I see what appear to be small crack marks on my hull. Is that common?

It's a '91 sunbird with a 20 feet fiberglass hull. The marks are about
4 inches in length and they run top-to-bottom (not from bow to stern).
They are mostly towards the stern of the boat on both sides of the
hull under the boat's waterline. When I run my hand over them, they
are perfectly smooth and flushed with the surface of the boat; you
can't feel them, one can only see them. They're all under the boat's
water line.

There is no water coming in the boat (or if there is, the bilge pump
sucks it out over night). The boat runs great with no trouble. I
purchased this boat used. It didn't have those marks when I inspected
it. I've been using it for 2 years and I just noticed them last week.

Is it expected to have small cracks when a hull gets old? Is there a
fix short of junking the boat? How would a marine mechanic fix them?
What would one expect to pay for that?

thanks.