You can convert fish whose worth is lower than 40 gold into fish fillets, increasing your profits considerably, since almost half of all fish you catch is worth ~25 gold.
IMPORTANT: I don't know if there's a fish merchant in the game yet (hopefully not) but if you are planning to add one, please make sure the purchase price of fish is lower than the selling price of fish fillets. Otherwise you would have an infinite money exploit in your game.
My recommendation would be to lower fish fillets' price to 10 gold as opposed to its current exorbitant price of 40 gold.