I'm sure everyone on occasion I have encountered at home or in a restaurant with a fish, a shellfish or a squid that gave off the smell of ammonia.
All theories are curious there to explain a fact that simply reveals the poor quality of the product. Sometimes they will tell you they have taken a preservative apparent that smell. In other I will say that is due to the ice. Other invent any kind of theory, but the truth is that this peculiar and unpleasant odor only one explanation, poor quality and lack of freshness.
As red meat fish is rich in protein, but unlike these fish meat more easily degraded mainly due to bacterial activity. This degradation occurs in two ways. The call is deamination resulting in the formation of ammonia and various hydrocarbon chains. Conversely call decarboxylation leads to 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 breakdown of adenosine monophosphate (AMP) in chilled seafood products.
I will say as an example that a squid stored two days after his capture will have minimal levels of ammonia but almost elapsed stored at 2.5 degrees Celsius ammonia content in ten days will increase 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 the counter action acids such as citric acid containing lemon.