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Trivia: During the film's opening number, pay attention to the shot when Kenai is riding the back of the mammoth, busting through the fishing net that his brothers are holding. If you look very closely, you'll see Nemo, from Finding Nemo, right there along the flying salmon. He's only in three short frames, so it helps if you pause the DVD and go frame by frame. (00:04:45)
Answer: The bear injures Sitka right before he sacrifices himself. We don't see the injury itself due to the film wanting to keep its G rating, but we do see the bear attack and most likely maul Sitka, heavily implying his injuries are fatal. This is why he's hunched over and breathing heavily when we next see him as the bear is moving towards Kenai and Denahi.