This may seem like a silly question at first. Fish live in water, so how could they possibly drown in it? But it turns out there are actually a few ways that fish can “drown” or suffocate from a lack of oxygen while surrounded by water.
How Do Fish Breathe?
To understand how fish can drown, we first need to understand how they breathe. Fish don’t have lungs like humans. Instead, they rely on their gills to extract oxygen from the water.
Gills are made up of filaments filled with blood vessels and capillaries. As water passes over the gills, dissolved oxygen diffuses into the bloodstream while carbon dioxide diffuses out. This countercurrent exchange allows fish to absorb 80-90% of the available oxygen from the water.
The gills must be constantly bathed in fresh, moving water in order for this gas exchange to take place. Stagnant water quickly becomes depleted of oxygen. Additionally, gills can be damaged by debris or pollution in the water, preventing proper oxygen uptake.
Reasons a Fish May Not Get Enough Oxygen
There are a few different reasons why a fish might not be able to get enough oxygen from the surrounding water:
High Water Temperatures
Warm water holds less dissolved oxygen than colder water. During hot summer months or heat waves, the water temperature can rise to dangerous levels for fish. Areas of stagnant, shallow water are especially prone to getting too hot and exceeding the oxygen carrying capacity.
If the water temperature gets above 77°F, oxygen levels can dip dangerously low. Fish living in these conditions may congregate at the mouth of tributaries where cooler, better oxygenated water enters the body of water.
Overcrowding
Too high of a fish population density can also lead to oxygen troubles. Each fish consumes oxygen as it breathes and releases carbon dioxide and ammonia as waste products. In aquaculture or other closed systems, the waste of many fish accumulating can quickly deteriorate water quality.
Overstocking ponds or insufficient aeration systems can cause low oxygen events that kill off large numbers of fish at once.
Pollution & Algal Blooms
Pollution from agricultural runoff, sewage, or industrial sources introduces nutrients like phosphorus and nitrogen into bodies of water. This sparks rapid, uncontrolled algae and phytoplankton growth called an algal bloom.
Algal blooms are a major source of oxygen loss for fish. At night and on cloudy days when photosynthesis shuts off, the algae stop producing oxygen and instead consume it as they respire. This creates wild swings in dissolved oxygen on a daily cycle.
As the bloom dies off and decaying matter accumulates, bacteria populations explode. Their decomposition of the organic matter further strips oxygen from the water, leading to hypoxic or anoxic conditions.
Density Stratification
During summer, the upper layer of water in ponds and lakes warms up and becomes less dense than the deeper cooler layers. This density stratification prevents mixing between the layers.
Without circulation, the bottom layer becomes isolated from atmospheric oxygen. Decaying organic material falling from above rapidly consumes the remaining oxygen supply.
Fish in these bodies of water must stay in the well-oxygenated top layer to survive the summer stagnation period. However, a sudden cold snap in fall can mix the layers before the fish migrate higher. The turnover causes a massive die-off as deep, oxygen starved water engulfs the fish.
Aquarium Mismanagement
Fish kept in aquariums and tanks are completely dependent on the equipment and caretaking to provide suitable water quality. Just a few mistakes can put aquarium fish at grave risk of oxygen deprivation.
Insufficient aeration, filters that are too small, and poor cleaning practices are some common pitfalls. Goldfish and other high waste species are especially prone to fouling their own water when kept in inadequate housing.
How Do Fish Drown?
So in what ways can a lack of oxygen actually kill fish? There are a few mechanisms:
Asphyxiation
Just like humans can asphyxiate from a lack of oxygen, so can fish if the water loses all its dissolved oxygen. Their gills are unable to extract any oxygen, and they suffocate.
During the day, fish may be able to survive very low oxygen levels by floating at the surface and gulping what little is available. But at night they often succumb. These overnight mass death events are common during algal blooms and de-stratification turnover events.
Organ Failure
Even before oxygen levels hit zero, critically low concentrations can inflict internal organ damage. The cells furthest from the gills like those in the heart and brain suffer first from lack of oxygen.
Prolonged low oxygen can lead to organ failure, loss of equilibrium, and eventually death.
Osmoregulatory Failure
The cells that line fish gills are sensitive not only to oxygen, but also proper water salinity. Hypoxia causes these cells to weaken and die.
With its osmoregulatory surface compromised, a fish can experience lethal changes in salt and water balance leading to failure of nerves and muscles.
Secondary Stressors & Infections
Low oxygen in itself may not directly kill a fish, but it causes substantial stress on their metabolism and immune function.
This compromised state makes the fish extremely vulnerable to secondary parasitic, bacterial, and fungal infections that can more easily turn fatal.
Warning Signs of Oxygen Deprivation
It’s not always obvious from the outside when fish are being deprived of oxygen. But there are a few key signs that fish are under oxygen stress:
– Gulping at the surface
– Heavy breathing/rapid gill movement
– Listlessness or loss of equilibrium
– Reduced feeding and growth
– Bottom-sitting away from the school
– Random spiraling or twitching
– Crowding near aerators, inlets, outlets, or tributaries
– Constantly shifting locations
If multiple individuals are displaying these symptoms plus water conditions are poor, an oxygen crash may be imminent.
Preventing Oxygen Deprivation in Fish
While fish evolved to handle short-term swings in oxygen, extreme or prolonged low levels can overwhelm their adaptations. Here are some measures fishkeepers can take to avoid suffocating their fish:
– Maintain water temperature below 77°F
– Avoid overcrowding fish
– Use appropriately sized filters and change media regularly
– Perform partial water changes to dilute waste
– Monitor ammonia, nitrite, and nitrate and keep levels low
– Check oxygen levels with a probe and maintain above 5 ppm
– Install bubblers, air pumps, powerheads or aerators for added water circulation
– Reduce or eliminate supplementary feeding to prevent organic waste buildup
– Shade outdoor ponds and tanks from excessive sun exposure
– Plant aquatic vegetation like hornwort, elodea, and duckweed to generate oxygen
– Use calcium salts or bubblers to prevent stratification in deep ponds
– Reduce fertilizer runoff that promotes algal blooms in natural waters
With proactive management and water quality vigilance, fish can be kept safe from suffocating in their own environment.
Can Fish Recover From Oxygen Deprivation?
If caught early, fish that are mildly deprived of oxygen can often fully recover with prompt intervention like a water change, increased aeration, or transfer to an oxygenated holding tank. Reducing crowding also gives them more personal space to capture the available oxygen.
However, severe or prolonged oxygen crashes will take a lethal toll before measures can be enacted. Additionally, the secondary effects like organ damage may be irreparable even if oxygen is restored.
For community ponds struck by summer kill or turnover events, restocking from outside sources is required to replenish the population. In aquariums, it’s best to remove and euthanize any extremely lethargic individuals that are unlikely to bounce back even if oxygen issues are corrected.
With quick action, many trapped fish can be saved and returned to good health. But waiting too long can allow irreversible damage to occur.
Unique Fish Adaptations for Low Oxygen
While we typically think of fish dying from insufficient oxygen, some unique species have evolved remarkable adaptations to survive in hypoxic habitats:
Air Breathing – Fish like bettas and gouramis can directly breathe oxygen from the air in addition to absorbing it through their gills. Some catfish and loaches can as well.
Skin Breathing – Eels and mudskippers can absorb oxygen directly across their skin when water levels drop.
High Hemoglobin – Blood of fish like carp and catfish binds oxygen very efficiently thanks to abundant hemoglobin proteins.
Reduced Metabolism – Slow moving fish like oscars and surgeonfish use oxygen sparingly by reducing their metabolic rate.
Piped Oxygen – Reedfish and snakeheads can gulp air at the surface and transfer it to their gills via a modified stomach.
Aerial Exposure Tolerance – Mangrove killifish and mudskippers can live for months out of water, slowing breathing until the tides return.
So while most fish would perish in O2 starved waters, these adaptations allow some species to carve out a niche in harsh shallows, swamps, and backwaters. Their unique skills help them thrive where other fish dare not go!
Conclusion
Fish need oxygen just as much as people do, though they access it through water rather than air. Despite living submerged, fish can effectively drown if the water lacks sufficient dissolved oxygen. Pollution, overcrowding, high heat, and stagnation are key factors that create low oxygen emergencies.
Paying close attention to water conditions and fish behavior are critical to prevent mortality events. With proper habitat management, most issues can be avoided before fish health declines. While not all species cope well with hypoxia, some unique adaptations allow a few fish to survive extreme low oxygen habitats.
So in the end, it is certainly possible for some fish to drown if the water they live in cannot provide enough oxygen – even though they are surrounded by it! Treating them to clean, well-aerated water allows fish to breathe easy and thrive.