This item can be fished in the sea waters of Darkshore. While it is the objective of a quest, it is not a quest item per se, and can be caught while not on the quest.
Darkshore Grouper as a quest objective
This item is an objective of  The Family and the Fishing Poleω τ ϖ, which rewards the fisherman with the [Blump Family Fishing Pole].
It is worthwhile to pick up the  Gaffer Jacksω τ ϖ fishing quest from Wizbang Cranktoggle on the second floor in the Inn and do them concurrently. Both require random catches from the same water.
There is no cooking recipe for this fish. It can be eaten raw.