MadSci Network: Zoology
Query:

Re: Do fish ascend and descend the same way as submarines?

Date: Mon Mar 1 19:56:11 1999
Posted By: Rob Campbell, PhD Candidate, Biological Oceanography, University of British Columbia
Area of science: Zoology
ID: 920234819.Zo
Message:

Hi Pablo-

No, strictly speaking, fish do not adjust their depth the same way that submarines do (by taking in and pumping out volumes of water), but they do use the same mechanism- that is, by adjusting their buoyancy. They can create buoyancy (an upward force, or "lift") a couple of ways:

  • Dynamic Lift is created by creating lift through motion. A good analogy of this is when you stick your arm out of a fast moving car and allow it to "fly". Negatively bouyant fish (like sharks) must swim constantly, or they'll sink.
  • Static Lift is created by creating buoyancy through density differences- this is the same mechanism a submarine uses (but submarines use liquid water while fish use gas). Buoyancy inside the fish is done with the swim bladder and regulated a couple of ways:
  • 1. They can suck in air at the surface, and regulate their buoyancy it by "burping" it out, or
  • 2. They can create their own gas, usually oxygen, with a gas gland. By secreting oxygen, or reabsorbing it into their blood, fish alter their volume slightly, and thus their buoyancy.

    Fish with sealed swim bladders (called physoclistous swim bladders) have to be careful, because their swim bladder will burst if they rise too quickly- the reduction in water pressure with decreasing depth causes the swim bladder to expand, and so the pressure must be decreased by reabsorbing the gas. If the swim bladder isn't sealed, the fish can simply "burp" out the extra pressure.


    Hope that answers your question,
    Rob Campbell, MAD Scientist


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