Once the fish is caught and in your inventory you can double click to "rip it open" and collect the goods inside it. Rewards vary according to the species you've caught so here's a short summary of fishes available.
Fish can be categorised in 5 types (in a manner simmilar to weapons for example):
Rainbow Trout - That would be the 'ancient' and most rare one. Extremely difficult to catch with the in-game rods. It's also a quest required items so it would be wise to keep some (at least 5) if you're not neccessarily selling it.
Sweetfish - Could be categorised as the 'unique' type, although it's quite easier to catch than Rainbow Trout. It's a quest item required for some guild quests (30 pieces) so if you're a guild enthusiasts i'd be wise to have some unless you have spare crones to buy them from retailers.
Salmon / Mullet- The 'rare' type. They have no specific use, although can be a goal of some fishing derbies.
Eel / Catfish / Carp /Goldfish The 'common' type fish that gives rewards with close to the level/price of the bait used (albeit a bit lower maybe).
Minnow - It's a booby prize type and gives rewards with lower level/price than the actual bait used to catch them.
Edit
/ Mullet 
- The 'rare' type. They have no specific use, although can be a goal of some fishing derbies.
/ Catfish 
/ Carp 
/Goldfish 
The 'common' type fish that gives rewards with close to the level/price of the bait used (albeit a bit lower maybe).
