Why does fish stink so bad after it has been cooked and what is the best way to get rid of that stinky smell?

Generally, the deterioration of fresh fish is characterised by an initial loss of the fresh flavour, which is species-specific but may in general be described as "sweet" and "seaweedy". However, the compound trimethylamine (TMA) is an important component of the typical fishy smell of marine fish that has been stored for some time.

TMA is a product of bacterial metabolism of the compound trimethylamine oxide (TMAO), which occurs in appreciable amounts in marine fish as part of their osmoregulatory system. More specifically, TMAO may help to prevent the fish from becoming dehydrated in a salty environment. The microorganism that converts TMAO to TMA is usually a cold-tolerant Gram-negative bacterium. So, in short, if a fish smells fishy it is an indication that it is going off.

https://en.wikipedia.org/wiki/Trimethylamine

Fresh fish should never smell "fishy" when puchased or cooked.

Fresh fish does not have a strong smell. When you put your nose next to a fresh fish, all you should smell is the sea. Freshwater fish should not smell strong, either. The worst myth about fish is that it stinks. Only old fish stinks.

https://nutrition.org/is-fish-smelly/

Most fish mongers (sellers of seafood) use Lemon juice as a soap additive.

http://www.wikihow.com/Get-Fish-Smell-off-Your-Hands

I speak from experience. The only problem is that I did not know that my roommate who managed a booth at the old Fulton Fish market in New York City kept his lemon juice in a shampoo bottle. One day I ran out of shampoo and I quickly found out about LEMON JUICE!

Trimethylamine is the "why".

" Well, it's because fish tissue contains an odorless chemical known as trimethylamine oxide. Once the fish is killed and the fish's tissues are exposed to air, the bacteria in the fish's body break down this chemical into two new chemicals that are derivatives of ammonia, and therefore smell pretty bad. "

https://indianapublicmedia.org/amomentofscience/why-fish-smells-more-than-meat.php

" Method of determining the quality of fish prior to processing and packaging thereof which comprises measuring the quantitative distribution of trimethylamine in the fish and more particularly, the extent of migration of trimethylamine from dark to light meat in the fish. "

http://www.freepatentsonline.com/4309185.html

The culprit seems to be TMAO.

The principal smell contributing to “fishiness,” says McGee, is a compound called trimethylamine, or TMA. It is derived from trimethylamine oxide, or TMAO, which is not in itself objectionable. TMAO is one of several amines and amino acids that ocean creatures accumulate inside their cells to buffer them against a fatal influx of sea salt. (Seawater is 3 percent salt; the optimal level of dissolved minerals in animal cells is about 1 percent.)

Once a fish is dead, TMAO is gradually converted to TMA by bacteria proliferating on the surfaces of the fish. (Freshwater fish like ayu live in an environment less salty than the inside of their cells, so they don’t accumulate amino acids and amines. Their flesh is mild tasting and slower to turn smelly.)

As for truly spoiled fish, let’s just say that by the time proteins are being broken down into skatole, putrescine, cadaverine, and hydrogen sulfide, you probably won’t want that fish in your mouth.

http://wildshores.blogspot.com/2008/09/why-do-dead-fish-smell-fishy.html