After circumstances make the shallow water in the oceans uninhabitable, great white sharks retreat to the deeper parts of the ocean, about 600 feet down (182 meters) to survive. The sharks can not go more than 450 feet up, as the water up there is uninhabitable. My question is, could sharks adapt to survive in this new environment?
There are sharks that live quite deep in the ocean; the goblin shark and frilled shark, for instance, have been known to live over 1,000 meters below sea level (Wikipedia claims great whites have been found at this depth, too). This is off the edge of the continental shelf.
Your depths (~180 meters) puts you only just off the shelf - and still certainly in the photic zone, where there's a good amount of sunlight. This means that your sharks will still be in an ecosystem similar to their old one. They'll still have some of their old food, and they'll be living in temperatures that aren't terribly low.
Deeper into the ocean, you have three issues:
- Food. Sharks are used to a certain lifestyle, with a certain amount and type of prey available. Different organisms frequent the lower areas of the ocean, and so sharks would see less of their favorite fish. Adapting to a new food supply wouldn't be too hard, but having less food overall would.
- Pressure. At depths on the order of a kilometer, pressure becomes an issue, although compounds like TMAO can mitigate it. Deep-sea creatures do have adaptations like TMAO to help them survive. At less than 200 meters, pressure might be a bit of an issue, but not a large one.
- Temperature. Below one kilometer, in the bathyal zone, mean temperatures hover just above freezing - not good for fish, although certainly not inhospitable.
That said . . . I don't see any of these being problems at the depths you give, for the simple reason that your sharks won't be living in deep water at all.
They are already adapted to such depths
The great white is an epipelagic fish, observed mostly in the presence of rich game, such as fur seals (Arctocephalus ssp.), sea lions, cetaceans, other sharks, and large bony fish species. In the open ocean, it has been recorded at depths as great as 1,200 m (3,900 ft). These findings challenge the traditional notion that the great white is a coastal species.
According to a recent study, California great whites have migrated to an area between Baja California Peninsula and Hawaii known as the White Shark Café to spend at least 100 days before migrating back to Baja. On the journey out, they swim slowly and dive down to around 900 m (3,000 ft). After they arrive, they change behavior and do short dives to about 300 m (1,000 ft) for up to ten minutes.
White deaths are opportunistic and adapt well to a large variety of prey. They have been seen eating sunfish and other sharks. If the top of the water column becomes inhospitable, many other fish will also dive under 450 feet and the white deaths will follow.
Many species of sharks can live for longer or shorter periods in various depths of water.
Sharks that are sometimes seen at the surface can sometimes be found thousands of feet or meters below the surface.
Great White sharks, for example, are usually found from the surface down to 825 feet, but have been found as far down as 4,200 feet.
Greenland sharks, which have been found in relatively shallow rivers and dived among by scientists, have also been found as deep as 7,200 feet, and the closely related pacific sleeper sharks have been found on the surface, have also been found as far down as 6,500 feet.
There is no problem with some shark species that are well known to the public because they are often seen at or near the surface spending part of their time below 450 feet. I am not sure how well they would survive if going above 450 feet was always fatal. And most of their food supply would be gone if all life above 450 feet died.