Story related and unmissable.
There are two pieces that need to be combined to unlock this achievement. The first is on some bookshelves in the back/right corner of the room (to the right of the desk).
The second requires several steps. Pick up the broken fishing rod on a nearby table, then interact with the nearby case. Input this code to unlock the case:
Place the broken fishing rod in the appropriate position and then pick up the key. Use the key on the top left desk drawer to receive a roll of fishing line. Put that fishing line in the round receptacle of the previously opened case and take the numbered base. Go back to the desk and pick up the ashtray, then use it on the glass wall behind the desk and pick up the fish head from the far right side. Place the fish head on the empty plaque on the wall and then the numbered base on the wall immediately below, then back up and interact with the wall full of fish to start a puzzle. Arrange the fish like this:
The bottom center fish will open its mouth and reveal part of a trophy that you can pick up. Combine the two pieces to create an intact trophy and unlock this achievement.