a small, fleshy fin, usually lacking rays, behind the main dorsal fin in trouts, catfishes, and other bony fishes.
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British Dictionary definitions for adipose fin
a posterior dorsal fin occurring in some fish, such as those of the salmon and catfish families
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012