From memory of the research I did on the subject of scent back in 2000-1 it was shown that animals such as fish and dogs have such highly developed olfactory senses that they can differentiate hundreds of different elements to any cocktail of scent. For example a dog can discriminate the scent of cannabis or cocaine buried in tins of coffee even though the drugs are wrapped and the tins sealed. A cadaver dog will indicate blood spots left at a scene years ago and a medic dog can pick out cancer cells being exhaled in the breath of a patient.
Regards fish, it was indicated that sight feeding fish such as trout and minnows have less developed senses than carp and barbel. But even then the trout and minnows have olfactory senses far, far greater than man's. Salmon are supposed to navigate their way to home rivers using the power of scent. If you imagine the vast volumes of water going down a river such as the Humber then what percentage of that would come from a small tributary such as the Wharfe or Don? How do they manage to follow that scent from the open sea?
I am convinced that you cannot rid yourself of human scent and any additional scents such as soap, tobacco and fuel will also remain even after rubbing hands with grass or other objects.
Then you face another quandry. Why would a fish avoid the smell of cigarettes, diesel or petrol if the item that was was on something they would normally eat? Bass have been caught on fag ends. Sea anglers douse baits in WD-40. Carp take cork balls. Chub baits include mouldy cheese. My dog would readily eat the left overs of a pork pie even if I gave it her with oily hands.
As I said at the start; I don't know of any proof to back up this oft quoted advice.
