I was flying back through Twilight Highlands and fishing along the coast, and ooh there were 3 pools near a boat. So I landed on the boat and started to fish. And when I clicked it there was a blue item with my fish, and I didn't quite get what it was until the achievement went off.
OMG OMG OMG OMG!
So after 16263 fish caught and other things, I got my Turtle mount. Though really, only 12293 of those fish count because they were fished from pools and you can only catch the turtle from pools.