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Old 07-25-2004, 10:40 PM
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Default Barometric Pressure and How it Affects bass

The Pressure Myth

by Dr. David A. Ross
illustrations by Jonathan Milo
Does a changing barometer truly affect our fishing success? Let science answer that question.




FISHERMEN SOMETIMES have ideas or opinions about the marine environment that do not stand up to scientific scrutiny. For example, many anglers believe that changes in barometric pressure strongly influence fish behavior—most notably their willingness to cooperate with anglers. Some have even written that fish can detect a change in barometric pressure before it occurs. An interesting notion, perhaps, though in almost all instances it is incorrect.
A rise or fall in barometric pressure, such as with an approaching cold front, usually means a shift in the weather pattern. And it is the change in the weather, not any fluctuation in barometric pressure, that affects both the fish and the fishing. In fact, most saltwater species probably aren’t even aware of barometric variations.
Pressure, whether in the air or in the ocean, is expressed by scientists as units of “atmosphere.” One atmosphere is defined as the pressure caused by the weight of all the overlying air at sea level—or 14.7 pounds per square inch (psi). Atmospheric pressure is often called barometric pressure because it can be measured by the height of the mercury column in a barometer. Changes in barometric pressure, therefore, indicate capricious weather. In general, low-pressure systems bring unstable conditions, often with precipitation and clouds. A rising barometer means high-pressure is approaching, the harbinger of stable and clear skies.
How much do fish respond to these day-to-day fluctuations? Consider that a normal value for barometric pressure is about 30 inches. Strong high pressure is about 30.70 inches. A powerful low, such as during a hurricane, can reach down to 28 inches or less. The difference between these two extremes (2.7 inches of barometric pressure) is equal to about .09 atmospheres. The barometric pressure difference from a simple passing cold front is only about .06 atmospheres.
The rate of a falling barometer also tells us how fast a low-pressure storm is approaching. A slow-moving storm would have a dip of about .02 to .03 inches of barometric pressure per hour; a fast-moving storm will drop the barometer about 0.05 to 0.06 inches per hour.
Simply stated, barometric pressure does not change quickly enough to magically turn the bite on or off. It certainly is one of the ingredients in the overall weather process, but temperature, cloud cover, wind direction and speed, and humidity can also affect fishing conditions. More importantly, the rate and amount of change in barometric pressure is insignificant compared to what’s going on below the surface.
Beneath The Squeeze

Pressure in the ocean, called hydrostatic pressure, increases with depth due to the weight of the overlying water. Water is almost 800 times denser than air; thus, hydrostatic pressure increases much more rapidly than atmospheric pressure. If you swim or dive just a few feet below the water’s surface, you feel this rapid increase in pressure.
At a depth of just 32.8 feet in the ocean, the hydrostatic pressure is equal to the pressure from the entire weight of the earth’s atmosphere as measured in pounds per square inch. In other words, at 32.8 feet, the total pressure, due to the weight of both the atmosphere and the water, is two atmospheres. At 65.6 feet it’s 3 atmospheres, and so forth.
Fish can tolerate hydrostatic pressure because they have a swim bladder containing a volume of gas, which they adjust to equal their environment. This enables most fish to comfortably make small and quick up or down movements in the water column.

In the ocean, four main factors can change the hydrostatic pressure in the fish’s world. First, a fish naturally changes pressure around itself by making movements associated with feeding, swimming about, avoiding predators or trying to loose a hook. A small move can result in a relatively large pressure variation. For example, going up or down just 3.28 feet will decrease or increase the pressure on a fish by 1/10 of an atmosphere. One tenth of an atmosphere exceeds any reasonable change that might occur due to a fluctuation in barometric pressure. Equally important, when barometric pressure rises or falls, it can take more than a day to equal the change in hydrostatic pressure that a fish experiences in seconds during its normal up or down movements.
Second, tides can alter hydrostatic pressure. Assuming the fish stays in the same position, even a small three-foot rise in tide will increase the hydrostatic pressure by about 0.09 atmospheres. A low tide would decrease the hydrostatic pressure by a similar amount. Thus, within about a six-hour period from high to low tide, a fish would experience a fall of about .18 atmospheres of pressure. This is about twice what could be expected from the barometric pressure going through a major drop during a hurricane.
Third, waves make rapid and continuous changes in hydrostatic pressure. Two-foot waves, for example, will produce a change in pressure of about .06 atmospheres. This rapid change correlates to the period of the waves—about four to six seconds. Higher pressure comes when the crest passes; lower pressure occurs under the trough. When a storm approaches a coastal area, the waves, and the increase in hydrostatic pressure, will be considerably higher than during calm-weather periods.
The weight of the air itself is the fourth influence on hydrostatic pressure, but its effect is quite gradual. Barometric pressure associated with a major storm will dip (depending on the system’s rate of speed) by only .002 to .02 atmospheres per hour. This gives fish considerable time to make any necessary adjustments. When compared to the effects of the tide, waves, and normal movements of the fish in the water column, changes in hydrostatic pressure caused by barometric-pressure are trivial for saltwater fish. Even a dramatic change in the barometer will be lost to the everyday pressure changes experienced by fish under normal oceanographic conditions.
It’s a happy notion that one could simply consult the mercury column each morning to know whether it’s a better day for work or fishing, but it’s unlikely that barometric pressure alone can trigger the sudden bite that angling’s common wisdom often asserts.

Dr. David Ross is a scientist emeritus at the Woods Hole Oceanographic Institution and the author of The Fisherman’s Ocean (Stackpole Books).
This article first appeared in Saltwater Fly Fishing Magazine. 2004 Dr. David A. Ross and Saltwater Fly Fishing Magazine.
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Old 07-25-2004, 10:40 PM
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Default Barometric Pressure and How it Affects bass / ? The Pressure Myth

By Bill Dance

I honestly believe that the main reason largemouth bass are such a challenging gamefish is because they seem to be more influenced by environmental changes than other species. Nearly everything that happens in the air and in the water has an effect on most freshwater gamefish, especially largemouth bass. Normally two, or more fluctuating conditions are in effect at the same time - simply because they are all related - and everything that happens in the atmosphere eventually affects the watery world of fish. In all my years of fishing, one of the key things I've noticed (particularly about shallow water fishing) is the dramatic influence that barometric pressure has on fish. Ironically, this is the one influence that biologists, ichthyologists and serious fishermen have studied least. This could be because the required equipment is so delicate that it is difficult to take to the lake. And the whole pressure equation really gets complicated when you try to measure it against water pressure which also changes as fish change depth. It is a well-known fact that even minor barometric pressure changes affect a fish's swim bladder. This air-filled sac is to a fish what the inner ear is to humans. When the barometric pressure rises quickly, it exerts pressure upon the bladder, thus affecting the fishes equilibrium making it hard for the bass to maintain perfect balance. Naturally, this affects their behavior and appetite. I'm sure you've heard the term barometric pressure many, many times, but do you know what it actually means? Simply stated, it is the pressure of the atmosphere at a given point and time. And it's measured by a barometer, which is an instrument for determining the pressure of the atmosphere and predicting probable weather changes.
About 10 years ago, I started watching the barometer very closely. I had a cheap version that worked fairly well, but just to be sure, I would also check with the local weather service before and after every trip. This improved my understanding of how pressure fluctuations affect bass behavior. All serious bass fishermen know that the barometric pressure has a dramatic and immediate effect on a fish's personality and mood. Without question, it is an important element that influences fish behavior, especially shallow-water bass. Deep-water fish are not affected as much by major pressure changes and this is why they are more dependable on those days. Something to keep in mind is that barometric pressure doesn't change dramatically during a period of just a few hours unless a major storm is moving your way.
Like fish, other wildlife can predict the weather better than The Weather Channel or the National Weather Service. Mother Nature has given her creatures the uncanny ability to accurately anticipate an approaching weather system as well as knowing how long it will last. As a general rule, I concentrate my efforts in shallower water during falling pressure and in deeper water when it rises. Normally, barometric fluctuations are most important during late fall, winter, and early to mid-spring (because that is when fronts that frequently move from both the northwest and due north are strongest). Fronts that occur during the summer and early fall seem to move more from the southwest or west, and have less effect. Plus the recovery time is much quicker during these warmer periods of the year. A lot of folks think that the perfect day to be fishing is a beautiful day, when the sun is out, the sky is blue, and there's not a cloud to spoil the view. But let me tell you, most of the time these are the worst conditions for catching fish, because these conditions normally prevail just after a front has passed through. This is the type day when the pressure goes up and up - and the fish either go down or move into thick cover and seem to get lockjaw. When these conditions occur, you have to really slow down and use lures that you can work extremely slow (those that appear less likely to escape). Worms, grubs, or jig-and-pork combinations are good choices.
How many times have you heard fishermen say Wind out of the east, fish bite the least. Wind out of the west, fish bite the best. Or Wind out of the north, don't venture forth. And Winds out of the south, blows the bait in the fish's mouth. Well, first of all, the direction of the wind doesn't directly affect fishing. I've caught fish in wind of all directions except when it was blowing so hard I couldn't get out, or perhaps when it was too strong to fish a particular area. However, there is some truth about the effects of wind direction which actually has its roots in the barometric pressure. That's right, it deals with fronts. A strong brisk north or east wind will generally indicate a fast weather change; therefore, a drastic change in barometric pressure. Gusty south or west winds usually indicate a slow changing weather condition; thus minor changes in the pressure. So it's not really that the wind affects fish behavior. Instead, it's the barometric pressure that affects the wind and, therefore, fish behavior.
I think it would be safe to say that most fishermen can remember times when they were really whacking the fish and all of a sudden the wind changed direction and the fish stopped biting. This happens often, but again, it's not actually the wind that makes the difference. A dramatic shift in wind direction is the result of a frontal passage or change in barometric pressure. If I've said this once, I've said it a thousand times: the best time to go fishing is any time you can go. But if you can schedule your trips to coincide with the best weather forecast for current conditions, it will certainly pay you to do so because this is when the fish will be the most active.
Let me take a minute to explain what my experience has been with different ranges of pressure both the good and the bad. Where I live in west Tennessee, our normal pressure is 30 inches of mercury. So naturally, any reading below 30 is low and any above 30 is high. An optimum range would be 29.98 to 30.02. Without question, some of my best catches and biggest fish have come from mid-spring to early fall after several days of normal pressure were interrupted by an approaching front that caused the pressure to fall extremely fast (more than 10 to 15 points) in a few hours time. Normally during such a period, you will see bad weather moving your way, and the bass will go on a feeding frenzy. Unfortunately, these feeding sprees are short lived and to take maximum advantage of them you have to take the unsafe risk of possible high winds and lightning.
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Old 03-12-2010, 10:00 AM
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Default Re: Barometric Pressure and How it Affects bass / ? The Pressure Myth

Bump day

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Old 03-12-2010, 10:04 AM
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Default Re: Barometric Pressure and How it Affects bass / ? The Pressure Myth

The author postulates that barometric pressure is so minute its a waste of time to use that.

But you can infer from his article that hydrostatic pressure is part of the equation that makes a fish put the feed bag on. And a falling barometer will trigger these events.

Front approaching - white water. bait everywhere.
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