Re: Why does vegetable oil smell like fish?

The short, quick answer is that your oil has gone bad. How do you store it
and how old is the oil?

I tested out the oils I have around the house: grape seed oil, olive oil,
and vegetable oil. None of these produced a fishy smell when heated. These
oils are less then a month old and are stored in a cool and dark cabinet.

Olive oil can last about 20 days at room temperature exposed to light. If
kept out of the sunlight then somewhere around 3-4 months. If you have you
oil near a heat source, like a stove or a furnace vent then it well degrade
even faster. The same would apply to your oil.

The taste or smell for rancidity can vary widely from person to person. So
a chemical test is used in industry to determine when oil has gone rancid. 

The action of air and enzymes will age your oil even faster. 

Yes, by applying heat you speed up the process, so your cold oil may be
somewhat rancid but by heating it you push it over the threshold and have
fishy oil.

For info on olive oil http://www.oliveoilsource.com/olivechemistry.htm

Some other reading
Chemistry, Flavor, and Texture of Lipid-Containing Foods
by Fereidoon Shahidi and Hugo Weenan

Lipids in Foods: Chemistry, Biochemistry, and Technology (Progress in Food
and Nutrition Science)
by F. D. Gunstone 

Canola and Rapeseed: Production, Chemistry, Nutrition, and Processing
Technology edited by Fereidoon Shahidi