pressure

[Dick Dastardly]

Steve to further confuse the issue you have touched on something that I have often wondered!

 

In fact its so confussing that Im having trouble trying to put it in to words! Please bear with me!

 

Its often sugested that different air pressure/the wether conditions it causes affects different species in different ways.I feel that this may not be as simple as it first seems.For example is the reason that silver fish fishing in the winter is generally poor in high pressure conditions more down to the fact that the same conditions are more suited for pike hunting (therefore the silvers keeping their heads down!) rather than the conditions directly affecting them? And why low pressure with its overcast skies lower light levels higher winds is better for silvers,not because the actual conditions are better for them to feed but "safer" as they are poorer for the pike? Generalising yes but do you see what Im getting at? The conditions only really affecting the predators and the prey fish merely reacting to the predators? After all a cyprinid would normally feed every day and for quite long periods as opposed to a predators shorter less frequent feeding cycle.

[johng]

It's interesting that nobody can show us one of the scientific studies mentioned by the two Steves that support the notion that fish can detect changes in atmospheric pressure. Surely such an unlikely result would be widely published?

 

The Cartesian diver works very well with big changes in atmospheric pressure, but if you want to know how it works in the real world, try it again without a top on the bottle.

 

It's a shame that the thread is changing direction into a general discussion on whether fish can detect or predict changes in weather (I certainly accept that they can ) It would be good to nail the pressure question and then we could figure out a more plausible explanation

 

 

 

The swimbladder is certainly subject to the effects of atmospheric pressure. I'm sure we all remember the Cartesian diver experiment from school.

[Steve Burke]

Yes, Budgie, I do indeed see what you mean. That's why I wrote "In fact, could this be a further reason why fish like roach feed well in rain? ". But you've explained it much better!

Edited October 17, 2006 by Steve Burke

[Dick Dastardly]

John even though it may well be the weather conditions that affect the behaviour I can see no other obvious reason that they could predict these changes other than by pressure.Just a case of elimination really.

 

Have you any theories as to how they can do this if it isnt by pressure?

 

I admit that my "human" senses are different from a fishs' but I know that my senses are far more receptive to change in water pressure the deeper I dive and my reaction to light less.Surely a creature who permenantly lives in such an enviroment would be far greater tuned to both?

 

I will recount the story of how several times Ive checked the barometric pressure when leaving to fish (only a matter of 50 or so yards away from the water in this case) the barometer showing no sign of rising.Not much expected on the Pike front.During the day the fish have totally unexpectedly come on the feed. Upon returning to the cottage the barometer is only just starting to show signs of rising and indeed continues to do so resulting in classis high pressure weather conditions.How did the fish know it was going to change?

[Steve Burke]

John, I'll see if I can track down the reference. It's something I've seen at least twice but I can't recall where. If it's in a book I've a decent chance of finding it, although as a dealer in fishing books I have more then a few to check! If I saw it in a scientific paper it could take a long time as we've just packed a lot of material away as we're planning to move very shortly.

 

Unfortunately there's insufficient exchange of information between the various styles of angling or between different countries. Wackett's little-known work is a good example of this.

 

There's even less exchange of information between scientists and anglers. Indeed there's often dangerously little exchange between scientists themselves as more and more specialisms develop.

 

Hopefully though the internet has begun to change the situation. In fact boards like these are a good example.

[Steve Walker]

It's interesting that nobody can show us one of the scientific studies mentioned by the two Steves that support the notion that fish can detect changes in atmospheric pressure. Surely such an unlikely result would be widely published?

 

John, I didn't mention any studies. I said that it's commonly stated that the weather loach is sensitive to changes in air pressure but that I couldn't find any references to primary literature showing that to be the case.

 

The diver won't work with an open top because you are then unable to alter the pressure in the bottle by squeezing it. It is subject only to atmospheric pressure. You could of course in principle use the same effect to make a barometer, using a set of divers with each calibrated to rise at a given pressure. In practice the gas volume is likely to be more affected by changes in temperature than in pressure. The point of mentioning the diver is that changes in air pressure are just as detectable under water as they are at the surface. Whether fish can detect them is another matter. Certainly there will be much larger swings in pressure as the fish moves up and down in the water column. Likewise, there are large swings in light intensity as we walk in and out of the shade, yet we can still tell when it's going dark.

[johng]

Budgie,

 

I have enjoyed this thread, it is certainly making me think!

 

Sight is the most likely sense that would work. In an earlier message I mentioned the 'Red sky at night' old wives tale, and I have found the following explanation:

 

'The origin of the saying is unknown, although a form of it appears in the bible (Matthew 16:2-3). It has some basis in science and is a fairly good predictor of-though no guarantee-of weather at the mid latitudes in the Northern Hemisphere, where storm systems generally follow the jet stream from west to east. A red sky in the morning indicates a sun rising in clear eastern skies casting its rays on storm clouds approaching from the west. At night the clear sight of the red setting sun would tell a sailor that no storms are to the west'

 

Fish have the observation skills and the long evolutionary timescale to make use of visual clues in the earths atmosphere. I am sure that there are many clues since changes in weather are preceded by visual changes.

 

Regards

 

 

John

 

 

Have you any theories as to how they can do this if it isnt by pressure?

[Dick Dastardly]

Yes Im quite prepared to acept that just because we cant visually see that the weather is going to change as early as it apears fish can tell that this may just be down to their sight being more sensitive to these signs than ours.Even though underwater we certainly feel pressure better than we can see light.

 

Trouble is that when we try to comprehend things like this we naturally relate to how our senses work and this might not be the same as a fishs.Not only due to the different physiology but also the different enviroments we live in.

 

A great example of this are the very big differences in the senses of taste and smell between us and fish due to the different enviroments.

 

Like Steve said the whole point of trying to find out why something is rather than just accepting it is that you can apply what youve learned to other things.Some times though it just gets to a stage though when you cant come up with any definate answers and must just rely on practical experience.

 

Another very good point is that the exchange of data on this sort of thing is very poor.A lot of it is because of the need or desire to know something.When Ive been doing several of my courses/workshops its amazed me (and some of my fellow students) the contradictions between some of the the acepted scientific facts re fresh water biology,fish behaviour etc and what I have seen (and also demonstrated) on the actual bankside. Mind you this works both ways and several things I have "learned" through actual angling experience have proved to be rubbish (In as much as the reasons why) when the actual science has been looked at.

[Steve Walker]

There is one paper which found that barometric pressure affected spawning activity in rainbow trout:

 

Peterson D. A. (1972) Barometric pressure and its effect on spawning activities of rainbow trout. Progressive fish-culturist 34: 110-112

 

I can't get my mitts on the original paper, so I'm summarising from other work quoting it, always dangerous.

 

Conversely, this paper (PDF) found that high or low pressure did not influence the movement upstream of spawning rainbow trout. They did find, however, evidence to suggest that changes in barometric pressure induced movement. I felt that their statistical treatment of this effect was lax. I would have liked to see a more rigorous consideration of time series analysis.

 

It has also occurred to me that barometric pressure has another fairly direct effect upon the fish's immediate environment; oxygen solubility. The effect is small, the difference in oxygen saturation between the highest and lowest ever recorded air pressures being 9.7mg/l to 12.1mg/l at 10C. Maybe enough to prompt a behavioural change though.

 

(using calculator at http://antoine.frostburg.edu/chem/senese/1...icting-DO.shtml )

[Vagabond]

the result was a theory that explained why barometric pressure affected fish. Briefly it was to do with the absorption of oxygen and carbon dioxide in the swim bladder.

 

Note that these are gases - and unlike water, gases are easily compressible. What's more the gases in the swimbladder are affected by atmospheric pressure.

 

Just found this thread whilst waiting for a plane to take me to my next fishing jaunt.

 

Yes, I think your Aussie researcher is nearly there Steve.

 

Although other posters have pointed out that a small change in water depth equates to an enormous change in atmospheric pressure, I think that is irrelevant. What I think happens is that changes in atmospheric pressure will affect the concentration of gases in the water as a whole (carbon dioxide perhaps more important than oxygen in this context ? ) and this is what the fish "sense".

 

You might argue that changes in dissolved gas concentrations caused by barometric changes are miniscule compared with changes caused by temperature or wind velocity. Nevertheless, if you go fishing often enough, as Budgie recommends you will, as he says, become aware that SOMETHING is happening.

 

What about fish reacting BEFORE the pressure change - as Budgie says ?

 

What the fish is "sensing", as Budgie points out, is an IMMINENT change to the water. Light levels changing as the low pressure area approaches - before the barometer drops. Not only fish "sense" such change - birds and insects do too. Probably not light changes alone, but a combination of changes are responsible - humidity, temperature etc etc.

 

After all, even we humans get a waterproof jacket ready when we see a big black cloud appear over the horizon! Just as we associate the black cloud with getting wet soon, so the fish "sense" that a good feeding time is about to arrive.

 

An analogy - dog gets excited when he hears Master's car in distance - perhaps a mile away. Master drives (say) a Landie Defender - why does dog ignore all other Landie Defenders with the same engine and same type of tyre? Obviously the dog is picking out minute differences in the engine note and tyre tread wear that are beyond human discrimination.

 

This is a bit rushed, and it may be a month before I am in front of a keyboard again - but these are my first thoughts on what may become a very interesting topic.