What am I supposed to sell? Really, I'm trying to understand the usage of High level fishing.

Silver shark can be fished at level 30 and those (were) the most selling thing out there.
Now because there isn't any other good fishing resource out there, silver shark price has fallen by 50% (from 3500 to 2000) and are not bought anymore, only HQ sells a bit.

ALL the other fishes (more rare obviously) like sunfish, titanic sawfish are not used for anything and are supposed to be the most rare fishes out there.

Not only this, I can't gather shard with fishing and I have no idea why but ALL high level recipes do not use rare fishes to be built, only some low level fishes which can be found for 1 gil on the market board.

Really, tell me the purpose of fishing, I've leveled culinarian and fishing and I found myself without a gathering skill, so I have to buy shard (because I can't gather those) and all materials for my recipes because mine are not used for best foods.