I have been keeping koi for the last 15 years and currently have a 10,000 gallon pond in PA. It was stocked with my 6-7 year old koi in 2016 which happily multiplied. This spring I caught a baby koi in the skimmer which lacked a dorsal fin like some of the fancy goldfish (lion heads, celestials, etc.). However, it still has the koi shape and although it doesn't have a perfect back minus the dorsal fin (very slight bumps on the back in two places) I can't find anything on the internet about koi without dorsal fins. This particular fish is currently in a 40 gallon breeder tank together with two other (perfectly normal) koi babies and they are all doing well. They eat well, they swim well and I can't distinguish any difference between the behavior of the normal koi versus the "abnormal" one. Has anyone ever come across a koi minus the dorsal fin?