I've got about 70% of the fish found in the game. At some point you get an email from one of the characters that says a great place to find rare fish is a subterranean lake near where he lives.
I've taken that to be the puddle/waterfall at the bottom of the ravine between the city ruins and the commercial center, at the end away from the elevator. However while I've caught a number of things there, they have been extremely common fish (killfish) and junk (RPG).
Am I in the right spot, or is there another location for rare fish? Is there anyway to increase the chances of getting rare fish?
I am playing on PS4 so answers which would require a mod to the game will not work.
Update: So the ravine just outside the commercial center which I'm asking about above is NOT Emil's home, which is what is referenced in Emil's email. Just to be clear, and that's why I wasn't finding some of the rare fish.
Best Answer
There are rare fish in some of the fishing areas that can't be found anywhere else. The specific rare fish that I believe the character was talking about in that email would be the Twinfish and Bloatfish, as those can only be found near that character's house.
Other rare fish would be the Basking Shark and Swordfish (and their machine lifeform variants) near the Missile site in the Flooded City, the Coelacanth (and its machine lifeform variant) near the coast in the same area, the Beetle Fish and Arowana Machine in the Oasis, and the Fur Carp and Arapaima on the path to the Ravine.
Those fish can only be found in those specific locations and some of them are fish I've never even seen.
For the full list of fish in the game, here's a link to the wiki.