Just realized the best fishing trick in Stardew Valley is to just stand still and do nothing for 2 in-game hours
I kept trying to chase bubbles around the mountain lake every morning for 3 seasons and my catch rate was awful, so last week I parked my farmer in one corner of the pier by the big rock and just cast straight down without moving an inch. In one spring day (6am to 4pm in game time) I pulled 3 largemouth bass and a legendary-ish sturgeon, which was more than I'd gotten in the whole prior season. Is this just RNG being nice to me or does standing in one spot actually matter for fish quality?
Have you ever watched a bobber sit in the same pixel of water for like 45 minutes while you question your life choices? That is basically my Stardew fishing experience, except I did the exact same thing as you and got mostly trash and a few chubs, so either your farmer has some secret luck stat or the game just hates me personally. But yeah, emmag22 is right that the fish roll happens when the bobber lands, standing still does not make the fish fancier. Still, there is something to be said for cutting out the walking, because every second you spend jogging between bubbles is a second you could be casting. I parked by that same rock for a while too and my catch rate went up, mostly because I stopped missing casts by being lazy and rushing. So keep doing it if it feels good, just know you probably won the RNG lottery more than you found some hidden mechanic.
Ugh hate to burst the bubble but you got lucky, not skilled. Standing still doesn't change fish quality at all. The fish you hook is rolled from a table the second your bobber lands, and it's based on your fishing level, the spot's zone, and daily luck. What standing still actually does is save you time since you're not walking around, so you get more casts per day. That sturgeon was pure RNG, spring is one of its seasons at the lake and you happened to roll it. Keep doing it if it works for you but the pier corner isn't magic, it's just a decent spot with less walking.