Forgive me for indulging in the vanity of repeating a previous post. When fish spawn, the bottom must be clear sand or else the developing ova would not get enough oxygen. The fish spend a considerable amount of time and energy agitating the water in the place they plan to deposit the ova, to "redd up" the layer of dead or living algae. I have seen sunfish do this. All along the edge of the pond there were light colored circles of their nests, outlined by the dark dead algae layer. I have a hunch the males do the work, to attract a roe laden female.So the name "redd"
for a cleared place for fish to deposit ova makes sense to me.