What fish are you referring to ?
The rare fish and crustaceans are used for
Magical Fish Pies which boost several modifiers and thus have indeed a value but also
other fish and crustaceans which can be fished up, are usefull for quests and they also have a market.
If you are referring to regular fish neither usefull for magical pies nor for quests then those are usually cut up.
By the way, regular crabs and lobsters (not named) which cannot be used for Fishmonger Quests they still have a use as turn ins for Library Collectibles or Clean Up points.