Golden trout are a type of fish.
Like all types of fish, each golden trout gives one fish fillet when caught.
Catching golden trout
Golden trout can only be caught with gold lures in fishing spot #3 (the reedy area under the waterfall).
Golden trout can be caught in small circles and therefore are harder to catch.
See Fishing List for more detailed information.
Golden trout in the fishing book
The golden trout is on page 13 of the fishing book:
A true pioneer, this fish has dreamed of conquering new lakes ever since its youth.