I'm sure everyone at some point you have encountered at home or in a restaurant with a fish, a shellfish or calamari that gave off a smell of ammonia.
Curious are all theories to explain that there is simply a fact that reveals the poor quality of the product. Sometimes they will tell you they have taken a preservative that gives off that smell. In others they will tell you that due to the ice. Other invent any kind of theory, but the truth is that this peculiar and unpleasant odor only one explanation, the poor quality and lack of freshness.
As red meat fish is rich in protein, but unlike meat on these fish degrade more easily, mainly due to bacterial activity. This degradation occurs in two ways. Deamination call is what gives rise to the formation of ammonia and various hydrocarbon chains. Instead the call decarboxylation leads to the formation of biogenic amines (histamine, tyramine, putrescine).
Ammonia is formed mainly by bacterial degradation of proteins, peptides and amino acids, but may also occur in the autolytic degradation of adenosine monophosphate (AMP) in chilled seafood products.
I will say as an example that a squid stored two days after his capture will almost minimal levels of ammonia but elapsed stored at 2.5 degrees Celsius ammonia content in ten days tenfold making it inedible.
You know that the seafood or fish served with lemon? You will be surprised but has a chemical reason. Both ammonia and the remaining amino groups are bases and can imagine their action as they counteract the acids, such as citric acid containing lemon.