A famous matriarch bear is missing from Fat Bear Week. Where could she be?

Welcome to Fat Bear Week at Mashable! Each fall, Katmai National Park holds a competition as Alaska’s brown bears finish fattening up for their long winter hibernation. This year, Mashable is getting in on the salmon-munching action. Check back with us all week as we follow the fat bear face-offs each day, and remember to get your votes in for each round. Happy fishing! A top competitor is missing from this year's Fat Bear Week contest. For a fleeting moment in June, rangers spotted Bear 410 — the oldest-known bear at Alaska's Brooks River — walking up the chilly stream where the eager bears spend months snatching five-pound salmon from the water.  The 30-year-old bear, easily recognizable by her prominent, thick brow and drooping lower lip, soon disappeared beyond the riverbank. She hasn't been seen since. Bear 410 snoozing in September 2017.Image: npsHer absence is striking, not least because 410 has been a formidable challenger in the last four Fat Bear Week contests. In 2014, she was a finalist, losing out to three-time champion Bear 480, Otis. The following three years — 2015, 2016, and 2017 — she was a semifinalist.  "Her absence is surprising, but it's certainly not out of the realm of possibility," Mike Fitz, a naturalist who spent the summer documenting Brooks River bear activity, said over email. SEE ALSO: An appreciation of Holly, the fat bear mom who adopted and raised an abandoned cub "We've seen many examples of fully mature bears, even some we'd consider to be old, change their habits," noted Fitz, who is a former Katmai National Park ranger.  During her brief June appearance, Bear 410 appeared healthy. "From all accounts, the lady seemed to be her usual self," Andrew Lavalle, a Katmai ranger, said over email. "That is to say she was slow, lumbering, and as perhaps the most human habituated bear, generally unfazed by humans." Her October absence is all the more notable because she has grown habituated to a very peculiar brown bear environment. Katmai is largely a wild place (with over 2,000 brown bears), but the specific Brooks River area is a place where, come summer, human visitors abound. "Her tolerance for crowds of people, airplane traffic, and vehicle noise allowed her to use habitat not available to many other bears," said Fitz.  Where might 410 be? Typically, brown bears in Katmai can live for around 20 years. So, 410 is certainly an advanced-aged bear. In short, there's a real chance she did die at some point after the rangers saw her this season.  However, there's also a good chance she's alive and well and has simply moved on to other fishing grounds. "I also can't ignore the possibility that she's found another place to make a living," said Fitz. "Katmai is a big park and while Brooks River is very important to the bears who fish there, it isn't the only place to catch salmon." The 2018 Fat Bear Week bracket is missing bear 410.Image: bob Al-greene/mashableEach year at the Brooks River large brown bears — that park biologists have never spotted before — show up at the salmon-rich waterfall. Perhaps, then, 410 discovered another rich fishing environment, similar to these bears coming to the river for the first time. "It makes one wonder, where were they all this time?" wondered Fitz. "How and why did they discover the falls just now?"  In any case, wherever Bear 410 might be, her absence is notable. She's an elder bear, a sort of matriarch, and one of the most successful known bears — and fattest — in Katmai history. "No matter the reason for her absence, for me, Brooks River will never be the same without 410," said Fitz. WATCH: A paralyzed man walks, with assistance, thanks to a new therapy that reactivates the spinal cord