Have you caught a marked or tagged fish?
If you have, or just want to find out more about these you can look at the DNR website. They have several kinds of markers and tags that are used for studying various types of fish species. Much like the waterfowl that are banded, you can send the information in when you find a tag/marker, and will get mail in return with the history of that fish. When it was tagged, approximate age, and an idea of how far it has traveled.