Why can fish withstand high water pressure but not humans?

Like me, you may have found it difficult to understand why humans have to go down into the depths of the oceans in super-strong underwater capsules because the pressure is so great down there that it would crush them in a second while deep sea fish happily swim around at depths of 10,000 feet and more. It seemed very incongruous to me so I decided to figure out why this discrepancy exists. It is all to do with the balance or equilibrium between what is inside us and what is outside us – a pressure differential. Although humans are made up of about 60% water, there is a lot of air inside us too including in our lungs and sinuses and so on. Even inside the bloodstream there is air.

Yokozuna Slickhead deep sea fish or sumo fish at 2 km down off Japan
Yokozuna Slickhead deep sea fish or sumo fish at 2 km down off Japan. See base of page for image credit.

It would be the air inside us which would be compressed in an instant by the huge column of water above us in the depths of the oceans. Fish do not have air that is compressible but water inside their bodies which is not compressible. Therefore, their bodies are in balance with the water above their bodies. They don’t have air pockets that can be compressed such as lungs because they breathe through their gills rather than through lungs. Fish are largely composed of water which means the pressure differential remains in balance.

On the surface of the planet at sea level we are subject to air pressure at 1 atm (one atmosphere). One atmosphere is the amount of pressure exerted by one-inch column of air as tall as the atmosphere which weighs 14.7 pounds.

At the deepest point of the ocean which is apparently just under 11,000 m deep, the water pressure above you would subject you to a pressure which is 1100 times greater what you experience in the open air. Without being inside a specialist deep sea diving capsule (submarine) your body would be crushed due to the thousands of pounds pressure per square inch. Ear drums would rupture and lungs would fill with blood and collapse.

The reason why some whales which have lungs can swim around 10,000 feet below the surface is because they collapse their lungs and remove the air from their bodies therefore maintaining a pressure equilibrium.

Scientists from the Japan Agency for Marine-Earth Science and Technology have discovered a giant sumo fish lurking in the depths and it is the biggest that they’ve ever found. It was found more than 2 km down on the seabed off Japan. It is called the Yokozuna Slickhead. It was 2.5 m long and caught by a camera next to mackerel used as bait by the researchers. The previous largest sumo fish was less than 1.4 m long.

The picture on this page is of a sumo fish. The agency found last year that the Pacific sleeper sharks in the area are the world’s slowest swimmers for their size at 0.9 km/h.

Photo credit: Scientists of the study: Detection of the Largest Deep-Sea-Endemic Teleost Fish at Depths of Over 2,000 m Through a Combination of eDNA Metabarcoding and Baited Camera Observations.

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