[chris (OP)]

Now this is a strange question, so please bear with me!

For some reason, while I was laying in bed last night I decided to start thinking about what would happen if I constructured a water manometer and put a fish into it.

Specifically, If I had an inverted, water-filled glass tube with a closed end uppermost (held vertically) and with the open end submerged in a bath of water, and I put a fish in the glass vertical tube - what would be the experience for the fish?

Would it have to work any harder to swim up the tube? What would it feel like?

[Colin2B]

Because the pressure at the top of the tube is the same as air pressure on the dish surface, the fish will feel only as if it were at the depth between top of water in tube and it’s position in the tube - equivalent to being at that same depth below the dish surface.

[Bill S]

Best make sure your fish is not a surface feeder if it's going to live in there.

[chris (OP)]

Because the pressure at the top of the tube is the same as air pressure on the dish surface, the fish will feel only as if it were at the depth between top of water in tube and it’s position in the tube - equivalent to being at that same depth below the dish surface.

So let's work this through: according to your post above, the fish swims downwards in the tube feeling increasing water pressure that starts off atmospheric at the top and increases as it goes down towards the dish at the bottom.

Then it gets to the bottom of the tube and swims out into the dish; there the pressure is also atmospheric pressure.

Clearly something's wrong here...

[Colin2B]

Clearly something's wrong here...

Yes, my misreading of your OP.
With the closed tube there will be a vacuum at the top end so the only pressure the fish will feel is that due to weight of water above it. Without the added pressure of atmosphere it will therefore feel les pressure than in an open tank at the same depth.
Question is whether the fish can balance it's swim bladder to maintain a stable depth, it not it would likely rise to the surface and be unable to sink down without swimming effort.
Interestingly this paper https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2121825/?page=1 suggests the swim bladder is not under fish control but gas can seep in and out over time. It also talks about problems encountered by deep sea fish in aquaria - maybe your fish would suffer from some of these??

[Bored chemist]

I rather suspect the fish would burst (assuming it has a swim bladder- some don't).
The swim bladder wouldn't be designed to contain gas under pressure.
As the fish ascended the pressure would drop and the bladder would expand making the fish more  buoyant, so it would ascend more...

Also, the fish would asphyxiate- the "vacuum" at the top of the tube would pull the oxygen out of solution.

Don't try this at home.

[chris (OP)]

This is a link direct to the PDF version of the paper cited by @Colin2B

This paper is from the good ol' days when scientists wrote much more clearly and the writing is easy to follow.

FISH-EXPLOSION.PNG (57.39 kB . 483x234 - viewed 3452 times)

It addresses a question I've often wondered about (above); but has this suggestion been confirmed or corroborated since?

[Colin2B]

Fascinating question
This pdf and some others I’ve seen suggest the swim bladder is under fish control (in those that have them) but I doubt it is instantaneous. https://www.saylor.org/site/wp-content/uploads/2010/11/BIO308-Wiki-Swim-Bladder.pdf

I had heard of barotrauma, and even rupturing, in fish brought back from extreme depths, but this is relevant describing barotrauma in fish brought from deep water to surface by anglers, so the depths here are not extreme https://www.stuff.co.nz/sport/boating/74819080/how-to-release-fish-caught-in-deep-water. These fish will have been brought to the surface very quickly.
See also http://www.fishingorangebeach.com/How-to-vent-reef-fish-the-right-way.htm


Your manometer tube will be 10m high so by the time the fish has got to 5m it will be at half atmospheric pressure and won’t be able to adjust quickly enough, so probably uncontrolled ascent. Just like a diver who can’t release air quickly enough from bcd.

[chris (OP)]

I wonder if we could do the experiment with a pretend fish, or even a recently-dead real fish (to avoid accusations of cruelty), to see the effect manifest.

So do we think it would, beyond a certain critical point, begin to accelerate up the water column and be incapable of swimming down again?

[Bored chemist]

It is important to recognise that the same would happen to a human diver if they found themselves in such a situation.
I can't see it happening in a natural environment, but something like the water pipes round a hydropower plant might give rise to such a situation.