When the algae blooms it says it is because of over fishing. Is that right (if you never fished, you'd never get algae) OR does the algae come at a particular number of fish or fisherman OR is it scheduled at a particular spot in the game?

I think the algae show up when you've collected a certain number of fish -- if you never fish, you'll never get it. It's possible to finish the game without getting the algae, if you are careful with how much you fish (apparently you can solve the algae puzzle even if you don't have algae, although I haven't tried it).