ventral fin

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A fin, such as a pelvic fin or an anal fin, that is found on the ventral side of a fish.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun ichthyology A fin, usually paired, located ventrally below the pectoral fins of a fish.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun either of a pair of fins attached to the pelvic girdle in fishes that help control the direction of movement; correspond to hind limbs of a land vertebrate