Fortunately, it isn’t necessary to answer that question, because divers, and fish, don’t get the bends (more accurately known as decompression sickness) from staying down too long. Divers get the bends by coming up too fast, but fish can indeed get the bends from other causes.
When the water pressure on a diver’s body is reduced too fast, gas bubbles can form in his or her bloodstream. That hurts, to say the least. And yes, the same thing can happen to a fish, but not from swimming upwards too fast. It happens because of changes in the characteristics of the water itself.
Oxygen dissolves to some extent in water and in watery liquids such as blood and body tissue fluids. That’s just great for the fish, of course, because they live on the oxygen that is dissolved in the water. But nitrogen, which is the major (78 percent) component of air and which is inert and useless to physiological processes, also dissolves in water and in blood.
Ordinarily, this causes no problem for either fish or man, because we extract the oxygen that we need for metabolism and throw away the nitrogen via gill or lung. But if for some reason there is too much dissolved air in our bloodstreams, we may not be able to “undissolve” the excess nitrogen fast enough, and it can collect into actual bubbles of gas, blocking the circulation and destroying local tissues.
The amount of air that will dissolve in water at a certain temperature depends on the pressure: the higher the pressure, the more gas will dissolve. When a human diver goes down, the increased water pressure forces more oxygen and nitrogen into his or her bloodstream via the lungs.
The oxygen is no problem because the blood’s hemoglobin eagerly latches onto it and delivers it to the cells. That’s its job.
But when a diver surfaces and the pressure decreases, it would be very nice if the excess dissolved nitrogen could depart via the way it came in: through the lungs. Unfortunately, that can be a very slow process. Instead, when the pressure is reduced too quickly, the excess nitrogen gas just bubbles out of the blood, just as the carbon dioxide does when you release the pressure by opening a bottle of soda pop.
The answer for divers, of course, is to come up slowly and give the nitrogen a chance to leave the bloodstream gradually, molecule by molecule, exiting via the lungs.
If a fish were to swim rapidly upward from a great depth to the surface, the same thing might happen, except for two differences: One, fish have more sense than to do that; and two, something even more drastic would happen, the fish’s swim bladder would expand so much that it would crush the fish internally and kill it. But we said that fish can get the nitrogen-bubble bends, and they can. Here’s how. Suppose that a fish is happily acclimated to its environment, swimming around in water that contains a certain amount of dissolved air. Its bloodstream will have adjusted itself to that same amount of nitrogen.
Now suppose that the fish wanders into water that for some reason (we’ll get to the reasons) contains a lot more dissolved nitrogen than is normal at that temperature and pressure. Before long, its blood will also acquire that same abnormal amount of dissolved nitrogen. This is a precarious condition to be in, though, because at any moment some of that excess nitrogen can pop out as a bubble and the fish will get the bends. It can only relieve itself by heading for greater depths, where the added pressure will push the bubble back into the blood.
How might a fish find itself in water that contains an abnormal amount of nitrogen? It turns out that it needn’t have anything to do with depth or pressure.
For example, a fish may be swimming in a river that contains the standard atmospheric-pressure’s worth of dissolved nitrogen, when it happens upon a region of warmer water that has just been discharged by a factory or power plant (Power plants inevitably throw away a lot of waste heat.)
By rights, the warmer water should contain less nitrogen, not more, because gases dissolve to a lesser extent in warm water than they do in cold. But if the plant’s discharge water had never been given enough time to lose some of its extra nitrogen when it was heated, and remember that the evolution of nitrogen can be a slow process, then it will still be carrying more nitrogen than is normal for the river’s normal conditions. The poor fish finds itself swimming in abnormally high-nitrogen water, and it gets the bends. That’s one way in which power plants kill the fish in a river “merely” by discharging warm water into it.
Another example: Have you ever bought a couple of goldfish, taken them home, put them in a bowl of nice, fresh water and then watched them get sick and die? Well, here’s what might have happened.
Your tap water has lots of dissolved air in it because it is cold and it was probably even sprayed into the air at the waterworks to aerate it. Then you put it into the fishbowl, where it slowly warms up to room temperature. But it may still retain its cold-water load of nitrogen because, as noted above, evolving excess nitrogen can be a very slow process. The water then will still contain an abnormal load of nitrogen when you put the fish in it. Bends and death ensue.
Can anything be done about the power-plant fish-kills and the thousands of goldfish murders that are committed every day? Yes, and it’s fairly simple. Just let the water stand for a long time before dumping it into the river or the fishbowl. Standing will permit any excess nitrogen to escape, and the water will come down to the just-right amount of nitrogen content for its temperature and pressure, and therefore the just-right amount for a fish at that temperature and pressure.
How do fish in deep ocean water get their oxygen? How much oxygen can there possibly be down there with the atmosphere so far above?
The oxygen doesn’t come only from dissolved atmospheric air. You’re forgetting about plants, which breathe in carbon dioxide and breathe out oxygen. Oceans contain an abundant variety of plant life, and the oxygen emitted by the plants dissolves directly into the water By constantly swimming and passing large amounts of water over its gills, a fish can “vacuum up” a lot of oxygen, even if it isn’t present in very concentrated amounts.
In areas where not enough plants exist to supply the fishes’ breathing needs, they just take their business elsewhere.