How are you so sure? Can you prove it's impossible? Have you examined the web feet of herons on a regular basis?
It's very likely fish eggs are in the area a heron walks, and we do know eggs can be sticky and adhere to things depending on when contact is made and the species of fish. I once took your postion, but when I was exposed to a very knowledgable fish farmer with a lot of education in the field, and he made a convincing case, I'm leaving my mind open to this.
