Science Questions

Do fish always swim anticlockwise in a round bowl? I’ve watched my sister’s fish and they always go anticlockwise. When they’re in an oblong tank they go in every single direction. Roy, Morden

We were first asked this question in the Naked Scientists Combating Climate Change Show.

Chris:  Izzy wrote to me to tell me why she thinks it is.  She says, ‘There’s a simple reason why fish swim in circles and not always in the same direction.  There’s a current in the bowl.  The caller said their fish swam anticlockwise that means that the current in the tank is going clockwise.  The fish swim into the current because they get ore oxygen forced into their mouths and through their gills.  If the current was going anticlockwise the fish would swim clockwise.  If they swam with the current so using less energy to travel they would actually end up worse off because they wouldn’t get enough oxygen to make up for the energy they’re using to swim at all.  It’s much more efficient for them to swim into the current.  This phenomenon is known as rheotaxis.

Helen:  I certainly see fish in the wild swimming into currents.  I always though it was so they could stay in the same place because you quite often see flickering away into the current.  Why is the current there in the tank in the first place?

Chris:  I wrote to her and said that.  And surely the fish swimming would make the water move in the same direction as them and she said, ‘My understanding is that when fish swim they push the water behind them in little eddies because nothing’s perfectly efficient in the natural world.  The eddies cause a spin and this makes the water travel backwards and also turn in the opposite direction to the fish’s motion.  That’s why you then set up a current in the opposite direction to the fish.

Helen:  Sounds a bit like fish and egg to me.  What came first?

Chris:  I think people have to put it into perspective.  Often people have bubblers in the tanks and a bubbler means the bubbles rise asymmetrically up the tank and this pushes the water round in circles.  I think she’s hit the nail on the head that the fish are going into the current if there is one.

February 2008