Adipose fin

The adipose fin is a very interesting fin in Salmonids, means all the salmon and trout, grayling, white fish get this little boneless fin. It's located on the lower back in front of tail, like a little finger shape. (Noted, lot's of different group of fish bare this fin too). It is cute and some are very colorful actually. The brown trout have a exaggerater adipose fin refer to size and coloration in male mature brown trout. What is the function of those small/helpless wing/fin? good question. There is one paper published in Europe showed that the female brown trout actually assess the "size" of those adipose fin to make a mate choice decision. I will come back to this mate choice topic again and again... maybe I should look up that paper again before I write any further...