Fish and decompression

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Liquid

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Scuba Instructor
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Atlit/Eilat/beer sheba israel.
I went through this forum, and found out it's the best place for a question that bugged me for a long time.

How can fish defend themselves from decompression?

A fish fast ascending from 60 feet when completly saturated, should have quite a problem when reaching 3 feet.

I had a talk about this with a friend and we came to several defferent ideas about it-

-Decompresion is not as dangerous for fish as for mammals, as their blood system works diferently.(my favorite).
-the amounts of N2 dissolved in the water are not enough to couse it (seems to me phisicaly not right, as it is suposed to have the same ratios as in air. if the oxigen is enough to sustain life, the N2 should pose a problem).
-the gill system absorbes only O2 (also dosent seem right to me).
-Fish tend to stay in a relatively restricted area of depths (my second favorite).

I guess it can also be a mixture of the first and last.
I also know of a certain kind of embolism that affects fish, and would like to know more about it.
 
The water gets its oxygen and nitrogen at the surface, where the pressure is one atmosphere. Taking water with one ATM of gasses in solution down deep still leaves the gasses disolved at one ATM as the liquid is incompressible. So fish, breathing water, don't build up the high gas tensions we do breathing compressed air.
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Embolism is another matter, however, and bony fish with swim bladders can embolize quite easily if brought up too fast. We see this frequently with snapper taken at depth and hauled to the surface with heavy tackle - the fish are uninjured from the hook, but catatonic from the overexpansion injury.
Rick
 
Damn me, I cant remeber the name of the law, but the amount of gas desolved in a liquid, in constant temperature, is in direct relation to the surounding pressure. That's what couses DCS, that's why at 30 meters, with surounding pressure of 4 bar, there should be 4 times the amount of dissolved gases. However, I do acknoledge it, that water will be saturated at sea-level, but wont be deeper. i am not sure of this issue, and would really like to know the answer here.
 
It's the pressure of the gasses in contact with the liquid. The sea is in contact with the air *at the surface* - so that's the pressure that counts. When you breath compressed gas at depth, your respiratory system is in contact with the gas at the surrounding pressure, so *that* is the pressure that counts. If you could somehow take air down and bubble it up from the bottom *then* you could get the increased tensions, but as the only contact is at the surface you can't. (there are exceptions in some lakes with volcanic activity, and in the immediate vicinity of some oceanic volcanic vents etc, but in the open ocean the total pressure of the dissolved gases is 1 ATM).
Hope that clarifies it for you.
Rick
 
Hi Liquid:

Your question regarding DCS in fish can provoke confusion. Others have wondered also.

The gas law for which you are searching is Dalton’s law of partial pressures. It states that the amount of nitrogen, for example, dissolved in a liquid is proportional to the pressure of the nitrogen gas in contact with the liquid. Pressure of the nitrogen gas in contact with the liquid is what determines the number of molecules in the liquid. The actual pressure of the water does not change the number of molecules of nitrogen that can dissolve. If a quart of seawater at the surface has 1,000,000 molecules of nitrogen dissolved in it, and that quart of water is now taken to a depth of 60 ft., it will still contain the same 1,000,000 molecules of nitrogen. The pressure of the water itself, the hydrostatic pressure, can neither create nor destroy molecules of nitrogen.

There does exist another situation, however, in which fish can have DCS problems with dissolved nitrogen. This occurs when nitrogen gas is carried to depths (20 to 30 ft.) in water in the form of gas bubbles. This happens when water cascades down the spillway of a dam and air bubbles, entrained in the water, dissolve at depth.

In this bubble-filled water, nitrogen is supersaturated, and the fish take nitrogen in through their gills, and their arterial blood becomes supersaturated. Air bubbles then form in the arterial blood. The fish soon die of an arterial gas embolism. It is termed "fish gas bubble disease."

Dr Deco


 
Rick, Doc- Thank you both.
Stupid me, forgot that part of the law... :bonk:
And I am suposed to be an instructor.. hm.

Doc-what you said there about the "Fish gas bubble disease" was FSCINATING. Can you tell me please where i can find some more on that subject?

One question further now- Ill try to put it to words, I hope I'll ask it correctly (I'm not native to englsih)-

When descending to VERY deep deapths, and I mean REALLY deep like 4KM and deeper, I belive the water do get a BIT denser, so the distance between the water mollecules get's a bit smaller. In result- Can water in those depths contain a little less disolved gasses?

Just wandered about it. I must improve my physics :wink:
 
Hello,

Forgot on thing on fish. The swim bladder. Fish uses the swim bladder (basicaly a gas filled sac) to regulate bouyancy.

Ed
 
I understand the previous posts with regard to fish, however what happens with mammals such as dolphins and Whales with really large lung capacities? When they dive to extreme depths the air in their lungs should be put under pressure equivalent to the surrounding water pressure causing nitrogen loading to take place. If that is true then don’t whales and dolphins have to decompress just like people?

Just think, we need a computer or complex tables to perform that kind of decompression!
 
Diverl,

With mammals the amount of niterogen stays exactly the same all the time, I mean, the same number of molecules. Becouse of that, there is not enough niterogen dissolved to couse DCS.

It goes the same for skin divers that reach extreme depths (over 100 meters) and do not get either DCS or narcosis. simply not enough niterogen to couse it.
 

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