Yes, fish do open their mouth like they are yawning but not for the same reason that a human yawns.  

The reason that they “yawn” is to do territorial, sexual, and aggressive displays.  They also do that yawning thing in order to cleanse their gills in order to absorb maximum oxygen from the water.  This is done to meet their high requirements for oxygen.

Yawning: What Is It?

There are many theories as to what yawning is.  One theory is that mammals and humans yawn to cool their brains down.  As the temperature increases, your brain becomes fatigued and it is thought that when you yawn, your overall body temperature cools down so you feel more relaxed. 

Yawning will also speed up your metabolism and increases the level of oxygen in your body.  With fish, it is just increasing their high oxygen requirements, showing territorial and sexual displays.  It does nothing to cool a fish’s brain down.

Do Fish Yawn?

Fish do not yawn, per se, but they do open their mouth and it looks like they are yawning.  What they are really doing is opening their mouth to let the water in and then they flush it through their gills.  This is called buccal pumping. 

So yes, fish do open their mouths and look as if they are yawning but not for the same reason that humans and mammals do.  Fish are ectotherm, which means they are cold-blooded so they do not need to cool their brain down as humans and mammals do.  Their body temperature is dependent on their external environment.

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yellow fish swimming

Why do Fish Open Their Mouth Wide Randomly?

There are many reasons why a fish opens their mouth ride randomly.  

Reaction to Sexual and Threat Display

A fish will also open their mouth widely as aggressive behavior towards an intruder.  They have to protect their territories in order to be able to live there safely.  It is also another way that they can get access to their mates so the fish can produce offspring. 

They use this behavior to keep their territory safe for intersexual activity, especially males.  The females will “yawn” to attract the males by making sounds and then directs them toward their mates.  Other animals like birds and reptiles do this same thing to signal to mate and showing signs of aggression.

1. To Cleanse Their Gills

As everyone knows, fish breathe through their gills.  When the water passes through their gills small unwanted particles can get stuck to the gills.  As a result, this can hinder a smooth gas exchange and makes it hard for them to breathe. 

If the gills continue to clog, the fish can be in trouble.  To cleanse their gills, the fish will open their mouth wide to let water enter their mouth and leave via their gills.  By doing this, all of the unwanted particles are flushed away so the fish can breathe normally.

It is a natural process and something that all fish do.  They do not appear restless when they do this.  They just look like they are yawning and stretching, much like humans do when they yawn.

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2. Environment is Oxygen-Poor

If you are watching your fish and notice that they seem more like they are gasping for air and not as much yawning, this is because of the lack of not having enough oxygen in the water.  They find it difficult to breathe so they are keeping their mouths open restlessly. 

The water can become oxygen-poor if it is heavily populated and/or it is stagnant.  In aquariums, you can increase the level of oxygen by continuously producing water currents through the air pumps and filters.

3. Filter Feeding

Almost all fish, ranging from the cartilage to the bony ones, filter feed.  This means that they are literally filtering from the water small food particles.  They will open their mouths wide to let the water come into their mouths. 

They will filter the food particles from the water and then eat them.  The water is then sent back out through their gills.  When they are doing this process, they will have their mouths open for a longer time than necessary for a yawn.

4. High Nitrate Level 

Nitrates are very dangerous for fish and can be toxic to them if it builds up.  Nitrates are the result of old food, plants, and fish waste in the water.   If your fish seem to be a bit restless and are opening their mouth to properly breathe, this can be due to the nitrates in the water.  It is difficult for fish to breathe if the water is high in nitrates.

fish open mouth

When to Worry About Your Fish Yawning

If your fish appear to be opening their mouth wide, it can be a sign that your fish is asking for help and is restless.  If they are continually doing this while coming to the surface, it is time to totally change the water.  They are showing this behavior because of the lack of absorbed oxygen in the water and they are not able to breathe right.  Make sure you do not ignore this sign of help and increase the level of oxygen in the water.  You can do this by installing an efficient air pump and filter, change the water, or even add some plants.  All of these can make it easier for your fish to breathe better.

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Conclusion

  • Most everyone knows that yawning seems to be contagious because when you see someone yawn, you automatically do too.  Fish yawning is not contagious.  Contagious yawning is only seen in reptiles, mammals, humans, and birds.   
  • There is one fish that will open their mouth and it is accompanied by a flexed body and raised spines.  This fish is called the stickleback fish.  When you watch them it does look like they are yawning and then stretching.
  • When fish “yawn” it is more of a cry for help, that they are having trouble breathing, and you need to do something to make their water more oxygenated.
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