Often fish sellers line up longer fish together on the ice, usually the fish don't overlap (to preserve the individual tastes of the fish). If the salmons and the eels are touching is the salmon still Kosher, or does it become tainted by the eel?
The answer I just looked up on kashrut.com is that its ok if the juices from non kosher fish get on kosher fish. Just wash it off with water at home. Buying a whole fish and doing the necessary cleaning and preparation yourself at home more or less makes sure its not getting cut with non kashered knives and utensiles and that any juices are cleaned off.
link to an answer equivelent: http://kashrut.org/forum/viewpost.asp?mid=3996&highlight=shrimp