In Act I: Shot In The Dark, after separating into two groups and going to the right, you will enter a sewer. At the starting area of the sewer (before going through the archway), shoot the four valves (two on the left side and two on right side) until they start smoking to spawn the Fish Stick weapon. This weapon runs out of ammo quickly. Once it runs out of ammo, your character will drop it. It can only be used again by reloading the chapter and activating the Easter Egg again.