If you mean "Ragnar the Terrible", then you pick up that mission from an NPC at Drubman Marina (after liberating it). But there are no fish in the game that reward perk points for catching them. At least not including the fishing 'challenges' anyway.
http://orcz.com/Far_Cry_5:_Ragnar_the_Terrible
And if you mean the 'hard' fishing spots, they are revealed by finding maps in-game. Or by googling for a map of their locations.
I'd attempt to describe all the locations for you, but it would be much quicker and easier to just google for a map of them.
The record breaking fish are always found at the hard locations.
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