Nees to be run only after fishing is discovered;

A villager spots a large fish swimming slowly off the shore. Does he a) get out of the water or b) try to catch it?

Possible outcomes can be either-Doh! It was a shark. He's been eaten, or Yay! It was a large Tuna fish +400 food units!

I also like the idea of having certain achievement unachieved. Fixing the ruins could be undone by an earthquake, unblocking the stream could be blocked by a landslide.