I've added a section to the WikiNas page on Fishing. It focuses primarily on safe fishing areas with good skill ups that produce decent vedorable fish. I'm currently level 49 but will be updating as I go. Hope it helps!

Silver shark in sauromogue champaign will sell for about 500 a pop, needs meatballs but the cost of bait in negligible, or getting cooking up to that level is negligible if you are already fishing that hard. Cliffs are fairly safe the Dodo could in theory wander over I guess but I've never seen it personally. You will catch some garbage but not much and the ones you do vendor/ah well except buckets (mythril swords and rusty swords). Don't fish on ferry, ferry is lamesauce.

Could also do bream once you hit 66 or so, they vendor for like 550 or so but you catch a lot of garbage along with them. Best place I found was gustaberg but you get a ton of bycatch (ogre eels, cobalt jellies), port windurst and port bastok just seemed super slow comparatively. Use a shrimp lure for this.

After that you're right, sole npc for 700 gil, rarely worth waiting on auction for the slight gain you'll make if the ever sell, which they rarely do because ppl making sushi tend to fish em themselves. Fish em at the same spot as the sharks though, you don't get jellies there.

Oh and all water in the zones seems to be more or less the same on Nas, makes it very difficult to target certain fish, but you can be super lazy as any puddle will yield the same fish.

I actually plan on trying out Silver Sharks at 56, and I'm also going to try out breams and black ghosts to see which I prefer. Not too sure about the ghosts though--I don't want to get my ass kicked, so I'll only try them if I can find a safe camp. Thanks for the tips!