Pacific is the name of an ocean. An ocean is where you can catch fish. "Pacific" sounds close to "specific". Since the joke is discussing a fishman catching fish, the word "specific" was replaced with "pacific" which relates to an ocean where fish can be found. The joke is playing on the word "pacific" because it sounds close to "specific". 
The fisherman kept bragging about the big fish he caught , but he would not be very pacific about where he caught it .