I'm sure everyone at some point you have encountered at home or at a restaurant with a fish, a shellfish or squid that gave off a smell of ammonia.
Curious are all theories out there to explain a fact that simply 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 say it is due to ice. Other invent any 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 rich in protein, but unlike these fish meat more easily degraded 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 results in the formation of biogenic amines (histamine, tyramine, putrescine).
Ammonia is mainly formed 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 their capture have minimum levels of ammonia but almost elapsed stored at 2.5 degrees Celsius ammonia content in ten days shall be multiplied 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 as the counteracting action acids such as citric acid containing lemon.