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The fin on the back of a fish is the dorsal fin. It runs along the top of the body and helps stabilize the fish, preventing it from rolling as it swims. Other fins have different jobs: the fins on the sides near the gills (pectoral fins) help with steering and braking, the fins on the belly (pelvic fins) assist with stability, and the tail-fin (caudal fin) provides most of the propulsion. So, the dorsal fin is the one located on the back.