I saw an expert Japanese chef fillet a mackerel (Scomber scombrus). He cut off the head and the whole intestinal sac came out with it as a single unit. Then he took the knife and scraped something from inside on the back. The object(s) were dark and bloody. Would those have been the kidneys?
You can see the video here: https://www.youtube.com/watch?v=udTHiRg34KI. The moment is at 2:20.