.org, or this Woobox link if the primary site — due to voting popularity impacting the page — is down.

Your vote matters.