"Simple" question re:fish & currents

Ulf Andrick ura at strix.ruessel.sub.org
Fri Dec 31 12:29:16 EST 1993

Jim Webb (Jim.Webb at launchpad.unc.edu) wrote:
: I am on staff at a local science museum, and have been asked to modify an
: exhibit there. I have a frustratingly simple question about this piece
: that, so far, has not been answered to my satisfaction. (Actually, it's
: sparked some rather interesting discussions on animal behavior and
: instincutal vs. learned responses.) As there is no icthyologist authority
: on staff, and my colleagues tend to disagree with one another, I thought
: I'd pose the question here.
: In this particular exhibit, visitors press a button which starts a current
: flowing in a nearby aquarium. In this aquarium, tropical fish (namely,
: tinfoil barbs) group up and swim into the current, as close to the source
: of the current as possible. (Later, the motor resets and the visitor can
: press the other button and get the current flowing in the opposite
: direction; the fish swim towards that end.)
Some fish, I think salmon or so, are grown in circular tanks with
a rotating water current. They swim against it all time.
Such a reaction is even known from flying insects: Flight
experiments in which you want the insects to fly constantly are 
performed in wind channels. Such wind channels are also used for
experiments with birds. So, it seems to be a common trait of most 
organisms capable of active movement in a fluid by their own 
force that they move against the current.

This is certainly some reflex designed to keep the position and
posture of the animal stable.

: My question is: Why do they swim against the current?
: Obviously, if they didn't, they would get swept "downstream" (or knocked
: into the opposite end of the aquarium). However, is this behavior merely a
: defense mechanism (so they won't get hurt), or does it have greater
: significance?
: Is it difficult for them to breathe when swimming with the current? 

I don't think so, as fish do have an effective mechanism for circulating
water over the gills.

: What
: are the physiological reasons for this behavior? Is this merely a reaction
: to oncoming water, and a way to conserve energy? (One of my colleagues
: offered this analogy: when you're at the ocean and a wave comes, it's
: easiest to dive into the wave than to try to block it. Is this a fair
: comparison?) Perhaps this behavior is utilized in their natural environment
: (i.e., for catching food)?
: Or is it a combination of all these things and not the wonderful, elegantly
: simple explanation (tailored for a science exhibit) that I was hoping for?
I think the orientation reaction I mentioned above plays an important 
role here. It's possible that there is more behind it, as the fish
don't simply keep their position stable, but swim towards the source of
the current as you say. But on the other hand, you might have created an
unnatural situation, that fools the fish in some way, just as we
are deceived by an optical delusion. Perhaps they could also be
deceived optically as in the stirfish experiment:

How to stir a liquid with a fish (without any magnetic device!):

Put the liquid to be stirred into a transparent vessel of appropriate
size. Place the glass on a fixed table inside a revolving striped 
cylinder. Add a guppy to the liquid and turn the cylinder around as 
fast as you can.

              |                                   |
              |                                   |
              |                                   |      revolving
              |                                   |----- striped
              |    \                       /      |      cylinder
              |     |                      |      |
              |     |                      |      |
              |     |______________________|      |
              |     |        ___           |      |
              |     |    \  \   \          |      |
              |     |     \/   ( O         |      |
              |     |     /\      -)       |      |
              |     |    /  /---/       ---|------|---- liquid
              |     |                      |      |
              |     |                      |------|---- transparent
              |     +----------------------+      |     vessel
              |    ==========================     |
              |                 |                 |
              +---------------  |  ---------------+
                                |---------------------- fixed table

Here, we have a situation that the fish will probably never encounter in
its natural environment: The whole surroundings are spinning around.
Normally, it's the fish which moves relative to its environment and
not vice versa. To compensate for its apparent drift, the fish
follows the movement of the stripes.

But now, I have drifted off topic and wish a Happy New Year to

Ulf Andrick
ura at strix.ruessel.sub.org

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