Good question. When I filet fish at the pond's edge, I'm surrounded by our dozen layer hens, all trying their best to swipe the goods. They get the carcasses and quickly clean them up. I cut the skins into bite sized pieces. The birds particularly enjoy these treats.
The uneaten portions go back into shallow water. Next morning the skeletons are picked clean by the crayfish. If I were to shine a light onto the carcasses at night, I'd see big crays working over the remains.
My pond water is not overly fertile, so the small amount of nutrients returning to the water doesn't seem to be harming anything. I view the process as fish feeding.
