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Do Trouts Have Lungs?




As a marine biologist, one of the most common questions I get asked is, do trouts have lungs?

The short answer is no, trouts do not have lungs. Instead, they have gills that allow them to extract oxygen from the water.

However, the more in-depth answer to this question is quite fascinating and sheds light on the unique respiratory system of fish. In this blog post, I will explore the physiology of trouts, their gills, how they extract oxygen from water, and much more.

The Respiratory System of Trouts

Gills: The Key to Underwater Breathing

Trouts, like all fish, have gills that allow them to extract oxygen from water. These specialized organs are located on either side of the fish’s head, just behind the eyes. Gills are made up of thin, flat filaments that are covered in thousands of tiny, finger-like projections called lamellae. These lamellae increase the surface area of the gills, which is necessary for efficient gas exchange.

How Gills Work

Gills work by allowing water to flow over the gill filaments, which contain a dense network of blood vessels. As water flows over the gills, oxygen from the water diffuses into the fish’s bloodstream while carbon dioxide, a waste product of metabolism, diffuses out of the blood and into the water. This process is known as respiration, and it is essential for the fish’s survival.

To ensure that water continuously flows over the gills, trouts have a unique breathing mechanism. They open their mouths and take in water, which then flows through their gill chambers and over the gill filaments. As the fish closes its mouth, the water is forced out through the gill slits, ensuring that a constant flow of water is maintained over the gills.

Why Don’t Trouts Have Lungs?

Evolutionary Adaptations

The absence of lungs in trouts can be traced back to their evolutionary history. Fish are among the oldest living vertebrates, and their respiratory systems have evolved over millions of years to be perfectly adapted to their aquatic environment. Lungs, on the other hand, are a more recent evolutionary adaptation that first appeared in some fish species before giving rise to the terrestrial vertebrates, like reptiles, birds, and mammals.

Efficiency of Gills in Water

Gills are incredibly efficient at extracting oxygen from water, which is crucial for trouts and other fish species. Water contains far less oxygen than air, so fish need a highly effective respiratory system to survive. Gills provide this efficiency by having a large surface area for gas exchange, thanks to the countless lamellae on the gill filaments.

Additionally, the flow of water over the gills creates a constant supply of oxygen-rich water while flushing away carbon dioxide. This constant flow is essential for fish, as it allows them to maintain a high level of oxygen in their blood even in oxygen-poor environments.

Do Trouts Ever Need to Breathe Air?

Supplemental Air Breathing in Some Fish Species

While trouts do not have lungs and rely solely on their gills for respiration, some fish species have developed supplemental air-breathing abilities. These fish, known as facultative air breathers, possess modified swim bladders or other specialized structures that allow them to extract oxygen from the air when the oxygen levels in the water are too low.

Trouts in Low-Oxygen Environments

Trouts, however, do not have any specialized air-breathing structures. If they find themselves in an environment with low oxygen levels, they must rely on their gills’ efficiency to extract as much oxygen as possible from the water. In some cases, trouts may increase their ventilation rate, or the frequency with which they pump water over their gills, to help maintain their oxygen levels.

How Do Trouts Adapt to Different Oxygen Levels?

Changing Metabolic Rates

One way that trouts can adapt to different oxygen levels in their environment is by adjusting their metabolic rate. In areas with low oxygen levels, trouts may reduce their metabolism, which decreases their oxygen demand. This allows them to survive in less-than-ideal conditions by conserving energy and using less oxygen.

Hemoglobin Adaptations

Another adaptation that trouts have for dealing with varying oxygen levels is the presence of multiple forms of hemoglobin in their blood. Hemoglobin is the protein responsible for transporting oxygen from the gills to the rest of the body. Different forms of hemoglobin have different affinities for oxygen, allowing trouts to adapt to different oxygen levels by changing the relative amounts of each form in their blood.


In conclusion, trouts do not have lungs, but instead rely on their highly efficient gills to extract oxygen from water. Their respiratory system is perfectly adapted to their aquatic environment and has evolved over millions of years to provide them with the oxygen they need to survive. Here are some interesting facts about trouts and their respiratory system:

1. Trouts do not have lungs but instead possess gills for respiration.
2. Gills are made up of thin filaments covered in tiny projections called lamellae.
3. The large surface area of the gills allows for efficient gas exchange between the water and the fish’s blood.
4. Trouts maintain a constant flow of water over their gills by opening and closing their mouths.
5. Lungs first evolved in some fish species before giving rise to terrestrial vertebrates.
6. Gills are incredibly efficient at extracting oxygen from water, which is essential for fish survival.
7. Some fish species have developed supplemental air-breathing abilities, but trouts are not among them.
8. Trouts can adapt to different oxygen levels by adjusting their metabolic rate and altering the forms of hemoglobin in their blood.
9. Trouts may increase their ventilation rate in low-oxygen environments to maintain oxygen levels in their blood.
10. The respiratory system of trouts is a testament to the power of evolution and the incredible adaptability of life on Earth.


Why is Bell’s called two hearted?

Bell’s Two Hearted Ale is named after the Two Hearted River in Michigan’s Upper Peninsula, which is known for its excellent trout fishing. The beer is brewed with 100% Centennial hops, which are known for their floral and citrusy flavors, and are a perfect match for the river’s natural beauty.

Why do trout have 2 hearts?

Trout have two hearts because their circulatory system is adapted to living in cold, oxygen-rich water. The two-chambered heart pumps blood to the gills where it is oxygenated, and then to the rest of the body.

How long can trout breathe out of water?

Trout can generally survive out of water for a few minutes to a couple of hours, depending on various factors such as temperature, humidity, and the health of the fish. However, it is important to note that trout are aquatic animals and are not adapted to living out of water for extended periods of time.

How long till a fish dies out of water?

It depends on the species of fish and environmental conditions, but most fish will die within a few minutes to a few hours out of water.

Do salmon have 2 hearts?

No, salmon do not have 2 hearts. They have a single heart, like most other fish species.

Do trout have hearts?

Yes, trout have hearts. Like all fish, they have a two-chambered heart that pumps blood throughout their body.

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