WEBFISHING: Fish drop tables (and how it works)

Fish chance refers to how quickly a fish will bite your bait.
All current bait gives a base fish chance of 0.06.

“Rod Chance” upgrades to your rod add a flat value to your fish chance:
Upgrade 1: 0.02
Upgrade 2: 0.04
Upgrade 3: 0.06
Upgrade 4: 0.08
Upgrade 5: 0.10

Several further factors can increase your fish chance:

If you have recently slow-reeled your line, your chance is multiplied by 1.1.
Using the Attractive Angler multiplies your chance by 1.3.
Fishing in rain multiplies your chance by 1.1.
If you have the Catcher’s Cola buff, your chance is multiplied by 1.15; for ULTRA or DELUXE buffs, it’s 1.3.

The maximum possible fish chance is 0.36 (rounded up).

When you fish, the game rolls a random integer between 0 and 1 (like rolling a d100) to determine what you catch:

Without the Magnet Hook,
0.01 or 0.02: You will get a treasure chest.
0.03 to 0.05: You will roll the Trash pool.
0.06 to 1: You will roll the Saltwater/Freshwater pool.

Magnet Hook:
If you’re using a Magnet hook, it doubles the chances for treasure and trash:
0.01 to 0.04: You will get a treasure chest.
0.05 to 0.10: You will roll the Trash pool.
0.11 to 1: You will roll the Saltwater/Freshwater pool.

If it’s raining, rolling 0.01 to 0.08 will roll the Rain pool.
Rain takes priority over treasure or trash, meaning you typically won’t be able to catch trash nor treasure chests during in the rain.

When a pool is chosen, you’ll roll three separate integers for each fish to determine its type, tier, and size. If you use worms as bait, you cannot catch tier 3 fish.

If you are using the Fly Hook, you will be given the smallest-sized fish.
If you are using the Large Lure, you will be given the biggest-sized fish.*
If you are using the Sparkling Lure, you will be given the highest-tiered fish.
If you are using the Golden Hook, you will be given, if applicable, a rare fish. Only the Golden Bass, Golden Manta Ray, Diamond and Leedsichthys are considered ‘rare’.

If no specialised hooks are used or multiple fish have the same values, the first rolled fish is selected.

*This means catching fish like the Whale is more predictable with the Large Lure, not the Sparkling Lure. Tangentially, larger fish will give you more XP.