Believe it or not, seafood is not actually supposed to smell "fishy." It should have a fresh ocean aroma. If shrimp does smell like ammonia, it's usually a sign of protein degradation, but not always. If the shrimp hasn't reached the expiration date, the smell may be coming from purge—harmless, liquefied amino acids that seafood releases in the package, similar to the pink liquid in a steak package.
Checking for Freshness
Check for sliminess on the shrimp, a telltale sign of protein degradation that usually appears in tandem with the fishy odor. If this is the case, toss them. Fresh shrimp is usually still good for two days past their sell-by date.