Big Fish in a Small Pond is the fishing daily quest in Fallout 76, given by Captain Raymond Clark at Fisherman's Rest after gaining access to fishing. The objective is to catch 7 fish in the region assigned by Captain Raymond Clark.
Travel to the marked region and look for a valid fishing spot near water. You do not need to equip the Fishing Rod from your Pip-Boy — when close enough to fishable water, a Fish prompt appears on screen. Use bait, catch the required fish, then return to Fisherman's Rest and fill out the Captain's Log to finish the quest.
Completing this quest can reward bait, Legendary Scrip, supplies, Fishing Rod mod plans, fishing camp plans, and fish-based cooking recipes. It also counts toward lifetime fishing challenges that require multiple daily quest completions.