The thing we know for sure is that if you fail to click a bobber that has splashed when fishing a pool, the total number of fish left in the pool doesn't change. You actually have to bring up a loot window to reduce the pool's supply of fish. (And no, I can't remember what happens if you bring up the loot window and then fail to take the loot from it.)
Based on the behaviour of latent or laggy servers, the loot is generated by the server when you click the bobber, not when it splashes: The client has to go and ask the server something. Presumably it asks what the loot is - well, what else does it need to know? But then there's a delay when casting while the client checks something with the server. I assume the server was randomising the cast location, but it could be randomising the catch too. And pools (which basically use the same underlying code as ore and herb nodes - which I guess is why the left-over test pool above Northshire contains Peacebloom) might have their contents generated at pool creation time, a long time before anyone catches anything.
On a slight tangent, the Fishy Experiment thread at Elitist Jerks has some interesting loot generation theories. The theory goes that the "contents" of a container are generated at the time loot is created in the world. However, storing the precise contents would be needlessly data-intensive, so the contents are stored as a single seed number. That means if the loot table of the bag changes (as happened with fished crates at patch 2.3), the seed number stored may yield different loot before and after a patch.
Generally Blizzard's data structures seem very simple, but are so clouded by apparent randomness that almost all the players remain confused. For example, at release there were only 6 variations that determined the fishing skill and fish across every zone in the game - yet when I started researching El's guide nobody understood any of that.
So in answer to your question... 42? The virtual fish that did not get caught may or may not have ever existed. But if the generation of a quest fish is truly random, it's best not to worry about whether what you didn't catch ever existed, and instead focus on reducing the chance of a fish getting away ;-) .
