🚀 go-pugleaf

>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.)
> 
>My question is: Why do they swim against the current?
> 
Hi Jim,
  I think that most fish respond simply to maintain their position, as you
suggest.  This mechanism in certain fish is a standard tool for evaluating
a fish's performance, since the fish or the environment can easily be
altered to affect performance.  For example, you could change the temperature
or oxygen concentration of the water and see how this affects the ability
of the fish to maintain swimming performance, either short-term burst
swimming or long-term swimming.  This is in fact a lab that we ran at 
University of Toronto for many years.  One time when I was instructing
the lab, we encountered a very 'smart' fish, who rather than swim rapidly 
against the current, discovered that there was a small thread in the
apparatus that he could grab and hold in his mouth.  He wrecked the
experiment: no matter how fast the current against him, he could always
maintain his position by darting up and holding onto the thread -- on the
other hand, we learned something quite interesting about fish intelligence!
-- Ron Coleman
   colemanr@garnet.berkeley.edu