As a quest objective
This fish is found in the Verdantis River in Feralas. The river is in central Feralas, west of Camp Mojache, where the mountains dip south. Make certain the label on your mini-map says "Verdantis River" or you are in the wrong place.
The Verdantis River is difficult fishing for a 225 skill character and you will receive many, many failures. Its probably advisable to pick up a few or even some Aquadynamic Fish Attractors to speed up the search.