According to Blizzard:
"There are special fishing pools located throughout the world that allow you to fish up special fish. To fish them you must continue to cast until your fishing bobber lands inside the animated circle. If it is outside, you will not get the special fish. You can fish the pool 4-6 times before the pool is depleted and disappears."
If you haven't seen one of these schools of fish here's one I found in Stranglethorn Vale:
Another day I found this:
It's a pool of "floating debris" which yielded me a few Rumsey Rums.