That right there is something I stumbled upon on the last trip. Sad sight to see, that big of a rainbow, rotting with eyes picked clean. I venture to guess that this was probably the result of a botched catch and release. I don’t really know. This fish may have died from old age. Who really knows.
My guess is unnatural causes. It was found in a peculiar spot. Something to keep in mind when you are practicing catch and release. Do the fish a favor and bring it in as quickly and cleanly as you can. Have the camera ready and don’t pull it out of the water for very long. One other thing, use gear appropriate for the conditions. A little too heavy is better than a little too light.