Noun
adipose fin (plural adipose fins)
(ichthyology) A soft, fleshy fin located behind the dorsal fin and just forward of the caudal fin, found fish of certain families, believed to have some sensory function.
Anatomically, the pikes are characterized by the presence of shark-like, maxillary teeth, a mesocoracoid, and the absence of an adipose fin, breeding tubercules, and pyloric cecae. Source: Internet
Trout have fins entirely without spines, and all of them have a small adipose fin along the back, near the tail. Source: Internet