Fish Value is a minor currency, only being obtainable from Fishing.
Fish Value is the value of the collected fish, and can be used for upgrading Soul Orbs via Sacrificial Offerings.
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.