I can see how someone might get the the whitefish and the fallfish confused. They both have forked caudal fins and long semi slender bodies, they are close to the same color and both have distinctive scales, but they have a few field marks that can help you tell the difference. The lack of the adipose fin on the fallfish is one identifying characteristic. Another is the different sized scales, the fallfish is larger of the two. You’ll also notice the whitefish has a very distinctive difference in the size of the head in relation to it’s body girth. The head seems too small for the body.
Fallfish
http://pond.dnr.cornell.edu/nyfish/C...e/fallfish.jpg
Whitefish
http://pond.dnr.cornell.edu/nyfish/s...e_whtefish.jpg