What Makes a Fish a Fish?

Brown trout

Brown trout

Compared to other animals it is usually pretty obvious when you see a fish.  If you look in a river, lake, or ocean and see something swimming around underwater without coming up to the surface to get air then it is most likely a fish.

But, do you know exactly what makes a fish a fish as opposed to some other animal?  It is not as easy as you think answer that question.

Here are the main things that make a fish a fish:

  • Fish live in water;
  • Fish have a backbone;
  • Fish breathe underwater using gills;
  • Almost all fish are cold- blooded;
  • All fish have fins and most have scales.

Fish breathe oxygen – Just like animals that live on land fish breathe oxygen. However, rather than take the oxygen out of the air, fish have gills (located on the side of their head) that take the oxygen out of the water. Fish take water in through their mouth then force the water out their gills.  As the water passes through the gills the oxygen that the fish need to breathe is removed.

Fish are cold blooded – This is unlike mammals, which are warm blooded, but similar to reptiles, insects, and amphibians.  This means that fish can’t maintain their own body temperature.  Instead, as the day gets hotter they become hot.  In order to cool off they either go deeper under water or find a shady spot.  For fish, this means that if the day is too hot or too cold they become sluggish and less active.

Here is a basic diagram of the main parts of a fish:

fish diagram good good

Did you know? Fish have a line that runs down each side of their body called a lateral line. This line allows them to sense sounds or vibrations in the water. Fish use this lateral line to avoid predators and find food.dog fish 8

What do fish eat?



Most fish are opportunistic feeders, which means they will eat whatever they can find. They will eat aquatic insects at the different phases in their lifecycles including nymphs, emergers, and adults. There are many different kinds of aquatic insects, for example mayflies, caddisflies, stoneflies, damselflies, and midges.

Fish will also eat grasshoppers, Insects, beetles, and small worms that fall in the water.  There are even a few very large fish that eat mice.

What Eat Fish?

There are many different kinds of animals that eat fish.  Birds of prey such as Osprey and Eagles eat trout.  Many mammals also eat fish, including river otters, minks, bears, and raccoons.  Even bigger fish will eat smaller fish (There is a reason that we all are familiar with the saying of the biggest fish in the pond – because it is true!).