A lot of deep sea creatures have very slow metabolisms. It is one of the many reasons sea dredging and mining should be held with such disdain: these are ecosystems which may take thousands of years to recover.
We don't even appreciate how long it takes a forest to recover, much less one with glass sponges that are thousands of years old.
When H Melville stuffed the middle of Moby Dick with a "cetology" -- BEFORE The Origin of Species, famously saying "a whale is a fish" -- he didn't forget the Greenland Shark. I think all the time about how many of those sharks swimming around in 1851 are still swimming around today.
Note that Melville was well aware of the reasons that "whales aren't fish", and went over those in detail, then said he was going to call them fish anyway.
There's a business lesson in the longest lived creatures being the ones that move slow, abide small insults, and make themselves generally unappetizing.
Jeremy Wade, host of the TV show "River Monsters", has an episode where he investigates the Loch Ness Monster and concludes it's likely a Greenland Shark that swam up an underground river from the North Atlantic to the lake. He likens the shark's horse-like face and the distribution of the low fins on the shark's back to descriptions of the monster. A solitary long-living fish could explain the occasional sightings, and the scientists' findings that there is not enough food in the lake for a breeding population of large carnivores.
As a sibling comment notes most sharks cannot live long in freshwater, and moreover this is soecifically true of Greenland sharks, though they do sometimes spend time in brackish river mouth environments, so, unless it developed the weird behavior of migrating quickly up the relevant underground river to make a quick appearance and then inmediately rushing back down the river to the ocean, that’s one answer we can be fairly certain is wrong.
There are a few sharks that can live in freshwater, but they tend to inhabit warmer oceans.f_
That's the kind of rare and highly luck based curiosity they ought to give you a plaque for. "From this shore in 1973 local angler..." Slap it on the same sign post as the flood high water markers they put up.
He is likely wrong (most sharks cannot live long in fresh water). But given the show, he has to conclude it is a fish of some sort, and it is not going to be 10k arctic char in a dinosaur suit.
The most likely explanation for the Loch Ness Monster, of course, is that it's entirely made up and didn't require an actual sighting or a real physical phenomenon, ever, to trigger people's imaginations.
David Foster Wallace has always irritated me, in that I've greatly enjoyed all of his essays that I've read, including this one, but I bounced off the novel hard.
We don't even appreciate how long it takes a forest to recover, much less one with glass sponges that are thousands of years old.
See also https://news.ycombinator.com/item?id=46511555
There are a few sharks that can live in freshwater, but they tend to inhabit warmer oceans.f_
https://www.columbia.edu/~col8/lobsterarticle.pdf
https://en.wikipedia.org/wiki/Consider_the_Oyster
https://m.youtube.com/watch?v=KuVEbBmu1EM