Here's a disgusting experiment you can try. If you leave out a hunk of sirloin and a piece of cod for a few days, the fish smell will overpower any smells coming from the steak. Here's why fish stinks so much more than meat.
Fish tissue contains an odorless chemical known as trimethylamine oxide. Once the fish is killed and the fish's tissues are exposed to air, the bacteria in the fish's body break down this chemical into two new chemicals that are derivatives of ammonia, and therefore smell pretty bad.
Follow Your Nose
Meat, however, doesn't contain high amounts of trimethylamine oxide, like fish does. This chemical is especially common in the flesh of cold-water surface-dwelling fish such as cod. So, cod would start smelling faster than, say, catfish.
Interestingly, people who don't eat fish find its smell a lot more offensive than people who regularly consume it. Plus, because we associate rotting fish with food poisoning, we're conditioned to find its smell bad rather than appetizing.
Ray, C. Claiborne, "Rotten Fish" at New York Times on the Web Learning Network At http://www.nytimes.com/learning/students/scienceqa/archive/000523.html. Accessed May 11, 2003.