Fish living in aquatic environments occasionally rise to the surface, appearing to gulp air. This behavior might seem unusual for creatures that primarily breathe underwater. It is a response to environmental conditions or an inherent biological adaptation. Understanding why fish come to the water’s surface provides insights into their physiology and habitat health.
How Fish Obtain Oxygen Underwater
Most fish rely on gills to extract dissolved oxygen from water. Gills are intricate respiratory organs located on either side of a fish’s head, protected by a bony plate called the operculum. Water enters through the fish’s mouth and is pumped over these gills.
The gills consist of filaments, supplied with capillaries. As water flows over these filaments, a highly efficient process called countercurrent exchange occurs. Oxygen from the water diffuses into the fish’s bloodstream, while carbon dioxide is released into the surrounding water.
When Gills Aren’t Enough: Low Oxygen Conditions
A common reason for fish to rise to the surface and gasp is a shortage of dissolved oxygen, known as hypoxia. This indicates the aquatic environment cannot provide enough oxygen through gills. Fish seek the surface because oxygen levels are often higher there, as atmospheric oxygen diffuses into the water.
Several factors contribute to low dissolved oxygen. Elevated water temperatures significantly reduce the water’s capacity to hold oxygen. For instance, warmer summer temperatures decrease dissolved oxygen while fish metabolic rates increase.
The decomposition of organic matter, such as dead plants, uneaten food, or pollution, consumes oxygen. Microbes involved in decomposition utilize dissolved oxygen, reducing its availability for fish.
Overpopulation of fish also leads to oxygen depletion, as more fish consume available oxygen faster than it can be replenished. A lack of water circulation or aeration prevents oxygen from adequately mixing into the water body. Poor water quality, including high levels of ammonia or nitrites, can also impair gill function, making it harder for fish to absorb oxygen.
Fish That Need to Breathe Air
Some fish have evolved specialized organs allowing them to breathe atmospheric air. These adaptations enable survival in environments where water oxygen levels are consistently low or fluctuate. Such fish fall into two categories: obligate air-breathers and facultative air-breathers.
Obligate air-breathers must periodically access atmospheric air to survive; they may drown if denied surface access, even in well-oxygenated water. Examples include lungfish, which possess lung-like organs. African and South American lungfish are obligate air-breathers, relying on their lungs for respiration and surfacing frequently, sometimes every 30 to 50 minutes.
Arapaima, a large South American fish, also has a highly vascularized gas bladder that functions as a lung. Some catfish species, like air-breathing catfish, have specialized suprabranchial organs.
Facultative air-breathers can breathe air but do not strictly need to if water oxygen levels are sufficient. They use air breathing as a supplemental strategy when water conditions deteriorate.
Gouramis, bettas, and other labyrinth fish possess a labyrinth organ, an intricate structure located above their gills. This organ allows them to absorb oxygen directly from the air. This adaptation is advantageous in stagnant or densely vegetated waters where dissolved oxygen can be low.
Implications of Surface Breathing
Observing fish coming to the water’s surface provides important clues about their health and environment. For fish species that typically breathe underwater, this behavior is a significant warning sign. It indicates that dissolved oxygen levels are too low, threatening their well-being.
When this behavior is observed in non-air-breathing species, immediate action is often required. Checking water parameters like dissolved oxygen, temperature, ammonia, and nitrite levels is a first step to identify the problem. Increasing water aeration through pumps, fountains, or waterfalls can improve oxygen levels.
Reducing the number of fish in an overcrowded system or performing partial water changes can alleviate stress. For obligate air-breathing species like lungfish or labyrinth fish, surfacing for air is a normal and necessary part of their respiration. Distinguishing between these scenarios is important for proper care and environmental management.