Deadliest Catch

(Location Name): Deadliest Catch is a radiant quest in the Fallout 4 add-on Far Harbor.

Detailed walkthrough
One can get this quest from any settler at one of the Sole Survivor's settlements. While in the settlement, the player character will be approached by a random settler who informs them that a giant hermit crab has been spotted in the area and they'd like the Sole Survivor's help killing it. If one chooses to accept, the settler will then lead the player character to one of the five locations on the island where a hermit crab can spawn.

Upon reaching the location, the Sole Survivor will need to find and kill a leveled hermit crab. The crab stays hidden in its "shell" (a ruined bus or trailer), but can still be targeted in V.A.T.S.