adipose fin

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

• noun A fleshy rayless fin located on the back of a fish between the dorsal fin and the caudal fin, found in certain fishes such as salmon, trout, and catfishes.

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

• noun ichthyology A soft, fleshy fin found on the back of a fish behind the dorsal fin and just forward of the caudal fin.