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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.

barometric pressureFISHERMEN 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.

barometric pressureIn 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.

MidCurrent Fly Fishing
Dr. David Ross is a scientist emeritus at the Woods Hole Oceanographic Institution and the author of The Fisherman’s Ocean (Stackpole Books). He is also a regular columnist for Saltwater Fly Fishing Magazine. He can be contacted at © 2004 Dr. David A. Ross and Saltwater Fly Fishing Magazine.
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  • Durval Filho

    Excellent article Dr. Ross.
    Shouldn’t be an increase the hydrostatic pressure by about 0.9 atmospheres as result of three-foot rise in tide?

  • Jodi

    I’ve been fishing around the internet for an explanation of the effect of barometric pressure on fish so thanks for clearing that one up for me.

    It seems you’re explaining this specifically for the effects on saltwater fish though. Does barometric pressure have an effect on freshwater fish?

  • mike

    Just curious as to what your opining would be as to how freshwater fish may be affected, if at all

  • Mike

    I would like to point out that the article is presented with ocean fish in mind. Inland fishing is quite different and here is an example of how that may affect fish behavior. A bullet shot into a volume of water slows more rapidly than one shot into the air. If external pressure surrounding a volume of water is increased/decreased the density of the water should increase/decrease causing a bullet to slow more/less quickly than a volume of water with stabile pressure. If fresh water fish are more sensitive to changes in weather conditions they are sensitive to changes in atmospheric pressure. Sensing a potential unknown duration of hybernation caused by weather changes should cause fresh water fish to feed more aggressively. Think like a fish, if a low pressure weather change was to cover your state, town, home for an unknown period and you knew you might be hybernated in your basement for an unknown period without food because of the atmospheric condition, wouldn’t you, based upon experience and instinct, consume a bunch of food to preserve yourself first? Declining presuure spurs aggressive feeding, low pressure is slow fishing. I don’t have a degree in this stuff just some 40+ years of fishing and sinus that trigger when the bp changes.

  • David Douglass

    I have been charting data on an Excel Spreadsheet for ten years on every bass over four pounds. And atmospheric pressure is one of the 13 points of data for each fish. I have fished in Central Florida exclusively in all the popular lakes as a Bass Fishing Guide. Here is what my data indicates.
    When the pressure moves in any direction, right at the beginning of the movement, “The Fish Move” in order to adjust to the change. Simple bass biology; when bass move they use energy, they require food to sustain themselves and thus there is an increase in hunger. As the fish move, they come into contact with one another. Since all fish are ‘opprotunistic feeders’ they by moving, have the opprotunity to feed. Since hunger has increased due to adjustment, and there is more food sources presently moving, they feed. It is that simple.
    Now as to the weather situation, all the elements do play a part in where fish will ‘move’ to. Each body of water is different and therefore fish will, as they move to adjust, use the area that they have to their advantage. The type of cover, gradient, and food source, will determine the way they use an area.
    Bright conditions cause fish to stay close to protective cover. Variable cloudiness causes them to move to cover when bright light, and more away from cover when cloud cover occurs. An even mix of clouds and sun causes fish to move more often, which in turn has the same affect as pressure change.
    Atmospheric pressure change does NOT cause fish to feed, their need for food does. However anything that causes fish to move, causes fish to feed more often. It is that simple.
    So barometric change is beneficial and non-barometric change, is not. There is nothing more miserable than when atmospheric pressure does NOT change for extened periods, of days. Especially when water temperatures are low in the lower sixties or fifties. Temps in the seventies and lower eighties causes fish to need foor more often, as digestion speeds and metabolisms increase enough to cause fish to feed every other day or even every day.
    The angler’s “wise-tales” are exaggerations of a much more simple biological fact.