Can fish drown? The Surprising Answer

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If you’ve ever peered into your aquarium and wondered “Can fish drown?” you’re in the right place. We wondered the same thing when we first started managing aquariums.

Read on to find out what fish can actually drown, how fish use oxygen, and how humans stack up in our breathing abilities.

Can fish drown?

Can fish really drown
Can fish really drown?

You might be surprised to know that yes, fish can drown, but it’s actually referred to as suffocation. While there is a large difference between the two for the purposes of this article we’ll refer to this occurrence as fish’s drowning, since that is the more commonly used term.

Drowning vs. Suffocation

According to the definition of drowning from Oxford languages, to “die through submersion in and inhalation of water” fish can not technically drown due to them already being submerged and inhaling water. However, fish can suffocate under water when there is not enough dissolved oxygen.

NOTE


Dissolve oxygen (DO) is the scientific measurement to determine how much oxygen is in the water to be used by living organisms at any given time.

Similar to atmospheric oxygen, dissolved oxygen is the usable form of oxygen that fish and other aquatic organisms use to function. Suffocation occurs when there is a severe deficiency of this dissolved oxygen, which can be caused by a multitude of issues we’ll cover later on in this article.

How can fish drown?

The most common way for fish to drown, or suffocate is if their gills are damaged or water can’t move across them. Like our lungs, gills are fish respiratory systems and allow oxygen to enter their body and move throughout their systems. When these gills are damaged by things like parasites or infections, or blocked by a thick coating of slime, fish lose the ability to breathe underwater, thus inducing fish suffocation.

Why can fish drown?

Much like humans, fish require oxygen for their inner organs to function. Unlike us, most fish can’t use atmospheric air for their oxygen needs, instead they have to use a form of oxygen found in the water.

Do fishes breathe?

Most fish don’t breathe through lungs, but through gills and respirate through a process called countercurrent gas exchange. This process is more efficient (explained in “How Fish Breathe!”) in collecting oxygen than the human respiratory system, and is the topic of much research.

Do fish need oxygen?

Fish need oxygen for their hearts, brain, cells, kidneys, livers, and other vital organs. Surprisingly, despite being cold blooded and needing less oxygen than warm blooded animals, fish gills are more effective at oxygen absorption than lungs. While we generally think of breathing air being mostly oxygen, our lungs fill with other substances such as nitrogen and carbon dioxide. On the other hand fish are able to only extract oxygen in the water increasing their efficiency.

Maybe you’re wondering how much dissolved oxygen fish actually are able to harvest. Using countercurrent breathing fish are able to harvest 2-8 parts per million oxygen from the water, whereas above the surface of the water the air is 210,000 parts per million oxygen. In other words every breath humans take contains 1/210,000 oxygen whereas fish breathe as much oxygen as ⅛ per every breath.

How they breathe in water

How do fish breathe
How do fish breathe?

During countercurrent gas exchange fish take water into their mouths, the water flows past the gills on either side of the fish’s head. This allows the water to exchange oxygen with the fish’s gills, and for carbon dioxide to exit back into the water. The gills are filled with tiny blood vessels that then take the oxygenated blood to vital organs.

The reason that the countercurrent method is so efficient at taking oxygen from the water is that it is taking into account an invisible gradient. More oxygen is available in the water before it flows past the fish, oxygen, like any element, wants to exist in equilibrium so it is pulled backwards into the aquarium fish despite the water movement.

Movement in the opposite direction allows oxygen in the water to exchange with that of the body. As the water moves past the fish backwards carbon dioxide will also be exchanged.

What fish can drown?

Lungfish are actually capable of drowning because these fish breathe air. Lungfish have what is called “labyrinth organs.” Labyrinth organs are a form of modified lungs that allow the fish to breathe. Lungfish also have gills so they are capable of harvesting oxygen in the water, but need to supplement it with oxygen from the air.

Lungfish used to be much more common and their reduction in numbers is partly due to wetland habitat destruction. An important aspect of labyrinth fish is being able to keep their organs moist even when not in the water. During dry spells they will actually burrow into substrate and enter into a state of quadri-hibernation. Though they do breathe air, lungfish are generally not considered to be air breathing animals, and are classified as typical fish.

An argument can also be made for fish drowning due to lack of swim bladder. Swim bladders (also called an air bladder) are an organ that help keep fish buoyant, those that don’t have them have to keep moving or risk not being able to pull oxygen from the water. If they don’t continue to move, or swim into a eutrophic zone the fish can suffocate.

FUN FACT

Betta fish are also labyrinth fish! Like the lungfish these fish breathe air, and will swim to the top of the aquarium to take a breath when there’s a lack of oxygen.

Still curious about the abnormality that is the labyrinth organ? Check out this research on how this organ could affect territorial and mating behaviors.

Does a fish drown in the air?

Fish at the surface
Fish at the surface

Above the surface of the water, fish suffocate due to their inability to pump water through their mouth to extract oxygen from the atmosphere. Additionally, most aquarium fish will experience gill arch collapse when taken out of the water thus leaving blood vessels unexposed and incapable of receiving enough oxygen.

When you see a fish rapidly opening and closing its mouth on land it is because it is trying to pull water into its mouth so it can pull oxygen from the water. Gills can only absorb dissolved oxygen that is pushed through them, thus the need for water.

How long does it take a fish to drown?

The rate at which fish suffocate depends on the species and their natural habitat. Freshwater fish species have more fragile gills and smaller bodies than saltwater fish, meaning they’ll likely suffocate quicker. On average, fish can survive for about 10 minutes out of water, and 3-4 minutes with no gill movement. Some saltwater species have been known to survive up to 20 minutes out of water, whereas freshwater fish that panic will only survive for 1 minute.

In the aquarium trade it is suggested that you keep your aquatic life in the water as much as possible. If you are transferring fish between tanks, be sure that you are prepared to move quickly and efficiently to reduce time spent outside of the water column.

How can fish drown in water

The most common reason for fish to drown in water is damage or infection to their gills. Fish are regularly exposed to maleficent bacteria populations and parasites that can cause a lack of oxygen.

Damage to fills can include a cut from a fish hook or a fight, or slime or ink being caught in gills also resulting in a lack of oxygen.

Cause of Drowning

Causes of drowning aren’t always due to environmental factors, sometimes it’s in the genetic code or due to unforeseen circumstances such as diseases.

Breed Trait

Betta Fish Tail
Betta Fish Tail

Selective breeding can potentially cause drowning dangers in most fish species. For example, betta fish have been selected to have long beautiful tails, these tails can cause the fish to be weighted down and incapable of swimming to the surface to gulp air.

Other fish, like members of the Cyprinidae family are more susceptible to gill flukes such as Dactogyrus, meaning that they potentially have a higher risk of drowning due to gill damage.

Parasites and Disease

There are a plethora of parasites, bacterial diseases and infections that can cause fish to drown. Some of the more common include;

  • Amoebic gill disease
  • Ich
  • Bacterial gill disease
  • Swim bladder disease/gas disorders

Low Oxygen

One of the most common reasons fish drown in aquariums is due to low oxygen levels in the water. Caused by a variety of reasons, low oxygen levels in your fish tank is easily solved, but can cause many problems if it goes unmanaged.

NOTE

For a fish to absorb oxygen, the oxygen level in the water must be higher than the oxygen level in its bloodstream.

Cause of Low Oxygen in Water

Low oxygen within the water column is generally a result of environmental conditions or overpopulation. Below, we explain some of the most common reasons for this condition.

Algae Blooms

Algae blooms, often caused by excess nutrients, can consume all of the usable oxygen as it dies. Depleted oxygen levels can be long term as blooms tend to change water chemistry and lead to your fish suffocating.

RECOMMENDATION

If this sounds like a familiar problem, check out our article on algae in your aquarium and what to do.

Overcrowding

Within your fish tank there is only so much oxygen to go around. If you place too many fish in a tank you’ll start to notice rapid gill movement, indicating labored breathing, even when they’re staying in place. This is an indication of low oxygen levels, and can easily be avoided.

When planning fish for your aquarium be sure to pay attention to space requirements for each species. These requirements aren’t just so your fish can comfortably swim around, but also to ensure that there is enough oxygen in the tank.

TIP

If you begin to notice labored breathing and suspect overcrowding, remove some of your fish. It is better to have two underpopulated fish tanks than one where many fish are dying.

High Water Temperatures

Water temperature and oxygen requirements become a hot topic, especially in a more tropical fish tank. Warm water holds less dissolved oxygen than cold water, if you’re raising your fish tank temperature for processes like breeding you might end up having too many fish for the oxygen levels present.

Warmer air temps (above 25℃) also make it more difficult for oxygenation to occur between water and atmospheric oxygen less efficiently. The ideal temperature range for dissolved oxygen saturation is between 18℃ and 25℃.

Poor Water Movement

Aquarium Water Bubbles
Aquarium Water Bubbles

Water flow and movement is how water becomes oxygenated from surface air. Stagnant water leads to low dissolved oxygen levels.

Normally, water flow comes from your filter, however depending on your tank size the filter might have too low of a setting. Lower parts of the water column usually experience the effects of low oxygen levels first.

RECOMMENDATION

If you suspect that low dissolved oxygen in your tank is due to flow, you should keep an eye on your bottom-dwelling fish.

Not Enough Surface Area

If the opening to your tank is too small there will not be enough aquarium surface area for air to water gas exchange. Most fish can’t exist in small bowls or vases due to their small opening. However, many fish with a labyrinth organ, like betta fish, can survive in these settings because of their ability to swim to the surface and breathe air. Most fish need a tank with a large surface area provided by the opening.

If you think your aquarium opening is too small we suggest an air pump or bubbler to ensure there is enough oxygen.

Too Much Plants

Live plants can potentially cause low oxygen level issues in your tank if there is not enough lighting available. Live plants will use light to photosynthesize, however in low lighting environments they will begin to consume too much oxygen in order to make up for low photosynthesis rates.

Less oxygen can affect your fish in the long term, however this is a fairly easy fix as the solution is just adding more light to your tank. Be careful, aquatic plants will thrive with the added light but it can sometimes result in algal blooms which can also lower your oxygen concentration.

Waste

Not only will waste affect your water parameters, it might also leave you with not enough oxygen for your fish. As waste decomposes it requires oxygen for the decomposition process. The longer waste sits in your aquarium the more oxygen it requires and the less available to your fish.

Chemicals

Some water additives used to treat water fish diseases or modify pH can decrease oxygen concentrations. For the duration of using any water additives you should increase water circulation and read all warnings.

FUN FACT

If a fish realizes that the tank is becoming low in oxygen it might try to jump. Check out this article on other reasons why fish jump.

Can a fish drown when unconscious?

Just like humans don’t stop breathing when you fall unconscious, fish have a natural response to breath even when unconscious.

However, fish with swim bladders who have to keep moving for water circulation around their gills could potentially drown if they fall unconscious.

Can a fish drown itself?

A fish cannot drown itself intentionally by holding its breath, or stopping gill movement. However, an argument could be made that fish can drown by swimming into dead zones or jumping out of water. These responses are either made unknowingly, or in an attempt to escape a situation suggesting that fish die because of a desire to live, not to consciously drown themselves.

Conclusion

In conclusion, fish can’t drown but suffocation is a common issue faced in the aquarium world. Luckily, if noticed in time this can be easily solved through a variety of methods. We hope this helped satisfy your curiosity regarding the respiratory system of our favorite fishy friends!

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