Fresh fish doesn’t smell fishy at all. It’s only when proteins and amino acids in fish start to break down, releasing stinky nitrogen and sulfur compounds, that the funk sets in. There are a few reasons that this sort of smelly decay is more common with fish than with chicken, beef, or pork. Fish frequently eat other fish, so their digestive systems contains enzymes that can break down the proteins found in fish. So if some of these enzymes leak out, or if you’re slow to gut your fish, those enzymes will go to work…on its own flesh. Fish also generally have higher levels of unsaturated fats, which are less stable than saturated fats to oxidation. Acids, like lemon juice, can slow the enzymes down, and convert the amines into less odorous ammonium salts, which is probably why we’re all used to squeezing a lemon wedge on fish.