I was watching a documentary about a scientist fishing for giant stingray in Thailand. They ran into a fisherman on the river late at night. The fisherman said he was fishing for giant river shrimp (the things were the size of lobsters). The scientist then asked the fisherman if he was fishing to eat or to sell. The fisherman laughed and said he could not afford to eat the shrimp. How can I relate this to economics and far to explain the fisherman's statement that he "could not afford" to eat the shrimp he caught?