Conservation in the 13th century

I attended a 2 day work seminar in a castle on the banks of the Rhine this past week, and in between the work events we got a tour of the castle. In one corner of the tower, was a stone with three lines, the lines labeled with F, H, and B:

It turns out that these were the minimum size limits set for fish from Rhine at the time the castle was built, sometime around the year 1200. F for Forelle (Trout), H for Hecht (Pike) and B for Barbe (Barbel). As you can see, the minimum size was tiny by today's standards..I imagine there were more fish, too.