Why does sea fish not float up after an explosion: the physics of the process

Many people mistakenly believe that any explosion in the aquatic environment should lead to a massive ascent of fish due to damage to swim bladders or death of organisms. However, in reality, especially with powerful underwater sonograms or explosions of ammunition, a paradoxical picture is observed: fish often do not float to the surface, remaining in the water column or sinking to the bottom. This phenomenon contradicts ordinary logic, but is fully explained by the laws of hydrodynamics and biology.

The key factor here is not just the presence of a blast wave, but the complex interaction of shock pressure with gas cavities inside the fish's body. The swim bladder, which is the main hydrostatic organ in bony fish, under the influence of a sharp surge in pressure may not burst outward, but collapse or, conversely, remain intact, but lose its functionality. As a result, the fish loses control of buoyancy, but does not necessarily become lighter than water.

In addition, the very nature of the explosion and the depth of its epicenter play a decisive role in where the fish carcass goes after death. If the shock wave goes predominantly deeper or has a specific configuration, it can literally drive fish into the bottom layers. Understanding these processes is important not only for ichthyologists, but also for military experts assessing the consequences of using hydroacoustic systems or underwater ammunition.

The mechanism of barotrauma and the behavior of the swim bladder

The main reason why a fish may not surface is due to physics. barotrauma. The swim bladder is an elastic sac filled with gas (oxygen, nitrogen, carbon dioxide). With a sharp increase in external pressure caused by the shock wave of the explosion, the gas inside the bubble instantly compresses according to the Boyle-Mariotte law. If the pressure of the wave is high enough, the bubble can collapse, decreasing in volume several times.

At the moment of collapse, the total volume of the fish decreases, and its density increases. Since buoyancy directly depends on the ratio of the volume of displaced water and body mass, a sharp decrease in the volume of the gas bubble makes the fish heavier than the environment. Instead of floating, she begins to sink. This phenomenon is often observed during deep explosions, where the static pressure of the water is already high, and the additional pressure of the wave is critical.

However, the opposite effect is also possible, which also does not lead to ascent. If the wall of the bladder fails and bursts, the gas escapes into the tissues or abdominal cavity. There it can be quickly absorbed into the blood or dispersed into the surrounding water through damaged tissue without generating additional lift. In some cases, gases escape through the gills or mouth, which also deprives the fish of its “life jacket.”

• 💥 A sharp compression of gas in a bubble leads to an instant loss of buoyancy.
• 📉 The rupture of a bubble is often accompanied by the release of gas, rather than its accumulation.
• 🌊 The depth of the explosion determines the initial pressure, which affects the degree of compression.

⚠️ Attention: Not all fish have swim bladders. Sharks and rays (cartilaginous fish) rely on huge livers loaded with fat for buoyancy. The blast wave can damage their internal organs, causing internal bleeding, which will make the carcass heavier than water, and they are guaranteed to sink.

Effect of shock wave on muscle tone and orientation

A shock wave in water travels at tremendous speed and carries colossal energy. For fish caught in the blast zone, this is not just a mechanical impact, but also a powerful stress factor. Instant concussion nervous system leads to complete paralysis of the muscles, including those responsible for the functioning of the fins and orientation in space. The fish stops rowing and simply floats by inertia or obeys the currents of the water.

It is also important to take into account the direction of the force vector. The blast wave often has a complex structure, including not only frontal pressure, but also subsequent rarefaction phases. The fish can be “thrown” towards the bottom with a force exceeding its own buoyancy. If tissue rupture occurs at this moment, water begins to actively replace gases in the body cavities, making the body heavier.

There is also a "stun" effect, where the fish is alive but disoriented. In this state, she can instinctively swim down to calmer layers of water or to the bottom, escaping from the source of noise and vibration. Many fish species, under severe acoustic stress, tend to the bottom to take refuge in the bottom relief, which also explains the absence of floating individuals immediately after the event.

Muscle spasm caused by an electrical discharge (if the explosion was accompanied by it) or a powerful acoustic wave can pinch the gill covers or swim bladder, mechanically preventing expansion. This creates the effect of a “locked volume”, not allowing the fish to surface even theoretically.

The role of depth and hydrostatic pressure

The depth at which the explosion occurred is a critical parameter. At great depths hydrostatic pressure so large that the volume of gases in the swim bladder of the fish is initially minimal. Even if the bubble bursts, the amount of gas released will be negligible to create positive buoyancy at the surface. Gas released into tissues at depth will dissolve in body fluids much faster than in shallow water.

In addition, when ascending from a great depth (even if the fish began to ascend), the gas in the bubble would have to expand. But if tissues are damaged by an explosion, the gas comes out while still in the water column. As a result, having reached the surface, the fish no longer has a supply of gas to keep it afloat. It becomes a passive object whose density is close to or higher than that of water.

Temperature gradient also plays a role. At depth, the water is colder and denser. If the explosion occurs in the bottom layers, the cold water filling the damaged cavities of the fish has a higher density than the warm surface water. This creates additional negative buoyancy.

Differences between fish species and their buoyancy

The response to an explosion varies greatly depending on the species. Physophyllia fish (not having a swim bladder connected to the intestines) and physostome (having such a connection) react differently. Physostomes (for example, carp, salmon) have the ability to quickly release gas through the esophagus if the pressure changes sharply, but during an explosion this mechanism often does not have time to work or works the other way around - sucking in water.

Bottom-dwelling fish, such as flounder and gobies, initially have a density similar to, or even heavier than, water because they do not rely on buoyancy to survive. An explosion for them is simply mechanical destruction of tissue. They will not float up because they do not have a significant gas reservoir that could act as a float.

Pelagic fish (living in the water column), on the contrary, are maximally dependent on the swim bladder. They are the most vulnerable to barotrauma. However, as mentioned earlier, the destruction of their bladder more often leads to a loss of buoyancy, rather than to its hypertrophied acquisition. Their bodies become “wet sponges” soaked in water.

Type of fish	Presence of a bubble	Reaction to explosion	Probability of ascent
Bony fish (perch, cod)	Yes	Bladder rupture, barotrauma	Low (drowning)
Cartilaginous (shark, ray)	No	Liver damage, contusion	Very low
Bottom (flounder)	Reduced	Mechanical damage	Missing
deep sea	Yes (often)	Instant compression	Zero

⚠️ Attention: Deep-sea fish, when rising quickly (even if they surfaced), often explode themselves due to the expansion of gases, but with an external explosion they simply die on the spot, since they are not adapted to pressure changes.

Secondary factors: cavitation and temperature shock

An explosion in water gives rise to a phenomenon cavitation — formation and collapse of steam and gas bubbles. These microbubbles, collapsing, create shock waves of the second and third stages, which crush fish tissue at the molecular level. This turns the internal organs into a homogeneous mass, devoid of gas pockets. Without gas pockets there is nothing to float.

The temperature factor also (cannot be ignored). Explosives release enormous amounts of heat when detonated. Although water quickly extinguishes the fire, local heating can cause coagulation of proteins in the tissues of the fish, changing their density and structure. “Cooked” fish by the shock wave and heat becomes denser and heavier.

In addition, the blast wave causes severe turbulence. Whirlpools and vertical currents can hold fish in the water column for a long time, preventing it from surfacing even with residual buoyancy. Mechanical mixing of water masses is a powerful factor that hides the consequences of an explosion from an observer on the surface.

Ecological consequences and the future fate of the ichthyofauna

The absence of surfaced fish does not mean that the explosion passed without a trace. On the contrary, this may indicate a more serious environmental disaster. Fish remaining on the bottom or in the water column become prey for bottom predators and scavengers, or begin to rot, consuming oxygen in the bottom layers. This can lead to local freezes.

Toxic products explosion (nitrates, nitrites, heavy metals from ammunition shells) dissolve in water. Fish that survive barotrauma often die later from poisoning or secondary infections because their immune systems are suppressed by stress. Such individuals also rarely emerge, remaining in their habitats until complete decomposition.

The long-term impact on the ecosystem is the destruction of the food supply. The blast wave kills not only fish, but also plankton, eggs, and larvae. Restoring the population in the explosion zone can take decades, and the absence of surfaced fish is only the first, superficial sign of deep-seated disturbances.

The myth of "all the fish floating to the surface"

There is a common belief that after any strong explosion the fish should float up "belly up". This myth is based on observations of dynamite fishing (poaching), where charges of a certain power and depth are used, specially designed to stun and float. In the case of military or industrial explosions, the parameters are completely different, which leads to the opposite effect.

Comparative analysis: explosion vs dynamite fishing

It is important to distinguish between the consequences of an industrial/military explosion and poaching. When poaching, the charge is selected so as to stun the fish, but not tear it to shreds, and often the explosion occurs in shallow water, where surfacing is inevitable due to the shallow depth and specificity of the charge. In deep explosions the physics of the process is different.

In a military explosion, the goal is destruction, not preservation of biomass. The energy of the shock wave is orders of magnitude higher than that required to cause stunning. This leads to complete destruction of the structures that provide buoyancy. Therefore, the comparison with poaching is incorrect here.

It is also worth noting the difference in types of explosives. Some compositions produce more gas products, others produce more shock waves. For an underwater explosion, it is the shock wave that is important, which, as we found out, more often “sinks” than “raises”.

Frequently asked questions (FAQ)

Is it true that fish always float to the surface when the swim bladder becomes depressurized?

No, this is not always the case. If a bubble ruptures at depth, the gas quickly dissolves in the blood and tissues or escapes without creating lift. In addition, when the bubble collapses, the fish, on the contrary, drowns.

Can a fish survive an underwater explosion?

Yes, if it is on the periphery of the affected area. Fish have an amazing ability to regenerate. However, in a nearby explosion, only a few survive, and often they receive internal injuries incompatible with life, dying later.

Why are fish not visible on the surface when filming military exercises?

Because the bulk of dead or stunned fish either drown due to loss of buoyancy, or are carried away by currents, or are eaten by predators in the first minutes. It is almost impossible to visually notice individual fish in a large area of ​​water.

Does the salinity of the water affect the results?

Yes, salt water (sea) has a higher density than fresh water. Theoretically, it would be easier to float to the surface. However, the mechanism of barotrauma and tissue destruction is so powerful that the difference in water density becomes a secondary factor. The main mechanism—loss of gas volume—works the same way.