pressure

[johng]

Just to take this thread a little further, it is worth having a look at the following site:

 

http://www.sengpielaudio.com/calculatorPressure.htm

 

I'm not very technical, but...

 

The total pressure exerted on the fish will be the sum of the water pressure above it and the air pressure above it

 

An atmosphere represents about 30 feet of water pressure and a millibar represents about 30 feet of air pressure

 

The conversion ratio between them is 0.0009869 or about one thousandth. I think.

 

So...If a fish has a sort of altimeter to measure changes in pressure and it moves up in the water column by one foot (a small change) that will be equivalent to the air pressure changing by (1 x 1000/30)=33 millibars (a massive change)

 

I suggest that it is only plausible to argue that fish can detect changes in air pressure if you assume that they are completely immobile for a long time, and that the water level does not change at all.

[ahammond]

Without wishing to be accused of rehashing old wives tales I can say with absolute honesty that as the weather starts to change I always have problems with an injured knee. So if I and no doubt millions of other people can feel a change in the weather and hence a change in pressure then surely so can fish and other animals.

Oddly I never feel anything when I fly and the pressure in an aircraft cabin is normaly set around the equivalent of ten thousand feet above sea level.

[Steve Burke]

Many years ago I accepted the received wisdom that water was incompressible and thus atmospheric pressure had no effect on fish. However my fishing experience showed me otherwise.

 

When theory and practice disagree my natural inclination is to suspect the theory. The subject intrigued me and so I did some research in the literature. Unfortunately this wasn't completed by the time we wrote "The Book of the Perch".

 

Subsequently though I came across an experiment, performed if I recall in France, on trout. Now these trout were indoors in laboratory conditions. The only variable was barometric pressure. The scientists found that in high pressure the trout swam high in their tank, but near the bottom in low pressure. This was despite the fact that the temperature and light etc hadn't changed.

 

Much later I read a fascinating book called "Studies of an Angler" by Wing Commander L.J.Wackett. This text is very little known, probably because it was published in 1950 in Australia. (I've also got a copy of his earlier "My Hobby is Trout Fishing" that is very rare, only 300 copies being printed).

 

Wackett took a lot of observations when fishing, and 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.

[Steve Walker]

What's more the gases in the swimbladder are affected by atmospheric pressure.

 

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

 

I've been trying to find a definitive study of weather loach behaviour. I can't find any primary literature, though the reaction of the loach to barometric pressure seems widely quoted.

[Steve Burke]

Steve - thanks very much for that link. I'm too old to remember that experiment, but it certainly makes sense.

 

DavyR - whilst I find Metcheck the most useful weather site I take little notice of it's rating for how good the fishing will be. This is because different fish prefer different weather conditions.

 

For instance, what's good for perch isn't usually good for pike. In general I prefer high, preferably rising, pressure for pike. For perch I prefer the opposite, although it may only be that low pressure often corresponds with low light - and light is the most important single factor in perch feeding.

 

I feel that pressure is just one of the many factors that determine whether fish will be feeding hard or not. It's rarely the most important, but I've no doubt that it does have an effect, often a major one. For instance, and not withstanding what I've just written, if the general type of weather has been constant for some time pike often seem to go on the feed immediately after the weather changes - whether the accompanying pressure change is up or down.

[MikeT]

I have a scientific and medical background, and I can verify the lay understanding that cold and damp weather (especially together) may exacerbate the ‘rheumatic’ conditions (like ahammond's dodgy knee). It is our common experience that joints ache and stiffen when the weather is dank and chilly. However, I am aware of no scientific study or research that explains the mechanism of this. Indeed, many of my colleagues pooh-pooh the whole idea, and write it off as psychosomatic; we feel miserable when the weather is miserable- and so notice our aches and pains more.

 

Maybe the business of fish behaviour being affected by weather conditions is likewise difficult to rationalise scientifically. Fish are immensely complex organisms, sensitive to all sorts of affects that we can hardly comprehend. Despite public assumption, doctors actually know very little about the functions of the human body, and I suppose this also applies to piscine biology and function (maybe even more so).

 

So, although it is most interesting to speculate about the reaction of fish to weather conditions, I am afraid we simply don’t have the scientific knowledge to understand it fully. Clearly, the function of fishes is ‘greater than the sum of its’ parts’- however we may wish to reduce it scientifically.

 

The science is lacking- we can therefore be justified in celebrating the art of fishing!

[Guest Rabbit]

Air pressure is I believe one of the important reasons why we either catch or not. Dull overcast weather is normally associated with low pressure, I would always feel more confident of catching most coarse species in these conditions.

The exception I suppose would be predators (sight feeders) who I believe resond better to lure/ bait in brighter conditions, although I am not entirely sure about the bait.... HELP Steve or Zedhead.

 

And to put another one in the pot, how many of us find that fish go off the feed in the rain?

[Dick Dastardly]

Exactly why I now acept that it affects fish behaviour rather than worry about the exact reasons of why and wherefore.

 

I dont see why people feel that just because a fish will be under different pressures at different depths that it wont be able to detect any changes? I would have thought that a fish would either instinctively or by experience know what pressure it should "feel" at any given depth so therefore notice any difference to this?

 

I would also imagine (based on a fishs anatomy/physiology) that it would be more "comfortable under certain pressure than others? Maybe why they move up and down in the water when the air pressure changes the water pressure?

 

I feel sure that the actual change in behaviour,especially feeding is more down to the actual weather conditions the pressure change brings.The different light values especially.But its how they know these conditions are changing thats interesting.

[Steve Burke]

I have a scientific and medical background, and I can verify the lay understanding that cold and damp weather (especially together) may exacerbate the ‘rheumatic’ conditions (like ahammond's dodgy knee). It is our common experience that joints ache and stiffen when the weather is dank and chilly. However, I am aware of no scientific study or research that explains the mechanism of this. Indeed, many of my colleagues pooh-pooh the whole idea, and write it off as psychosomatic; we feel miserable when the weather is miserable- and so notice our aches and pains more.

 

Maybe the business of fish behaviour being affected by weather conditions is likewise difficult to rationalise scientifically. Fish are immensely complex organisms, sensitive to all sorts of affects that we can hardly comprehend. Despite public assumption, doctors actually know very little about the functions of the human body, and I suppose this also applies to piscine biology and function (maybe even more so).

 

So, although it is most interesting to speculate about the reaction of fish to weather conditions, I am afraid we simply don’t have the scientific knowledge to understand it fully. Clearly, the function of fishes is ‘greater than the sum of its’ parts’- however we may wish to reduce it scientifically.

 

The science is lacking- we can therefore be justified in celebrating the art of fishing!

 

There's been considerable research done on the effect of weather on human health in Germany and Austria in particular. In fact this field of science even has its own name - biometeorology. I'd add that it's a lot less high profile in the UK.

 

Whilst I agree that angling is partly an art, the scientific side of fishing interests me greatly. I like to know why I've caught or not caught. I like to then apply what I've learned to improve my catches. As you say we don't understand it all. But it sure is fun trying to find out!

Edited October 17, 2006 by Steve Burke

[Steve Burke]

I feel sure that the actual change in behaviour,especially feeding is more down to the actual weather conditions the pressure change brings.The different light values especially.But its how they know these conditions are changing thats interesting.

 

I'm in full agreement here. The pressure change is important, but as I wrote earlier its only one of the many factors involved. For instance the low light that normally comes with the cloud a low presure system brings is probably more important in turning perch on than the pressure change itself. Or is it?

 

Rabbit, rain obviously affects rivers more than stillwaters, and current speed and the colour of the water are definitely factors that turn fish on or off the feed.

 

For most stillwater species I find that rain is usually a positive factor. But once again is it the low light that comes with the rain that's more important?

 

However, there's one fish that's an exception to rain usually being good, at least in my experience. I've very seldom caught sizeable pike when it's been raining. It has happened, but it's been rare. The only thing I can think of is that rain interferes with the pike's neuromast system and thus its ability to pick up vibrations.

 

In fact, could this be a further reason why fish like roach feed well in rain? Or is it the coloured water or the low light?

 

Zedhead, this may have been slow to get going but it's an excellent topic!