Why should I casting rod fish over fish nets or bait fishing?
Both nets and bait fishing are highly random with what they pull from a given fishing spot's available fish. They work well in the early game, but as soon as you move into midgame or beyond random fish drop off in usefulness. You really want quality food in the current system, so getting the best fish for your needs becomes important.
What does quality of my equipment do?
The quality of your rod/line/hook/lure are averaged together create a softcap for any fish you catch. Generally speaking though this isn't hard to get above average fish quality since it's rare to see anything above Q30. Equipment quality also reduces the chance of it being broken or lost. In my experience you will break your line/hook about 1/5th as often as losing lures given similar quality. However when you do, everything up the chain (line->hook->lure) gets lost as well. Even with Q60-80 gear, I typically lose 5 lures and 2-3 line/hooks per 20 fish I bring in at 30-50% catchrates.
What does my tackle combination do?
Fish have a static preference of tackle. If you find a 100% catch combo for a fish, it will stay that way at least for the world you found it in. Many fish don't care much about which line or hook you use, but the rarer the fish type the pickier they tend to get. Any of the options are interchangeable, so if a fish likes flax and hide straps you can use either in the tackle set. Go with what's cheap to make and good quality so it breaks less.
Why can I only see a couple different fish types?
How many options you can see of what's available is based on will*surv score. For two snapshots, I was able to see 2 options at 30*45 and 5 at 72*129. Realistically speaking, you're going to want to see 3-4 before you start trying to fish for food. When you're trying to find a fish node, the fish you're after will often be hanging in the 3-5 slots as most of the good fish have small nodes that you absolutely want to find the edges of. The average node will hit 30% base catch rate with perfect tackle, but good nodes can go up to 50-70%. The green highlighted option is what your character will try to autofish if you don't hit the button and just let the timer run out.
Why don't I always see the same fish while fishing at the same spot with the same tackle?
Which fish you see is weighted towards the ones you're most likely to catch. So fish density and your tackle play a role. However, it's still RNG based. So while you usually see the best fish you can catch, sometimes the unlikely catches will make it into the list. With spam casting and 4-5 visible options this is a good way to find rare fish nodes.
How do fish nodes work?
When you go fishing there's two aspects to your catch chance, the first we talked about is tackle combination. The second is fish density. This is a combination of your physical location, the time of day, and the moon cycle you're on. This might sound overwhelming, but it's actually simpler in how it works. "Location" is just like a quality node. There'll be a roughly circular location where that fish appears within areas where you see fish jumping. Moon cycle and time of day impact how big it is and how high the density peaks at as well as causing it to shift around slightly (rare fish in a bad time/moon combination have incredibly tiny nodes/bad density). So while you might find a big lake with fish jumping everywhere, only small circles within that fish node will spawn some of the rarer fish like eel. Generally speaking though, you'll find better densities in fish nodes with a lot of jumping fish.
Although the wiki makes a fairly big deal out of shallow vs deep, river vs lake, most fish don't observe that system anymore. I've been completely unable to provide proof that they do at all actually, all the nodes I've found bordering different zones cross it without any change in density other than what you'd expect from distance. That said, there is most definitely different fish lists for ocean vs fresh vs cave water. Some fish show up in fresh and ocean, but that list seems small (the only ones I know of are eel and salmon).
Why can't I find _____ fish?
Likely you're trying to do the fishing credo with low will*surv and no idea what tackle combo to use on a fish that has small/rare nodes. May jorb have mercy on your soul. Some people try bait fishing, but this still observes the node system as far as I can tell so good luck on that one. Sadly this means your best bet is to either ditch the quest, or go to the forums/wiki trying to find a tackle combination and pray. However there's significant amount of confusion on how fishing works, so often the information is pretty useless. It's also not uncommon to have your wanted rare fish buried under 2-3 common fish, even at their node peak.
Tackle combos I've figured out for anyone interested:
