This was not uncommon among many native peoples, including some Native Americans. I saw some kids doing this when I was a child, as well. They seemed to think that the fish scales would stick to the mud as they opened up the dried coating.

There was a cooking fad several years ago involving a porous clay casserole-type set up. One soaked the clay vessel, top and bottom, in water, added food, then baked the whole thing in an oven. Like the mud, the soaked clay released water to steam the food as the vessel dried again.

CM