Most fish breathe through their gills, typically on either side of the pharynx.

These gills communicate with the outside environment through a series of paired slits known as gill slits.

Within the gills are filamentous tissue structures called gill filaments, which increase the surface area for gas exchange between the water and the fish's bloodstream. When a fish breathes, it continuously draws water in through its mouth and expels it through its gills.

As water flows over the gill filaments, oxygen is absorbed from the water and enters the fish's bloodstream, while carbon dioxide is released into the water.

However, despite this efficient mechanism for obtaining oxygen from water, fish can still suffocate under certain conditions. The primary reason for suffocation is insufficient oxygen availability in the water.

Oxygen has poor solubility in water compared to air; under standard atmospheric pressure at 20°C, only about 9 mg of oxygen can dissolve in a liter of water, approximately 1/35th of the oxygen content available in the air.

The solubility of oxygen in water is influenced by temperature. As water temperature rises, the solubility of oxygen decreases. For instance, at 0°C, water can dissolve approximately 14.6 mg of oxygen per liter, whereas at 35°C, this reduces to only 6.9 mg per liter.

Consequently, in warmer water temperatures, the amount of dissolved oxygen may become insufficient to meet the metabolic demands of fish for normal respiration. When the level of dissolved oxygen in the water falls below a critical threshold, fish may suffocate due to oxygen deprivation.

The suffocation point for most fish occurs when the dissolved oxygen level drops to around 1 mg/L. Fish, particularly those with high metabolic rates, such as actively swimming species, are particularly vulnerable to suffocation under these conditions.

Therefore, during hot summer, it is not uncommon to observe fish congregating near the water surface, gasping for air. Their open-mouthed behavior is an attempt to increase oxygen intake from the surface layer of the water, where oxygen levels may be higher.

In addition to natural fluctuations in oxygen levels, unforeseen circumstances can also lead to fish suffocation. For example, fish may become entangled in fishing nets, aquatic vegetation, or debris in the water.

In such situations, the fish's ability to open and close their gill covers may be compromised, preventing water from flowing over the gill filaments for oxygen exchange. As a result, the fish may slowly suffocate due to oxygen deprivation.

Recognizing that fish are not as accessible to breathe underwater as commonly perceived is crucial. The availability of dissolved oxygen in the aquatic environment directly impacts their survival. Therefore, efforts to protect water quality and reduce pollution can help mitigate the risk of fish suffocation.

By maintaining healthy aquatic ecosystems with adequate oxygen levels, we can ensure the well-being of fish populations and preserve biodiversity in our waterways.

Maintaining optimal oxygen levels in aquatic environments is essential for fish's well-being and marine ecosystems' overall health.

Oxygen is vital for the survival of various aquatic organisms, including invertebrates, amphibians, and plants. Adequate oxygen levels support aerobic respiration and metabolic processes crucial for aquatic ecosystems' growth, reproduction, and nutrient cycling.