Why can't fish breathe out of water?
Dear Straight Dope:
I went fishing yesterday with my family and when we caught a fish I thought, "Why can't fish live outside of water?" I mean, they breathe oxygen in the water. Outside of the ocean, they should have more oxygen to breathe - no harm done! So why will taking a fish out of water kill it?
I do a lot of fishing, or maybe it is standing on the shore with a pole looking like an idiot. The other day, I did actually catch a fish, a largemouth bass to be exact. I am a catch-and-release kind of guy - but of course I had to take a few pictures, work the hook out of his mouth, carry him back to the water, etc. All told, the poor little guy was probably removed from his preferred environment for approximately five minutes. This makes me wonder, why can a fish survive for five minutes out of water while a human surely would not survive (or at least remain conscious) for five minutes submerged in water? Could it be that a fish is able to breath somewhat when it is out of water? Please help solve this fish tale, as I hate to think while I fish.
Fish gills are remarkable things, but the conditions under which they function are pretty specific. For one thing, they are rather delicate, and their tremendous surface area (the main thing that makes them work so well) is dependent on being immersed in water to support their weight. Out of water, the gills collapse like wet tissue paper, and very little surface area is left exposed for gas exchange. Most fish, therefore, can only survive a short time out of water before oxygen deficiency catches up with them and they asphyxiate.
If it were possible to keep the gills supported and moist without being submerged, a fish could survive quite a bit longer, but that isn't physically possible - even in a humid air-filled chamber at zero gravity, the gill filaments will simply adhere to one another. Water needs to completely fill the gill chamber to keep all of the filaments in operation. For that matter, the water has to be flowing in the mouth and out the gills in order for oxygen extraction to work properly. If you force water to go in the opposite direction, in the gills and out the mouth, the system only works at about 50% efficiency, since the water flow needs to go counter to the flow of blood for maximum oxygen uptake.
Many fish species have evolved mechanisms to work around this limitation (usually involving the development of lung-like structures in addition to the gills), and some can go for long periods out of water. But land-based critters haven't developed a comparable ability to breathe while submerged. The lungs of other vertebrates are simply not designed to extract enough oxygen for them to function underwater, where the oxygen concentrations are more than an order of magnitude lower. If water could hold about 20 times more oxygen than it does, things would be different - there are apparently a few liquids (though not water) that can hold that much dissolved oxygen, and one can breathe a liquid of this sort, as in the movie The Abyss. But maintaining those high oxygen levels for long in a closed system might be a major practical stumbling block, so I don't think liquid breathing systems are going to be easy to design or use.