The swimming of a dead fish (2018) | Hacker News Reader

The swimming of a dead fish (2018)

fyfluiddynamics.com

193 pointslord_sudo79 comments

That fish looks alive until you know it isn’t. That’s incredible.

I’ve wondered many times how it’s possible that fish in deep frozen winter rivers can survive given that they have such limited food and need to expend energy to stay in one spot… But I suppose this simplifies the equation. They can rest where the current allows for this phenomenon.

Alternatively, I learned in fluid dynamics that there's a gradient of velocity of the water circulating through any pipe; closer to the walls/floor it's speed is effectively 0, closer to the center it's at the maximum speed (in an ideal pipe/laminar fluid, that is barring turbulences/rocks/etc). In the real world there's definitely pockets of water within the river were the water speed is insignificant/still.

https://accendoreliability.com/fluid-flows-pipes/

https://en.wikipedia.org/wiki/Laminar_flow

That’s a great observation too. I wonder if areas like these would cause fish to congregate, which might explain why animals like minks can readily find fish under the ice. There’s so much I don’t know, haha.

> I wonder if areas like these would cause fish to congregate

Any fisherman, especially one who fishes rivers, has a wealth of intuitive understanding of what kinds of water features lead to greater concentrations of fish and indeed a lot of it has to do with depth and turbulence. You'll often hear them talking about whether a patch of water looks "fishy" or not, and that often has to do with how still or turbulent the surface is (along with many other factors).

Incidentally I grew up fly fishing and that’s something I (hopefully) know a bit about! I’m just not sure about winter, under the ice. I’ve never fished an icy river (or an icy anything, come to think of it).

Fish can stay in one spot with little effort also because they have a special organ called the swim bladder. They can fill it up with gas and this allows them to maintain their position without actually swimming.

Doesn't the swim bladder just help them calibrate depth in the water?

I'm not an expert, I just had to learn the basics recently to get my fishing permit. But the explanation given was that its function is allowing the fish to float / hold their position and save energy. Carps even have two chambers since they are bottom feeders and they can tilt their bodies mouth down more easily this way.

Yes. What you are talking about is a fish keeping a fixed depth.

Having a swim bladder enables the fish to attain neutral buoyancy. That way the fish doesn't need to expand energy to keep itself from sinking or floating up. What the article is talking about is position keeping against the current in flowing water.

Swim bladder is good for up-down position keeping, the article's phenomenon is good for forward-backward position keeping.

Interestingly there is a way to use a swim bladder like construct to propel one forward. Underwater gliders do this, and the process is very energy efficient.

The way it works is that the glider uses its variable buoyancy device (an artificial swim bladder) to set a negative buoyancy and starts sinking. The wings of the glider turn this downward motion into forward speed. At the target depth the glider expands some energy from its batteries to set a positive buoyancy and keeps the forward momentum as it is ascending. Because of this they only need to use energy at two points (at the top and the bottom) in their saw-tooth like swim profile, and they can travel thousands of kilometers on a single charge.

https://en.wikipedia.org/wiki/Underwater_glider

I think so. Scuba divers use a BCD (buoyancy control device) that sounds similar - it's used to control buoyancy only.

I don't think the parent commenter is saying the swim bladder is all that matters. There is an "also" hidden in their sentence :)

Yes, although it probably does have some role in swimming efficiency due to obviating lift generation to get from a to be at the same depth.

OTOH especially for laterally compressed fishes, there is a metabolic cost of all the paired and medial fin movement required just to stay oriented in the water column, and they may be able to save more energy by deflating and sitting on the bottom.

The fish is not merely holding position--it's actually accelerating upstream!

Reminds me of this Tadashi video: [How do fish swim so quickly?] https://www.youtube.com/watch?v=wYDh5d9pfu8

Tadashi shows that fish turn drag into thrust by swapping the positions of the vortices they shed. Low effort, high output.

TFA takes this one step further--fish bodies are shaped to effortlessly harvest thrust from drag. No effort, free energy!

TIL: The first law of thermodynamics has an exception for dead fish.

(JK, don't cite me in physics class!)

Well, actually...

In this case, the fish isn't a closed system. It's harvesting energy from the moving water, which requires external energy from the sun to keep the water cycle going.

Reminds me of Derek's video about a vehicle which is powered against wind: https://youtube.com/watch?v=jyQwgBAaBag

His claim about a sailboat not being able to exceed windspeed sailing directly downwind is deceiving because it is definitely not the fastest point of sail. I believe this is done for dramatic effect to make Blackbird's feat sound more incredible. Sailing close hauled, almost directly into the wind, can achieve double wind speeds near effortlessly on low draft racing multihulls, and possibly on land with whatever those sailing carts are called. Of course, directly downwind is the only course for this experimental propellor craft, thus the only criteria.

But that's the entire point of Blackbird: to show that it is possible to sail directly downwind faster than the wind. It is not immediately obvious that this is possible.

Pretty counter intuitive that it takes less energy to swim upstream than to move forwards in still water!

I wonder if live fish swimming upriver ever do this - just relax and enjoy the ride - rather than putting any effort into it ?

It depends on turbulence, though. If it's a wide uniform flow, there are no vortices to exploit, so the intuitive assumption holds. You need the vortices to work against, and certain configurations will have negative impact, so a fish has to pay attention enough to surge or turn when needed. Throw a dead fish in a turbulent river and it's 100% going to end up far downstream, even though it might take a few upstream jaunts on the way.

Besides energy saving behaviour, the metabolic rate has been depressed due to the physicochemical effects of cold, typically a reduction of 2-3x per 10 degrees.

In terms of having enough energy, energy stores accrued during summer and autumn go a long way, but many fish are indeed a negative energy flux state over winter. In fact, over-winter starvation in the first year or three is a common ecological bottleneck, where even if fish are capable of reproducing and adults are fine and can survive the first year, the 0+ age class may not have had sufficient time to store enough energy before winter to survive until spring.

> That fish looks alive until you know it isn’t.

Schrodinger's fish.

With my former biologist hat on I would ask the authors to define "dead"

Lots of cells and tissues remain alive for months after death. It's easily possible that the nerves and muscles of this fish are alive enough to trigger basic autonomic swimming responses that are powering it upstream.

I want to see an artificial fish model that shows this behaviour.

Where does the energy for muscle flexing come from in a dead fish? Without a heart circulating blood (and ATP), My expectation would be that the muscles would quickly run out of power.

If you do that test where you apply electricity to a dead frog's leg to get it to kick, it will only work a few times before it's out of juice.

That swimming video is pretty short. Kicking a few times is all that you need.

The energy comes from stores in the cells and when those stores are depeleated the cell will replenish the stores by digesting itself.

"months after death"? Can you share a citation here?

I misremembered, after a quick Google it's days not months.

I reckon efficient swimming is too complex a process for spontaneous innervation to be responsible.

A fresh dead and pithed fish might flop, and might even flop several times in a row but that's about as far as it goes.

E.g. Just a change in temperature can make an efficiently swimming fish's red muscles’ duty cycles so maladaptive they are doing negative work, swimming requires really specific firing patterns.

Also consider — chances are they were moving an anesthetised fish into the experimental apparatus without realising it overdosed, in which case innervation would have been negligible.

> To extend this conclusion from the dead fish experiments to mechanical devices, and to demonstrate the applicability of these results in an engineering context, we also show that a flexibly mounted two-dimensional rigid foil, through fluid-induced motion only, is capable of producing net thrust using a negative mechanical power input, i.e. all the required mechanical power is extracted from the flow.

from the paper, which was published in 2006. and re a downstream comment, they euthanized the fish with a bath of anaesthetic, also in the paper.

I didn't see that part. If it works with a foil fish then it is possible to have net forward thrust only by extracting the energy from the flow of the water.

One of my favorite things about keeping aquarium fish has been watching the ways they interact with currents in the water. Move the filter output and the flow changes, and the fish adjust their movements around the tank accordingly. Different species have different shapes and different behaviors leading them to move through the water in a variety of ways. I think we still have a lot to learn from the efficiency of fish.

Dead fish swimming upstream .. Reminds me a bit of this: a wind-powered vehicle than can accelerate directly into the wind...

https://www.youtube.com/watch?v=yCsgoLc_fzI

Actually, it's going with the wind, but faster than the wind. Veritasium also has a video of the dead fish going upstream though.

At first I thought this was related to the famous Dead Salmon study [0]. The article turned out to be completely different, and certainly much more amazing.

[0] Neural correlates of interspecies perspective taking in the post-mortem Atlantic Salmon: An argument for multiple comparisons correction — http://www.prefrontal.org/files/posters/Bennett-Salmon-2009....

I can't find links at the moment (what's up with search engines these days?) but there's a similar phenomenon where dead whales move forward due to the action of waves and the natural shape and movement of their bodies and flukes. There were folks building a kind of boat propulsion system shaped like whale tails.

It's not just fish. Our hands naturally tend to conform to the objects we grasp without our needing to think about it. And robotic grasping with hands designed to do the same is much, much easier than the other way around.

Fishing in Alaska is wild when the millions of salmon go upstream to lay their eggs and die. They just continually swim upstream. I thought they just looked dead (some are partially decomposed with pieces of flesh falling off). I just don’t know how they stay balanced, pointing upstream, in that case.

The bear love it. I counted 43 in 4 days. Lots of close encounters but you don’t feel unsafe when there is so much food in the water. You can’t cross the river without accidentally kicking fish.

> but you don’t feel unsafe when there is so much food in the water

Yeah that matches most deaths of humans by brown and grizzly bears, but only most.

Not everything is about an animal’s energy expenditure. Just like a human they can just off you.

The place I almost died was when I fell into the water and almost got sucked into a 50 meter broad whirlpool. Alaska is radical.

There's a whole new kind of fishing lure called a swimbait, of which there is a sub-type that's just a jointed fish shape. You cast it out, and reel it in, and it waves like a flag, looking exactly like a swimming fish. The only thing powering it is hydrodynamics.

Not sure if I would call it "new". Swimbaits have been around for as long as I've been fishing (several decades).

However there is definitely way more advanced ones then when I first started fishing.

That's not a new idea at all, unless there's a specific development that I've missed. Jointed and/or flexible swimming lures have been around at least since I was a kid and I'm pretty sure the ones my dad used were old already then.

My favorite lures though were the frogs, which had rubber bristles on the back which would pulse when pulled through the water to simulate a frog's legs swimming.

Reminds me of this Veritasium video were a wind powered car accelerates into the direction of the wind https://www.youtube.com/watch?v=jyQwgBAaBag

> one of the questions that remains is how the trout is able to sense its surroundings well enough to intentionally take advantage of the effect

Doesn't a fish have the capability to try swimming at different frequencies to find a resonant one that minimizes effort (or maximizes forward motion)? Swimming might not be the right word, maybe flexibilities. I obviously don't do research in the field so wouldn't know if this is actually a dumb question.

You're absolute right and the process is sometimes referred to as 'tuning'. A living fish is generally looking to hold station and avoids the region closest to the obstacle which draws it forwards towards the obstacle. The process is conducted by the natural elasticity of the fishes tissues but the fish does activate muscles in reaction to perturbations in the flow. So, to conclude, I think that the fish is mainly actively putting effort to first find the 'sweet spot' and then keep itself there, the actually act of 'tuning' is typically considered a passive process and can be seen exhibited in any well-positioned flexible object e.g. a ribbon or string placed in a flow will oscillate at the vortex-shedding frequency. So in terms of the question they ask, it is relevant because the fish needs to be able to detect small perturbations in the approaching flow in order to react to them and maintain its position, the goal being to minimise active muscular contractions and maximise efficiency.

Thanks for the detailed explanation. So then the fish seems to actively do better than 'start from scratch' when it falls out of the sweet spot which is the mystery of how.

No problem. As displayed in the video of the dead fish, although initially well-placed and enjoying the neutral flow region/sweet spot, perturbations eventually force it forwards into the obstacle. The live fish has to be able to detect those perturbations effectively enough to avoid similar situations. If not, the fish would have to actively swim back into the sweet spot to begin enjoying the benefits again, as you say 'start from scratch'.

I bet nature is full of these resonant dynamics, that life exploits - including in neurodynamics.

Sometimes it feels it takes more energy to pedal my bike when there is no wind. Now I start to wonder if a little wind can create a pushing vortex and it is indeed easier to ride in a little breeze than when there is no wind at all.

Trout are also notorious for only striking food that has more calories than the energy required to strike. It seems obvious … but if you want to catch big trout you must use larger bait or make it … effortless for the fish.

Here's a video that explains the effect visually https://www.youtube.com/watch?v=wYDh5d9pfu8

Hm. Not a perpetual motion machine. And that fish appears to be tethered - when it turns sideways it re-orients just like its on a thread.

So maybe it expends very little effort, but not zero.

Define "effort". This fish is dead. How can it be spending any non-zero effort?

They're saying that because the fish is dead it has to be tethered, otherwise this wouldn't last for more than a few seconds before the fish becomes unstable and the phenomenon stops. For a living untethered fish, some level of energy would need to be expended to keep the fish stabilized instead of the tether.

Add to that, some energy required to swim upstream.

If I understand the article correctly, the energy expenditure by the fish itself is zero. From TFA:

> "Under just the right conditions, there’s actually a resonance between the vortices and the fish’s body that generates enough thrust to overcome the fish’s drag. This means the fish can actually swim upstream without expending any energy of its own!"

That's correct. I actually recieved a copy of this video about a decade ago from a grad student at the MIT tow tank - they were working on their Robotuna design and we were discussing Thunniform propulsion and this video came up. I've used this video in many talks because it's so cool.

In a nutshell, fish are undulating foils. When an oscillating or undulating foils is submerged in a fluid, a trailing Karmen Vortex Street (1) is generated, which is a set of spatially offset vortices. One of the cool things about that is that as the foil "swishes" from, say left to right, it extracts energy from the vortex - the foil can propel itself forward by essentially "pushing" off of the vortex of spinning fluid. The result is that the vortex rotation slows down (that's where the energy to propell forward primarily comes from).

Side note: This is in contrast to a single rotating propeller that leaves a lot of used energy in the swirling trailing wake. Modern profilers can use things like contrarotating propellers or boss cap fins to recover some of that energy.

In any event, for this "dead fish" experiment, the Karmen Vortex Street (KVS) is being generated by the obstacle in the flow in front of the fish - this is due to the low pressure zone directly behind the obstacle. The flexible foil begins to undulate in concert with these vortices. If you look at the figure of the KVS, the region in the center line of the KVS is actually creating a flow in a direction that is opposite that of the vortices themselves. In other words, there's a flow in the center that's effectively sucking the fish towards the rock.

Nothing magic, no free energy sadly, but definitely some cool science! You can absolutely use this knowledge to design energy harvesters (generators) from flows, like rivers or deep ocean currents.

1. https://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_vortex_street

Awesome! I really appreciate you taking the time to write this explanation. Very interesting.

Agreed, but the tether is just to keep the fish on track, it's still no "effort" on the part of the dead fish in this experiment (effort == energy spent by the dead fish). The article does mention a living fish will spend energy in order to find the sweet spot of the current, and then the water flow will do the rest.

Also, when the dead fish "swims forward" and hits the obstacle, the tether itself is playing no part. It's 100% the water flow and the shape/flexibility of the corpse.

Well even without the fish moving itself it wouldn't be a perpetual motion machine because the running water would be an input to the system. Agree though that a living, untethered fish would need to expend energy to stay balanced and in the right spot.

Reframe the system to the moving water. The fish is then swimming upstream.

All frames of reference are valid. So yes, perpetual motion.

what about sailing up wind?

Reframe the system to the moving water and put a watermill or a hydroelectric power plant in there in there - perpetual motion and extracting energy. A flowing river is not a closed system.

I'm pretty sure the tethering is for us. The (living) fish wouldn't care about maintaining a central position in a tank.

Yet it makes the fish appear to be making progress in the moving water. Sure it's wriggling side to side, very neat. But keeping up with the current? Sure, if it's held there by a wire.

Right, but notice the part of the video where it actually swims forward and creates slack in the wire.

I saw this before. It shows that a lot of behavior may be just mechanics. It explains the left in any case. :P

> The researchers came across this entirely by accident, and one of the questions that remains is how the trout is able to sense its surroundings well enough to intentionally take advantage of the effect.

Should have been:

The researchers came across this entirely by accident, and one of the questions that remains is how ignorant we are about some of the most basic aspects of the world.

Hard disagree. "How is the trout able to sense..." is a good question and answering it will advance our understanding. Navel-gazing about ignorance will not.

Why are you conflating what scientists are doing vs what civilization as a whole has always thought about the subject?

What a strange thing to say on a post about researchers working to dispel ignorance.

For millennia people thought fish swam upstream until literally today. If that’s not sheer ignorance I don’t know what is.

> until literally today

you know this research was published in 2006, and is based on earlier work (from the citations you can see papers from the early 1970s onwards) which suggests the ignorance might be more localised ;)

I believe GP meant 'figuratively' (a common modern use of 'literally,' disconcertingly) and could also be mixing unrelated idioms such as swim like a fish and swimming upstream, though it is commonly known salmon swim upstream, I'm not sure this fluid dynamics epiphany changes anything there. If it could technically be incorrect to say salmon swim upstream, how should it be phrased correctly? Salmon get sucked upstream?

link to paper https://www.people.fas.harvard.edu/~glauder/reprints_unzippe...

This is the best metaphor I have ever seen for how creepy Christianity can be (next to the Resurrection scene in Jesus Christ Superstar).

I'd like to build a boat from this design, a boat that moves against wind and current without engines, without sails, and without propulsion of any kind.

I'm very confused how this relates to christianity

Simply, it is related both through the ichthys, the fish symbol, and the Resurrection, or reanimation of the dead.

Flow state

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