Fish Value is a minor currency, only being obtainable from Fishing.
$ FishValue = floor (Rarity^2 \cdot Quality / 10) $
$ Quality = 60 + (1\ to\ 100) $ with a 10% chance to keep doubling up to 10 times.
Theoretically you could hit the max quality of 153,600.
The expected (average) quality of a fish without upgrades is 138.125.
Fish Value can be increased with an upgrade in the Expeditions Upgrade Lab.