Do Fish Get Decompression Sickness? What Happens When They're Pulled Up Fast

Fish live their entire lives at pressure without ever needing a dive computer, a safety stop, or a slow ascent. It's tempting to assume they're simply immune to decompression sickness. They're not. Fish get bent too, and understanding why reveals exactly what makes human decompression sickness a gas-breathing problem rather than a pressure problem.

Why Fish Mostly Avoid the Issue

Human decompression sickness happens because we breathe compressed gas from a tank, absorbing extra nitrogen into our tissues under pressure, then risk that nitrogen forming bubbles if we ascend faster than it can safely leave our bodies. Fish don't breathe gas at all. They extract dissolved oxygen directly from the surrounding water through their gills, so there's no tank, no compressed air, and critically, no equivalent mechanism pumping extra nitrogen into their bloodstream the way ours does.

That single difference removes most of the mechanism behind human DCS. A fish swimming from 30 metres to the surface isn't off-gassing a tank's worth of absorbed nitrogen, because it was never breathing a nitrogen-rich gas mixture under pressure in the first place.

The Part That Surprises People: Fish Do Absorb Some Nitrogen

Fish tissue still absorbs dissolved nitrogen from the water itself through the gills, gradually reaching equilibrium with ambient pressure the same way any tissue would. Given a slow, natural ascent, that nitrogen leaves via the gills without incident, the fish's own slow decompression, effectively.

The problem appears specifically during rapid forced ascent, most commonly when a fish is caught and hauled up quickly rather than allowed to rise naturally. Documented cases in the reef fish trade show fish pulled up from just 10 to 15 metres developing genuine decompression sickness: nitrogen bubbles forming in blood and tissue, exactly the same underlying mechanism as human DCS, just triggered by a completely different route of nitrogen exposure.

The Swim Bladder Is the Bigger, More Visible Problem

What actually causes most of the dramatic effects you'd associate with a fish yanked up quickly, bulging eyes, a stomach pushed out of the mouth, a fish that can't submerge again, isn't decompression sickness at all. It's the swim bladder, a gas-filled organ fish use to control buoyancy, expanding under Boyle's Law exactly the way an expanding lung would in a diver holding their breath on ascent. As pressure drops, the trapped gas inside the swim bladder expands, and if that happens faster than the fish can vent it, the organ over-expands and can cause severe, sometimes fatal barotrauma.

This is functionally the fish version of pulmonary barotrauma in human divers: a gas-filled organ expanding faster than the body can equalise it. Different plumbing, same physics.

Why Sharks and Whales Are Different Again

Sharks generally don't have swim bladders at all, relying instead on their liver's oil content and continuous swimming for buoyancy, which is part of why sharks can typically move through depth changes far more freely than bony fish without swim bladder-related barotrauma being a concern.

Marine mammals like whales and dolphins solve the problem an entirely different way. They're air-breathing, like humans, but they don't carry a continuous supply of compressed gas at depth. A whale takes a single breath at the surface, and its lungs and rib cage are structured to passively compress under pressure at depth, limiting how much nitrogen gets forced into blood and tissue in the first place. They also shift to storing oxygen in blood and muscle tissue via unusually high concentrations of haemoglobin and myoglobin, rather than depending on lung air the way a scuba diver does. It's a genuinely different strategy from both fish and human divers, closer in principle to an elite breath-hold free diver than to anyone breathing compressed gas.

Frequently Asked Questions

Can fish get decompression sickness?
Yes, though it's mostly seen in cases of rapid forced ascent, such as fish caught and hauled up quickly, rather than during their normal, gradual movement through the water column.

Why don't fish normally get the bends like human divers?
Fish extract oxygen directly from water through their gills rather than breathing compressed gas from a tank, which removes the main mechanism behind human decompression sickness: absorbing excess nitrogen from breathing pressurised air.

What actually happens to a fish pulled up quickly from depth?
Most visible damage comes from swim bladder barotrauma, the gas-filled buoyancy organ expanding faster than it can be vented, rather than decompression sickness itself, though both can occur together.

Do sharks get decompression sickness?
Generally not in the same way, since most sharks lack a swim bladder and rely on liver oil and swimming for buoyancy, removing one major source of barotrauma risk that affects bony fish.

How do whales avoid the bends despite diving to extreme depths?
Whales take a single breath at the surface rather than breathing continuously under pressure, and their lungs passively compress at depth. They also store oxygen primarily in blood and muscle tissue rather than relying on lung air, closer to a breath-hold diving strategy than a scuba diver's.