It's a joke. But it wasn't always. In about 1917 it became illegal to fish from the back of an animal. Any animal. The law was not specific about the kind of animal, whether camel, zebra, rhino or giraffe. This item from the 1920 Fish and Game rules made it clear: "It is unlawful: ... ; to fish for trout from the back of any animal, or to travel up or down any stream on back of animal while fishing for trout." This restriction no longer appears in Fish and Game rules. But "chasing fish up or downstream in any manner" is still illegal.