- Four goldfish of different sizes
- Large fish tank
- Small fish tank or bowl
- Small fish net
- Tank divider
- Four friends of different sizes and ages
- First, buy four goldfish of different sizes from the pet store. After the experiment, you will need to give the fish a home. If you can’t keep the fish, find a friend who would like to adopt them.
- Set up your large fish tank. Give the goldfish a few days to settle in.
- After the fish have adjusted to the new tank, use the net to gently move one fish into the smaller goldfish bowl. Let the fish swim around in the bowl for an hour or so to make sure it’s relaxed.
- Set the timer on your stopwatch, and record how many times you see the fish’s mouth open and gills flap in one minute. Do this three times. Take the three numbers, add them together, and divide them by three to get the average number of breaths per minute. Now gently move that fish back into the big tank on the other side of the divider. Do the same experiment for each of the four goldfish.
- Now bring in one of your friends. Get your friend to lie down on the floor so it’s easy to see her breathing. Do the same experiment, watching her body for signs of breathing for one minute. Count the number of times your friend breathes. Do the experiment three times, and then average the number of breaths like you did with the fish. Do the same experiment with each of the people you’ve asked to help you.
What happened? Did adults breathe more or less frequently than children? Did the fish breathe more or less frequently than the people? Why do you think that this is the case?
The fish will breathe more times per minute than the humans will. Smaller fish breathe more times per minute than bigger fish, and children breathe more times per minute than adults.
Oxygen is lovely stuff. It’s what people and other animals need to breathe. We need it in order for our cells to do what’s called aerobic respiration, to create energy to stay alive.
Humans breathe through our noses and our mouths. Humans are mammals, and the breath that we take makes its way to our mammal lungs. These lungs are adapted to take oxygen from the air. Tiny air sacs move this oxygen into our bodies.
A fish breathes oxygen as well, but if you take a fish out of water, it will die. This is because its body is used to breathing through gills. When a fish breathes, water moves into its mouth and past feathery gills that are full of blood vessels. As water passes over the gills, the oxygen that’s dissolved in the water moves into the fish’s blood and makes its way into its cells.
Water is made out of hydrogen and oxygen, so at first it seems like water has a lot of oxygen in it. However, fish aren’t breathing in that oxygen. They breathe the dissolved oxygen in the water – the oxygen that moves into the water as it tumbles over rocks, for example. Air has around twenty times more oxygen than water does, and water is also heavier than air, so it’s harder to move into the fish’s gills. Since there’s less oxygen in the water and the water is harder to move around, fish need to breathe more quickly.
Smaller fish and humans breathe faster than larger fish and humans because smaller fish and humans also have smaller lungs and gills, which have less capacity to store and process oxygen than bigger lungs and gills do. So smaller lungs and gills compensate for their size by increasing the breathing rate.
Many different factors change the amount of oxygen that an animal uses. One is an animal’s thermostat. Some animals have an internal thermostat: they make their own heat, and they use oxygen in this process. Others have a body temperature that changes according to the animal’s environment. Fish are usually ectothermic, which means that their body temperature can change depending on changes in their environment. Animals that are ectothermic don’t need to heat their bodies using a process that involves oxygen. This means that they need to breathe less. How often do you think a fish would breathe if it also needed oxygen to keep itself warm?