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Obviously, humans can survive underwater for a short while. I've assumed this is because at some point it benefited us to stop breathing momentarily, and so we evolved a respiratory system that was able to take short breaks.

Similarly, fish can survive outside of water for a short while. However, I can't think of a single instance for most fish where they may have needed to evolve that ability. Even their ancestors originated from aquatic environments, so I don't think it's a residual ability they no longer need.

Why did fish evolve a respiratory system that could take breaks like ours can?

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Do not have too a "panselectionist" view of evolution!

You can survive in a bath of mercury for a little while. You can survive naked in the outer space for a while (see here). Yet none of your ancestors where exposed to such conditions. We can be tolerant to certain conditions without having been selected to tolerate it.

Similarly, you managed to survive in your very specific environment. Yet none of your ancestors ever encountered this specific environment.

In other words, evolution is more than just natural selection. Not every phenotype in every environment you can think of is the result of a direct selective pressure acting on it. A classical and easy to read paper on the subject is Gould and Lewontin (1979).

Why fish not die suddenly outside of the water?

The main reason why fish die outside of water is that they cannot intake oxygen from air (see Breathing under water; not considering lungfish). So once fish are exposed to air only, they stop intaking oxygen, consume the oxygen that they have left in their circulatory system and tissues and slowly die of asphyxie but there is no reason for the death to be direct and sudden.

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  • $\begingroup$ As I understand it, we could survive a mercury bath because we evolved a barrier (skin), and our homeostatic levels for mercury are far lower than instant death. The same goes for space and temperature (I don't die at 98.5 degrees). I'm pretty sure we evolved a resilience to things like that when we left the roughly-constant temperature and alkalinity of the ocean, since we needed to respond to huge environmental fluctuations. Fish do encounter a pretty standard environment though. Is it nutrition perhaps or other factors which led to them needing a response to changes in homeostasis? $\endgroup$
    – Lord Farquaad
    Oct 25, 2017 at 16:59
  • $\begingroup$ Or is your first point that there's not always a reason? $\endgroup$
    – Lord Farquaad
    Oct 25, 2017 at 17:00
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    $\begingroup$ I just mean fish still have oxygen in their tissues and there is no reason for them to die instantly. They were not selected (not that I am aware of) for surviving to this condition, they just happened to be pre-adapted due to various reasons, including the fact that they are big and therefore always keep a certain amount of oxygen in their tissues. $\endgroup$
    – Remi.b
    Oct 25, 2017 at 17:04
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    $\begingroup$ Do you consider lungfishes as fishes? When their pond dries they can survive hybernating in cocoon for up to four years. $\endgroup$
    – Maxim Kuleshov
    Oct 25, 2017 at 17:28
  • $\begingroup$ A lot of fish (not just lung fish) can breath air quite well they start dehydrating quickly however and no organisms breathing mechanism works onces it dries out. . $\endgroup$
    – John
    Nov 17, 2017 at 1:19
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The answer is simple. A fishes gills are exposed to air that is oxygen rich. Gills can take oxygen from the air. We do not inhale water, so humans under water must hold their breath. I am assuming the question is not referring to lungfish, who have modified their swim bladders (homologous to our lungs) to take up oxygen.

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    $\begingroup$ Gills cannot take up sufficient amounts of oxygen from the air - this is why fish cannot survive out of water for long. $\endgroup$
    – Chris ♦
    Nov 9, 2017 at 8:37
  • $\begingroup$ Also, do we know that lungfish modified their swim bladders to take up oxygen? I can't recall offhand if the current consensus says that lungs are ancestral to swim bladders, or that we don't know which is derived from which, but I'm pretty sure it's one of the two. $\endgroup$
    – Oosaka
    Nov 9, 2017 at 21:57
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Correct that they cannot take in "sufficient" amounts, but a bit is all it takes them from asphyxia for an hour or so. The question was not why can't fish live their entire lives out of water, but why they don't die immediately.

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Actually there is a huge group of fish that need to breath air, swamp living fish, stagnant water is very low in oxygen that is why many fish gulp air (air contains a lot more available oxygen than stagnant water) even house goldfish will do this is poorly oxygenated bowls (see below). in fact the early ancestors of terrestrial vertebrates show a lot of adaptations for swamp living, legs also work better in swamps than fins (at least above a certain size) that is why so many swamp fish develop leg like fins, such as the snakehead fish. These are not adaptations for land but for shallow stagnant water, air gulping for hypoxic water in particular.

Note most of these fish are not lungfish (or lobe finned fish) they are ray finned fish, they are using their swim bladder to extract oxygen. Aquatic surface respiration is actually a very widespread behavior in fish, and can be triggered in a very large number of ray finned fish. The behavior is quite well studied and has lead to better developed air breathing multiple times.

Fish that cannot do this (like sharks) will die very quickly on land as the suffocate, since gills don't work in the air. The risk for normal fish is dehydration of their breathing tissue, of while they are IN water this has a easy solution.

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