How fish swim

Question
How do fish change depth in water? Besides using their swim bladder or is that only what they use?

Answer
Hi Tina,
  Actually it is a misconception that fish use their swim bladder to change depth.  Fish actively swim up or down to go to a different depth.  The swim bladder allows them to stay at that depth once they get there.  So for example, when a rockfish is at 100 feet of depth and wants to go to 200 feet of depth, he actively swims down.  Once he goes down, he has to adjust his swim bladder to keep from sinking. You see, as he goes down, the increased pressure outside his body shrinks the swim bladder, causing it to be less buoyant. He therefore has to pump more gas into the bladder to compensate. This makes the bladder a little more buoyant and allows him to stay steady at 200 feet.  If he did not add gas as he went down, the surrounding pressure on the bladder would make it less and less buoyant and instead of stopping at 200 feet, he would continue to sink at a faster and faster rate.   

  Exactly the opposite occurs when a fish wants to go up.  As the fish swims up from 200 feet to 100 feet, there is less outside pressure on the gas bladder. This causes it to expand, making it more buoyant. If he does not pull gas out of the bladder, instead of stopping at 100 feet, he will continue to ascend at a faster and faster rate, ultimately popping up like a cork at the surface (which would be very bad).  

 Basically, using the swim bladder to go up or down would be way too slow; instead the fish uses it to maintain buoyancy at whatever depth they choose to swim to.

-- Ron Coleman
  rcoleman@cichlidresearch.com
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