Nigel first goes fishing in the Ordovician. Even though fish hadn't evolve yet there's plenty of creatures to catch if they don't catch him first. In the Triassic he finds plenty of fish but they're too big to catch so Nigel just annoys them.
Nigel follows a school of massive fish hoping to find history's the largest marine carnivore, Liopleurodon. Finally, in the cretaceous in a sea packed with predators Nigel searches for a the apex predator, Mausosaur.