I KNOW THE ANSWER:
The man was shipwrecked and survived on a life boat with a group of other people, sick and delirious. His companions fed him with broth, saying it was seagull soup. When he recovered he was within sight of land, but nearly all the people on the boat had disappeared. The survivors explained that they died of exposition, and they threw their bodies overboard.
After they reach shore, he went to a restaurant and asked for seagull soup. It tasted nothing like what he ate on the boat, so he knew what he really had was soup made of human flesh, and killed himself.