Selling the Sport Fish
You need to turn in a caught sport fishing fish at a fishing stand to get your gold. Because of this, you might want to fish close to a fishing stand or if at sea, go to a port with a sport fishing stand close by the sea.
For sea fishing, you should look into getting a fishing boat. There is one design available on Mirage Isle for 500 Gilda Stars and if you are really into fishing it is a must to have. The fishing boat has six slots to store trade items so you can go and get yourself a total of 7 sport fish in a single trip.