I did some fishing x5 just clicking around the windows manually and could get all 5 quite a few times having cast all 5 at the same time.
After you /cast fish there are only two different points in time at which the bobber will splash, so I usually got 2 of them on one of the times and 3 on the other, which made it manageable to get them all in time, but it got harder if 4 or 5 got the same timeframe.
I figure using an addon to make you recast fishing by rightclicking (or double) after you click the bobber on each one, you could have them not be synced so it would be a lot more manageable and more like whack a mole or even a nearly set sequence.

But yeah, got my 5 shamans to (at least) 150 fishing in one sitting once, haven't tried it since though since dailies make just as much money and doesn't depend on prices. And I hardly have time to even do dailies personally anyway.