This is not to say there are no more oysters left in the world, but that precipitous decline means they've lost their ecological niche. In many regions, they're dropped to less than 1% of their former numbers; now 75% of all oysters come from just five remaining oyster beds.
This is all the more ironic, since oysters have often been used as an example of nature's fecundity; cf. Avram Davidson's story "Or All the Seas with Oysters", which in turn takes its title from Conan Doyle's "The Adventure of the Dying Detective". As Holmes lies on his apparent deathbed, he rants ". . . I can't think why the whole bed of the ocean is not one solid mass of oysters, so prolific the creatures seem . . . Shall the world, then, be overrun by oysters? No, no; horrible!"
Horrible indeed -- not from an excess of life but from the vast emptiness left behind.