No one knows the catch ratio I think but it’s preeeetty low, I barely get double fish and got a full luck tree+rings on.
Intellect gives an advantage for finding nibbles, good for testing the waters, nothing else.
Fishing Potions: Always use age, it’s easy to age them and much more effective.
As for catch rates I think some are flatout bugged (clown fish in particular) but if I get to it I will do a separate post…
PS (Edit): Oh, as for Jelmiry I put an insane amount of time into finding patterns, hours per day in all the zones with notes and what not. To no avail. I completely gave up on fishing it, months ago, only fishing some in US timezone (middle of night for me) if I ran out and have to.
Reason is, there was a post of Landru which I found to be true - it likely does not even spawn if no one is in the zone at the time where it should. No clue if that bug is fixed by now, but it might have been the reason I never found anything during my playtime (usually there’s no one in shards when I play, with one exception, hs, and rarely in rainforest - but there you only get Jelmiry with lots of trash fish, and it barely is worth all the fuzz).