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J. Perelman
"Entertaining physics". Book 2.
Chapter 6. Properties of liquids and gases

THE PURPOSE OF THE FISH BLADDER

What role does the swim bladder of fish, usually speak and write - seemingly plausible is the following. In order to emerge from the depths in the surface layers of the water, the fish inflates its swim bladder; then the volume of the body increases, the weight of the displaced water is becoming more of her own weight - and, by law, swim, fish rises. To stop the rise or fall down, it instead compresses its swim bladder. The volume of the body, and with it the weight of the displaced water is reduced, and the fish sinks to the bottom according to the law of Archimedes.

This simplified view on the purpose of the swimming bladder of fish dates back to the days of scientists Florentine Academy (XVII century) and was made Professor of the market in 1685 for more than 200 years it was accepted without objection, managed to take root in school textbooks, and only works for new researchers (Moro, Charbonnel) was found complete failure of this theory,

The bubble is, undoubtedly, a very close relationship with swimming fish, as fish, in which the bubble was at experiments artificially removed, could stay in the water, only hard working fins, and at the termination of this work fell to the bottom. What is the true role? Very limited: it only helps the fish to remain at a certain depth, especially on the one where the weight of the displaced fish water equal to the weight of the fish. When the fish fins work falls below this level, her body, feeling great external pressure water, compressed, squeezing the bladder; the weight of the displaced water volume decreases, it becomes less than the weight of the fish, and the fish uncontrollably falling down. The lower it goes, the stronger the water pressure (1 atmosphere when lowering for every 10 m), the more compressed the body of the fish and the more rapidly it continues to fall.

Same thing, only in the opposite direction, occurs when the fish, leaving the layer, where she was in the balance, moves the fins work in higher layers. Her body, freed from the outer side of the pressure and still proceed inside of a swim bladder (in which the gas pressure was up to this point in equilibrium with the pressure of the surrounding water), increases in volume and as a consequence emerges above. The higher the fish rises, the more inflated her body, and consequently, the faster its further rise. To prevent this, "squeezing the bladder, the fish is unable, as the walls of her bladder devoid of muscle fibers, which could actively change its volume.

What is a passive expansion of the body is actually committed in fish, can be confirmed by experience. Bleak in colorforming state is placed in a closed vessel with water, in which there is increased pressure, closer to what prevails at a certain depth in a natural pond. On the surface of the water the fish lies inactive, belly up. Shipped a little deeper, it again POPs up to the surface. Placed closer to the bottom, it sinks to the bottom. But between the two layers there is a layer of water in which the fish remains in balance - not drowning and nothing POPs up. All of this becomes clear if we recall said now about passive extension and compression of the bladder.


Experience with bleak.

So, contrary to popular belief, fish are not able to arbitrarily inflate and compress their swim bladder. Changes its volume occur passively, under the action of increased or decreased external pressure (according to the law of Boyle - Mariott). These volume changes for fish not only not useful, but on the contrary, bring her harm, as cause or uncontrollable, all the accelerating fall to the bottom, or as unstoppable and accelerating ascent to the surface. In other words, the bubble helps the fish in a stationary position to maintain equilibrium, but the equilibrium is unstable.

This is the true role of the swimming bladder of fish, because we are talking about its relation to swimming; does it perform other functions in the body of the fish and what is unknown, so that this body is still mysterious. And only its hydrostatic role can be considered at present quite unknown.

Observations of anglers agree with this statement.

When fishing from a great depth, it happens that different fish half way released; but, contrary to expectation, it does not fall back into the depths from which it was extracted, but on the contrary, rapidly rises to the surface. In such fish, and notice sometimes that the bladder protrudes through the mouth.

Entertaining physics J. Perelman

 




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