Part of a fish outside the intestinal sac on the back?
Yes, those are the kidneys. They're quite different from mammalian kidneys.
This is a picture highlighting the kidneys in a salmon dissection from the Pacific Streamkeepers Federation (http://www.pskf.ca/sd/). A very different fish, but similar enough to show the kidneys.

You can also see a dissection of the fish urogenital system here: http://necropsymanual.net/en/teleosts-anatomy/excretory-and-osmo-regulatory-system/, though I can't tell what fish species they're dissecting.