The sea creatures are segregated by a land ridge. The females live on one side, the males on the other. During mating season, the males jump the ridge. There is a land species that for some reason wants to prevent this. They build a wall to act as a barrier over the ridge. The creatures are unable to jump it.
The prologue ends with a space ship taking specimens of the soon-to-become-extinct sea creatures. Possibly as some kind of ark operation? I was only able to browse the prologue of this novel and am desperate to know its title.