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Rotting fish seem to give off the same (very pungent) kind of smell, regardless of the kind (salmon, seabreen, tuna, etc).

What exactly is it that's responsible for this unique smell?

(Though I've answered my own question below, do feel free to post an answer of your own if you want.)

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The "fishy" odor that you're familiar with is brought about by a whole bunch of compounds, and not any single one. Then again, if we were to narrow this down a bit, we could say that simple nitrogen compounds are the main culprits.

But suppose we want to blame only a single compound for the delightfully pungent odor of rotting fish, and we couldn't be bothered with the nuances of olfactory physiology or food chemistry to go through a list of compounds running into the hundreds (if not thousands). In that case, we can pin the blame on trimethylamine (TMA).

enter image description here

With regard to fish, the TMA is a result of the degradation of trimethylamine oxide (TMAO) present in the body fluids (blood, lymph, excreta) of the fish.

Just so you don't get the wrong idea, TMAO is present in the body fluids of all vertebrates ̶I̶ ̶c̶a̶n̶ ̶t̶h̶i̶n̶k̶ ̶o̶f̶, not just fish (albeit in much lower concentrations, which is why you don't mistake ground chicken for tuna).

Now why is TMAO more abundant in fish?

Ever tried to pickle a cucumber (or anything for that matter)? Notice that the cucumber shrivels up over time? It's losing water to the pickling fluid by a process known as exosmosis: the water is drawn out from a less concentrated environment (cucumber's tissue) to a more concentrated one (pickling fluid).

Disclaimer: As user @lly pointed out in the comments, this particular bit has the potential to cause a bit of confusion. The inside of a cell (the cytoplasm I mean) is technically an aqueous colloidal solution (the pickling fluid is an aqueous solution as well). In the context of aqueous solutions, "concentration" is an indication of how much solute is present, not water. So in this context, if something is more concentrated, it has less water in it

enter image description here

Now the same thing is going to want to happen to a fish in salt water, which isn't so nice from the fish's perspective.

To counter the outflow of water, a fish has a significantly higher concentration of the familiar metabolite urea in its body fluids (as compared to terrestrial vertebrates). This establishes a situation where the fish's body fluids have roughly the same "concentration" as the salt water it swims in (the more correct term here would be "osmolarity", in place of "concentration")

That urea, if left to its own devices, will seek to destabilize macromolecular structures and inhibit cellular functions; the TMAO present alongside the urea helps counteract those detrimental effects.

Of course, going by this logic, saltwater fish should have a much stronger "fish odor" than freshwater fish (which is certainly the case).

Lets not forget the classic kitchen tip: Rubbing fish with something acidic like vinegar, b̶a̶t̶t̶e̶r̶y̶ ̶a̶c̶i̶d̶ or lemon juice helps reduce the odor (the TMA degrades under acidic conditions).

Also, some of you might know that a fish-like smell is observed in rancid food. That, apparently, is the result of the oxidation of omega-3 fatty acids.

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    $\begingroup$ This bit however--the water is drawn out from a less concentrated environment... to a more concentrated out--first off, you probably mean "one" there but, second, you have this backwards, right? or you're talking about the water being drawn to something with a more concentrated amount of sth-other-than-water? $\endgroup$
    – lly
    Commented Jul 20, 2018 at 16:25
  • $\begingroup$ @lly Thanks! Though I did take enough care to pick the right sources/references, there is of course always a possibility that I may have erred somewhere. And yes, I had a feeling someone or the other would raise the "concentration" problem :P $\endgroup$
    – paracetamol
    Commented Jul 20, 2018 at 16:26
  • $\begingroup$ @lly I've edited the answer to address that last issue. I hope that works O:) $\endgroup$
    – paracetamol
    Commented Jul 20, 2018 at 16:35
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    $\begingroup$ Shouldn't we be asking: Why have we evolved a sensitivity to the smell of rotting fish? If seagulls can smell it they certainly don't mind it. $\endgroup$
    – D Duck
    Commented Jul 21, 2018 at 21:12
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    $\begingroup$ @DDuck As far as I understand it, us evolving an aversion to the smell of rotten anything (not just fish) is Nature's way of telling us that some of the by-products of microbe induced decomposition of most foodstuff isn't meant to find its way into our mouths (much less our stomachs). Seagulls, vultures, flies, etc have mechanisms within them that nullify/greatly diminish detrimental effects of consuming such food (that would otherwise be seen in humans) ;-) $\endgroup$
    – paracetamol
    Commented Jul 22, 2018 at 13:59
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Putrescine, or tetramethylenediamine, is a foul-smelling organic chemical compound related to cadaverine, both usually produced during tissue decomposition. It is a toxic diamine formed by putrefaction from the decarboxylation of arginine and ornithine, and is largely responsible for the foul odor of putrefying flesh.

See reference.

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    $\begingroup$ Try to avoid using Wikipedia as a reference, instead please use something like a scientific paper, the CRC handbook of chemistry / physics "rubber book" or some other source which is better than wikipedia $\endgroup$
    – Nuclear Chemist
    Commented Jul 21, 2018 at 6:29
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    $\begingroup$ @NuclearChemist Wikipedia is known to have varying degrees of accuracy, so it is not to be used blindly. But that article has fourteen references, do you have any reason to recomend not using that particular article? $\endgroup$
    – Anonymous Coward
    Commented Jul 22, 2018 at 20:47
  • $\begingroup$ @Jose Yeah, but all 14 references don't back up the premise of this answer ("Putrescine is the cause of the rotting fish odor"). I gave that article a cursory look, and it appeared that most of the references (if not all of them) are references to the properties and biosynthesis of putrescine, and not to the fish-odor claim. :-) $\endgroup$
    – paracetamol
    Commented Jul 23, 2018 at 5:51
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    $\begingroup$ I upvoted this because I know putrescine to have a major role in the odor of rotting (generic) flesh. However, the distinctive odor of rotting fish is mainly attributed to TMA. In my answer, I did mention that other componds were involved in the odor as well, but TMA was the main factor. $\endgroup$
    – paracetamol
    Commented Jul 23, 2018 at 5:57

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