Can Fish Survive in Milk?

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Fish live in water, and skim milk is roughly 90.4% water, so could fish live, or at least respire, in milk?

A curious Redditor recently posed this simple, yet thought-provoking question to the AskScience subreddit. The simple answer is "no," but the nuanced response sheds light on how fish, and all other organisms, function.

Fish have evolved over many millions of years to survive in water with a certain amount of dissolved oxygen, acidity, and other trace molecules. So, though skim milk is nine-tenths water, it still would be entirely insufficient to support a fish for long. The differences in acidity and dissolved oxygen, not to mention all of the fat, proteins, carbohydrates, and other minerals in the milk that might clog the creature's gills, would quickly spell trouble. The animal would likely die within minutes, if not sooner.

If you think about it, skim milk is essentially just heavily polluted water. And even relatively mild pollution can bring about massive fish kills.

This skim milk hypothetical demonstrates why manmade climate change could be disastrous for ocean life. As the ocean heats up, it holds less dissolved oxygen, and as it absorbs more carbon dioxide, it grows more acidic. Both changes disrupt the delicate environmental balance that fish and other marine organisms rely upon to thrive. 

Expecting a fish to live in milk or a drastically altered ocean is like expecting a human to thrive on Mars. Sure, Mars' atmosphere contains oxygen, but the concentration is insufficient to supply our cells with the amount they need to create energy.

Humans also couldn't live in an environment with a significantly higher oxygen concentration, either. Earth's atmosphere is a pleasing mixture of mostly inert Nitrogen (78%) and reactive oxygen (21%), plus trace amounts of other elements, and it's what all land-dwelling life is used to. As Jane Hawke explained at BBC Science Focus Magazine:

Our blood has evolved to capture the oxygen we breathe in and bind it safely to the transport molecule called haemoglobin. If you breathe air with a much higher than normal O2 concentration, the oxygen in the lungs overwhelms the blood’s ability to carry it away.

The result is that free oxygen binds to the surface proteins of the lungs, interferes with the operation of the central nervous system and also attacks the retina.

Air for us is like water for fish. Change it too much (or turn it into milk), and we'd be goners.

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