[nexuist]

External pressure. It might be easier for you to survive at 130ft because you have a sturdy rib-cage and skull that keeps your internals together. Fish do not get this luxury. It might be entirely possible for a fish to swim down to 3000 meters, but I think evolution probably played a role in killing off all the fish who decided to risk it and swim that deep. As a result, "modern" fish probably experience more anxiety or similar emotions as they go deeper.

[LeifCarrotson]

This is not correct. Your rib cage and skull are scarcely able to contain the immense pressure at 130 ft. When SCUBA diving, you inhale air at the same pressure as the water at your current depth, there's no force on your rib cage. When free diving, your flexibility allows your organs to compress into the space previously occupied by your lungs.

A fish's swim bladder is full of air that's been extracted from the water. As long as they don't go up and down too fast, they'll be fine at whatever depth.

But the other side of the equation is the partial pressures of various gasses dissolved in the blood. Humans experience nitrogen narcosis and oxygen toxicity just from breathing regular air and need special mixtures, fish need special biology to cope.

[jimmyswimmy]

I think this is wrong but I am not a biologist. Fish do not contain air, which is the compressible fluid which limits our ability to go to great depths unprotected. Water is very incompressible, and is most of what fish are. So I don't think depth is the issue.

I would hazard a guess that it has to do with where food is.

[glofish]

Fish control their buoyancy via a swim bladder.

Suddenly bringing up a deep water fish would make it explode. every 33ft up the volume of air doubles.

Hence that rule of diving of never holding your breath and continuously letting the air out as you surface.

What I find incredible that a seal could dive that deep, the volume of air at that depth would be a tiny fraction of the volume of air at the surface. It is as if its lungs were completely collapsed and empty.

[koudelka]

> Suddenly bringing up a deep water fish would make it explode. every 33ft up the volume of air doubles.

I could be missing something, but I don't think this is quite correct.

Coming up from the first 33ft (10 meters) of depth, the pressure would go from two bars to one, or half, so the volume would double.

But any deeper, the ratio of pressure between the current depth and ten meters shallower isn't double, it's n/(n - 1).

[glofish]

I used my recollection from diving instruction and looks like I was off, every 33ft (10 meters) adds one ATM pressure, which, in turn, works as you said.

Every ten meters the volume goes down like so 1, 1/2, 1/3, 1/4, 1/5

PS. Fish could still explode tho

[taneq]

Correct, pressure is linear in depth.

[clort]

Yes the lungs are collapsed completely. A seal, as I understand it, has extremely haemoglobin rich blood and stores all its oxygen there for the dive.

[madhadron]

For diving mammals, most of the oxygen is carried in the muscles by myoglobin. It turns out that the myoglobin density in muscles is very precisely related with the diving ability of the mammal, and the the density is also very precisely correlated with the surface charge (and thus the exact sequence) of the myoglobin protein. There's an amazing paper about this that then uses myoglobin sequence to infer the diving abilities of extinct mammals.[1]

I'm not in general willing to be a guinea pig for genetic manipulation, but toothed whale myoglobin and being able to dive with empty lungs for half an hour or 45 minutes? Sign me up.

[1]: Mirceta, S., Signore, A. V., Burns, J. M., Cossins, A. R., Campbell, K. L., & Berenbrink, M. (2013). Evolution of mammalian diving capacity traced by myoglobin net surface charge. Science, 340(6138), 1234192–1234192. http://doi.org/10.1126/science.1234192

[notduncansmith]

I read that some seals can also force air into respiratory cavities in their skull, which keeps them from suffering decompression sickness.

[clort]

I am not sure how deep they would be able to go with this technique but it might be helpful in shallow water. I mean, to equalise at 20m they would need to generate 2bar (~30psi) of pressure. That is quite a lot! For reference, humans can normally push about 0.1bar (1-2psi) by blowing.. and their sinuses would need to withstand that pressure at the surface, before they dive.

Also, this has nothing to do with decompression sickness (aka 'the bends'). That is to do with dissolved gases in the blood which turn into bubbles when the pressure is lowered.

[notduncansmith]

Idk, I was just referring to Wikipedia where it says:

> Air is forced from the lungs during a dive and into the upper respiratory passages, where gases cannot easily be absorbed into the bloodstream. This helps protect the seal from the bends.

https://en.m.wikipedia.org/wiki/Earless_seal

[zaarn]

>Also, this has nothing to do with decompression sickness (aka 'the bends'). That is to do with dissolved gases in the blood which turn into bubbles when the pressure is lowered.

Dissolved gases dropping out of solution and forming bubbles is literally decompression sickness.

[glofish]

this is a good point, there is a lot more to diving really deep than storing a sufficient amount of oxygen. Basic physiological processes work differently at high pressures.

[glofish]

ah, fascinating! now it makes sense, it stores the air somewhere else!

[wtracy]

It's enough of a problem that there's special needles made specifically for puncturing a fish's swim bladder and relieving the pressure:

https://gillz-gear.com/blogs/news/39139841-smart-anglers-und...