@viralundead
General consensus is Big Flatfish and Big Mackerel are either NOT currently in the game, not currently spawning, or spawn in a very specific spot that's constantly despawning them with the wave/high tide bug, etc. etc.. so don't even bother looking for those right now. Between Youtube videos, it's comment sections, reddit, etc., no one is finding them currently. PC players might have "options" to get around it.

Every other fish like Big Cod are there. Dunno about a Big Eel as I haven't seen any quests asking for it.

IF looking for small/regular flatfish, there's an inlet in northern Cent, right where the river and ocean meet up. The inlet is shaped like Florida and I think west of Raculf Monastery and the fastravel anchor to it's left. It spawns nothing but both in the inlet and the water is shallow, so they can be hit with melee.

Also, FYI, if you go somewhere like Folkanstan (fast travel) the fish usually despawn with 5 sec. of entering. Meditate 5-7 times, using Odin's Sight after each meditation to see if fish spawned. On occasion, around 5 or 6 meditations, tide will subside and calm briefly and fish will respawn briefly. Works in known spawn locations, though not 100% of the time. Got two big Sturgeon in Folkanstan, and 2 big Bream on the tiny rock island south of 7 Sisters, for example. Though not worth the headache tbph until they fix this.

In original post, no.6, arrows were retrievable from red fish in Norway when I went back there. Could also see the arrow sticking out when it was floating. So no arrows might be location or fish dependent, dunno.

@bushwickedly

Just thought I'd add this
I just caught a fish outside Ravensthorpe. Nice you'd think
Except it was a Haddock - a salt water fish, in a river miles from the sea.

@viralundead Have had same issue. Fish reeling-in difficulty is wonky, not balanced, and needs fine-tuned. Have had regular sized fish that can't be reeled in and big variants that come in with minor effort. Due to line tension and amount of time it takes to reel in, you have to stop reeling briefly or risk breaking the line (nylon line breaking from modern fishing pole reels, understandable; cotton rope the diameter of your thumb breaking from a 2 pound fish is laughable and poor research on part of coders that made this, and the "reeling in" sound effect from a modern reel is out of place and silly while pulling rope in) and somehow the fish ignores the rope length (40' of rope pulled 30' in, means it has 10' of rope and yet it'll swim back out 40-50' away which makes ZERO sense as you aren't giving up rope length).

I've had fish I couldn't reel in due to that nonsense. Almost need a turbo button which, if we are being honest, you can't dive underwater and kill 10 fish with your hatchet in the amount of time reeling one regular fish takes.