Adipose Fin

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What makes a trout is the adipose fin. That’s not completely true but only the Salmonidae, Characins and Catfishes have an adipose fin. The adipose fin is found on the top back rear between the cadul (tail) and dorsal fin.
It was originally thought to have no purpose. Recent studies demonstrate that the adipose fin may actually serve one. This means the millions of hatchery fish with clipped adipose fins may have it a little tougher.

http://www.cosmosmagazine.com/news/removal-trout-salmon-fin-touches-a-nerve/

http://jeb.biologists.org/content/208/1/v.2.full

http://www.nrcresearchpress.com/doi/abs/10.1139/z04-069#.Uh4n4husgyo

The Diversity of Fishes: Biology, Evolution, and Ecology

Trout: An Illustrated History

Lampanyctodes hectoris (Hector's lanternfish)

The anatomy of Lampanyctodes hectoris
(1) – operculum (gill cover), (2) – lateral line, (3) – dorsal fin, (4) – adipose fin, (5) – caudal peduncle, (6) – caudal fin, (7) – anal fin, (8) – photophores, (9) – pelvic fins (paired), (10) – pectoral fins (paired)

By Lukas3 at commons.wikipedia [CC-BY-SA-1.0], from Wikimedia Commons