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Thread: Article on Barotrauma

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    Member SkinnyD's Avatar
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    Default Article on Barotrauma

    Passing up shots on mergansers since 1992.


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    webmaster Michael Strahan's Avatar
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    The first paragraph contains a technical error. Strictly speaking, "barotrauma" is any result of rapid depressurization. It's an umbrella that covers several types of pressure-related issues. Swim bladder over-inflation is different from the bends.

    "The bends" is technically "decompression sickness", or "caisson disease" (it was first documented in construction workers who were excavating for bridge footings underwater many years ago). They were working in "caissons" (hence the name), or large containers that were pressurized to keep the water out. Upon returning to the surface, some of the men became ill with severe joint pain and some died. Excess nitrogen had built up in their systems over time under pressure, and when they ascended to the surface, the nitrogen came out of solution in their blood, reverting back to a gaseous form. These nitrogen bubbles often collect in the "slower" tissues of the body (joints and ligaments) and cause great pain. It's called "the bends", because the victim usually bends his joints to relieve the pain. I was a Divemaster in Hawaii before moving to Alaska many years ago, and we ran into this on occasion. The nitrogen comes from the air we breathe, which is a mix of approximately 80% nitrogen and 20% oxygen.

    Breath-hold divers are immune, because they are breathing air at the surface, not pressurized air at depth. This is also why whales and dolphins don't get the bends, even though they spend some time at depth.

    Another related condition is an embolism, which is when air breathed through a SCUBA tank comes out of solution in the victim's bloodstream. The resulting air bubbles travel through the bloodstream and become lodged in capillaries that are too small to allow the air bubbles to pass through. The tissue downstream of the blockage cannot receive oxygen and dies. Embolisms can occur anywhere capillaries are present, including the brain. In such cases, the result can be death. Another term, "pneumothorax" refers to a condition where the air sacs in your lungs (alveoli) rupture, allowing air bubbles into the bloodstream. This can happen if a person takes a deep breath from a SCUBA bottle and ascends. In the right conditions (full breath at 33', for example) a person could experience pneumothorax with an ascent of only four or five feet.

    What happens to fish is completely different. The swim bladder expands, forcing the stomach to protrude out of the mouth. Of course this only happens with fish that have a swim bladder (rockfish are the most common fish in Alaska with swim bladders that are affected in this way). Lingcod and halibut don't have swim bladders and are unaffected by this. Technically fish may get "the bends", but from our perspective, we are recompressing fish because the float bladder has over-expanded, preventing the fish from descending.

    All of these conditions are forms of barotrauma, but they are not all the same. All of them can be prevented by slow ascent, which allows the nitrogen to remain in solution until it dissipates naturally. That's why divers are trained to use charts that outline how long they can remain at different depths before they have to start doing decompression stops on the way to the surface.

    Nobody is going to reel up a rockfish slow enough to allow the nitrogen to remain in solution, so we use descenders after-the-fact to return the fish to a depth where the gas bubbles are reduced back into solution. Usually that means returning the fish back to the depth it was when it was caught, but even if you get the fish below 30', they can often swim back down on their own. 33' is sort of a "magic number", because that first 33 feet in the water column is the same weight per square inch (psi) as the weight of the earth's atmosphere at sea level. One square inch column of atmosphere at sea level weighs 14.7 pounds. That's why we call it one "atmosphere" of pressure. So while I sit here at this keyboard, near sea level, my body is under 14.7 pounds of pressure per square inch. A column of seawater measuring 1" x 1" x 33' weighs the same, therefore a fish at 33' below the surface is under two atmospheres of pressure. That first 33 feet offers the greatest change in pressure (a fully expanded float bladder will be half the size it was at the surface). At 66' it will be 1/3 the size, and so forth, for each 33' descended.

    We put a whole section together on this topic a while back, which goes into the causes and treatment, along with different kinds of release methods. You can find it AT THIS LINK.

    Some in our forums are better scientists than I, so I am sure some correction of my perspective might be in order here...

    -Mike
    Last edited by Michael Strahan; 01-06-2015 at 13:51.
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    Member SkinnyD's Avatar
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    Thank you, Mr. Strahan. Your write-up was more interesting than the NPR article.
    Passing up shots on mergansers since 1992.


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    Yup, they blew it in the first paragraph......They say a good reporter can report on anything, but as a person with science and journalism majors, I disagree and knowing something about science would lead you to ask the question "So, Dr. Soandso, is barotrauma the same thing as the bends?" Then this article is worth reading, but as it is it just started an urban legend that rockfish stomachs blow out when they "get the bends". Too bad because it's an important topic.

    Excellent information Mike, thanks, shoulda had you write the first one

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    webmaster Michael Strahan's Avatar
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    Quote Originally Posted by Catch It View Post
    Yup, they blew it in the first paragraph......They say a good reporter can report on anything, but as a person with science and journalism majors, I disagree and knowing something about science would lead you to ask the question "So, Dr. Soandso, is barotrauma the same thing as the bends?" Then this article is worth reading, but as it is it just started an urban legend that rockfish stomachs blow out when they "get the bends". Too bad because it's an important topic.

    Excellent information Mike, thanks, shoulda had you write the first one
    Yeah, I forgot about the stomach thing too. A lot of people say that the fish's stomach expands. Strictly speaking this is not true. It's being pushed out the fish's mouth (the path of least resistance) by the expanding float bladder. I guess that's a good thing, because the float bladder is ultra-thin and if it were exposed even to the fish's teeth, it would probably be punctured and the fish would die.

    It's almost like someone designed it that way, dontcha think? Hmmm...

    -Mike
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    Thanks Mike. Great explanation!

    Fish do get the bends, however. In the Columbia River, when spill from the dams is too high, the total dissolved gases in the water column exceeds the saturation limits. Any fish in the area will "inhale" gas super-saturated water, and the gases come out of solution in the fishes bloodstream and muscle tissue. They appear to be covered in bubbles, just under their skin. We call it gas bubble disease.

    But the solution is the same. Go deeper. At deeper depth, the higher pressure (e.g., depth compensation) prevents this from occurring.

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    webmaster Michael Strahan's Avatar
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    Quote Originally Posted by Cohoangler View Post
    Thanks Mike. Great explanation!

    Fish do get the bends, however. In the Columbia River, when spill from the dams is too high, the total dissolved gases in the water column exceeds the saturation limits. Any fish in the area will "inhale" gas super-saturated water, and the gases come out of solution in the fishes bloodstream and muscle tissue. They appear to be covered in bubbles, just under their skin. We call it gas bubble disease.

    But the solution is the same. Go deeper. At deeper depth, the higher pressure (e.g., depth compensation) prevents this from occurring.
    The skin issue you are talking about sounds similar to "subcutaneous emphysema"; another form of barotrauma. Strange though, because the fish are not under pressure, therefore they cannot absorb gasses faster than they can dissipate in those conditions. The reason barotrauma (in its various forms) occurs is because the body absorbs gasses under pressure, then experiences a rapid decompression, which forces the gasses out of solution and back to a gaseous form. I don't think super-oxygenated water could have that effect, because it is being absorbed and released under the same pressure. I think all that would happen is that the body (of the fish) would reach a saturation point and the fish would not need to breathe as often.

    I would speculate that there's another cause. I could be incorrect though.

    -Mike
    LOST CREEK COMPANY: Specializing in Alaska hunt consultation and planning for do-it-yourself hunts, fully outfitted hunts, and guided hunts.
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    webmaster Michael Strahan's Avatar
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    ADFG has an article on "Gas Bubble Disease" AT THIS LINK. Interesting reading; you learn something new every day!

    I stand corrected on my observations on the Columbia River post-

    -Mike
    LOST CREEK COMPANY: Specializing in Alaska hunt consultation and planning for do-it-yourself hunts, fully outfitted hunts, and guided hunts.
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