So tell me...what happens once the whales are back to levels
where the Derwent is one solid mass of whales, sinking small
boats and a threat to larger shipping? (1830s)

Whales will begin attacking boats again (Melville, 1851), and small boat
numbers will begin to shrink. Hopefully some environmentally conscientious
whales will begin to enact laws to protect the small boats during their
migrations, and their numbers will be preserved for future generations of
whales to enjoy. Of course, Japanese, Russian and Aleutian whales may
reserve the right to hunt boats in small numbers to protect their way of
life.