Most fish can only survive for a very short period outside of water, as their physiology is adapted for an underwater existence. While some might last a few minutes, many perish in under a minute, with around 10 minutes being a common maximum for many freshwater species before severe damage occurs.
Immediate Challenges for Fish Out of Water
When removed from water, fish face immediate biological challenges. The primary reason most fish cannot survive long is the collapse of their gills. Gills are delicate structures designed to extract dissolved oxygen from water, which provides buoyancy and support for their intricate filaments. Without water’s support, these filaments stick together, drastically reducing the surface area available for gas exchange. This collapse prevents efficient oxygen absorption from the air, even though air contains more oxygen than water.
This inability to extract oxygen from the air leads to rapid suffocation. Fish gills are not equipped to function like lungs, and without constant water flow over their surfaces, they cannot facilitate oxygen diffusion into the bloodstream. It is similar to trying to breathe with a collapsed, dry sponge. The lack of oxygen causes cellular damage throughout the fish’s body, leading to death.
Another challenge for fish out of water is desiccation. Fish have thin, permeable skin, and their gills are highly exposed, leading to rapid moisture loss to the air. This dehydration can quickly lead to organ failure. The combination of suffocation and desiccation creates a physiological stress response.
This stress can result in the buildup of lactic acid and other metabolic disruptions, further compromising the fish’s ability to survive. Fish experience a cascade of physiological events when removed from their aquatic environment.
Factors Influencing Survival Time
While most fish have a limited time out of water, their exact survival duration varies based on several environmental and biological factors. Species differences play a role, as metabolic rates and gill structures vary, leading to marginal differences in how long a fish can last. Some species, like goldfish, might last up to ten minutes, but stress can shorten this to under a minute. Saltwater fish may generally survive slightly longer than freshwater species, sometimes up to 20 minutes if kept moist.
Temperature affects survival time. Colder temperatures can prolong survival by slowing the fish’s metabolism and reducing its oxygen demand. Conversely, warmer temperatures accelerate physiological decline by increasing metabolic rate and oxygen demand, while also reducing dissolved oxygen capacity in water.
Humidity and the moisture of the surface the fish is on also influence survival. High humidity or a wet surface can prevent rapid desiccation, offering a slight extension of survival time compared to dry conditions. If a fish lands on an absorbent surface, it will dry out and die much faster.
A fish’s size can also be a factor. Larger fish generally have a higher oxygen demand and may experience gill collapse more severely, potentially affecting their survival compared to small fish. Fish already stressed or injured before removal from water will likely have even shorter survival times. Handling fish carefully and returning them to water quickly is important to minimize stress and improve their chances of survival.
Remarkable Adaptations for Terrestrial Survival
Despite the general rule, some fish possess adaptations that allow them to survive for extended periods outside of water. These species have evolved alternative oxygen uptake mechanisms beyond their gills and strategies to prevent desiccation.
Lungfish are an example, known for their modified swim bladders that function as lungs. These fish can breathe air directly from the atmosphere, and some species, like the African lungfish, can enter a dormant state called estivation during droughts. During estivation, they burrow into mud, secrete a mucus cocoon to prevent drying out, and can survive for months or even years in this state, breaking down their own muscle tissue for nutrients.
Mudskippers are another group able to spend up to three-quarters of their lives on land. They breathe primarily through their skin and the lining of their mouth and throat, which must remain moist for cutaneous respiration. Their large gill chambers can also trap water to facilitate breathing when out of water, and their strong pectoral fins allow them to “walk” or “crutch” across muddy surfaces.
Walking catfish and similar air-breathing catfish species have specialized labyrinth organs or suprabranchial arborescent organs located above their gills. These accessory respiratory organs function much like a lung, allowing them to take up atmospheric oxygen. This adaptation enables them to survive out of water for up to 18 hours and even move between water bodies.
Eels, while primarily gill-breathers, can also absorb oxygen through their skin, especially when out of water. This ability allows them to make short overland migrations, particularly in moist conditions. Some species, like swamp eels, have reduced gills and rely more heavily on skin and mouth lining for air breathing, which helps them survive in low-oxygen environments.