There's no shortcuts - just requires patience and luck. As long as you are catching the occasional cherry salmon/char/golden trout, you know you are at least fishing in the correct area of the river.
In my experience of previous AC games, I have always found it to be one of the rarest fish in the game - I think it is the case in NH again.
At a nook island it is easier to catch them without bait. You just have to scare away any fish on lower tiers and wait for a big fish to spawn on a 2nd or 3rd tier area on that island
I've caught like 3 and the easiest place is on a nook miles island where the fish respawn quicker. Otherwise you burn through like 50 baits on your own island and it's frustrating Don't give up though!
took me about 30 bait/15 minutes! if small fish spawn form the bait just keep throwing out bait until the big shadow forms! i still need to catch a char and a golden trout though
I found a mystery island with cliffs. You guys are right, fish span like crazy there and I didn’t even have to use bait/fish food. Just walk around a bit. I think I tried for like 30 min, then boom, it was there. Caught a few sturgeon too.