The start of the annual Fat Bear Week contest in the US was delayed this week after one of the potential contestants was killed by a rival.

Both bears were in contention for the 10th anniversary of the tournament pitting the heaviest bears against each other in an online vote, as they finish bulking for hibernation.

Caught on a live webcam, the brutal attack involved a male known as Bear 469 and a female called Bear 402 in a river at Alaska's Katmai National Park and Preserve.

A naturalist for explore.org, which streams footage from the park, said the male bear's challenge appeared "predatory" - as if he was triggered "in the middle of something".

"This is a difficult situation to witness. We love to celebrate the success of bears with full stomachs and ample body fat, but the ferocity of bears is real," Mike Fitz added.

"The risks they face are real. Their lives can be hard and their deaths can be painful."

The video, at around the seven-minute mark, shows the bears fighting in the river, with the female seemingly overpowered and appearing to drown, before the male drags her body to land.

After the incident, the reveal for Fat Bear Week bracket - which confirms the 12 heavyweight bears who qualified - was postponed by a day to Tuesday.

"National parks like Katmai protect not only the wonders of nature, but also the harsh realities," park spokesperson Matt Johnson said.

"Each bear seen on the webcams is competing with others to survive."

Meanwhile, legendary fat bear Otis - Bear 480 - is missing, according to explore.org's TikTok feed and media reports.

Otis won the first tournament and went on to win another three to become the most decorated bear in the competition's history.

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Fans voting in each round are encouraged to not only consider the biggest bear, but the one they believe "best exemplifies fatness and success in brown bears".

There are more than 2,000 brown bears in the preserve, with only a select few making it to the competition every year.

Reigning champion Bear 128 - known as Grazer - is already in the final eight, while fan favourite Chunk - Bear 32 - appears to be the largest this year.

Chunk, a 20-year-old male, once ate 42 salmon within 10 hours.

More than 1.3 million votes were cast last year and voting is open until 8 October, when a champion will be crowned.