Hi! So in THE DOG Island I managed to decode most of the fishing system - enough to determine what fish can appear where, their sizes, etc. Turns out Petz follows a near identical fishing system.

How it works is like this: when you enter an area, the pier will randomly choose a fish setup to use. This determines the chances of each fish appearing, if at all. I haven't looked into it much in this game yet, but in the dog island there were almost always 3 setups per pier, and south pawville has 4 setups. This means it actually IS true that sometimes, you'll be able to enter the area with certain fish completely unavailable unless you leave and come back again. (Alternatively, some fish will just be more common or rarer than usual)

I'm hoping to find out which fish are exclusive to setups. So far, I've confirmed that Goatfish is indeed not available in some of south pawville's setups, so there's a free 1.5h timesave on my 100% run right there :P Although it seems like a bit of an issue, perhaps some fish are mutually exclusive so at least you have an indicator when you're on the wrong table. This was the case in THE DOG Island for a lot of fish so hopefully it's the same here.

Hello again! In the past few weeks we've discovered how to determine the exact chance of every fish in the game, as well as their potential lengths in cm. I'm putting it all together here: https://docs.google.com/spreadsheets/d/1FfAg6qm_FDZlUl0SvoC4RgJE6RF1ko447YRXytijoT8/edit?usp=sharing

As of the time of posting, I've only done the setups for South Pawville, and only their chances of appearing - no lengths yet. However, I do intend to do at least chances for every fish in the game, so this should be an incredibly useful resource for 100% that will make the fishing RNG a less intimidating by a bit. Hopefully, this should avoid any more goatfish incidents...

EDIT: Table now has all areas and some lengths.