Pressure Points: The Highs And Low’s Of Fishy Weather - The Fisherman

Pressure Points: The Highs And Low’s Of Fishy Weather

Storms caused by low pressure systems usually trigger fish to feed aggressively. While these conditions are not always fishable by boat, you can certainly fish them from the surf.

Use weather predictions to increase your odds of catching.

Some of the more knowledgeable anglers I’ve had the pleasure of fishing with over the years have been stubbornly set in their thoughts and theories concerning barometric pressure and the influence it has on fish behavior. No matter how the dice tumble, a barometer is far from a crystal ball for the angler; in the right hands though, a barometer can provide a good basis for how the day might unfold.

Of course, there are other factors such as wind direction and its velocity, tides, air and water temperatures and lunar phases that need to be blended together to plan out your day on the water. For some, this may sound too complicated or even, downright ridiculous, but for the savvy and well-seasoned angler, solid and reliable predictions are second nature. With that said, you may want to keep on reading if you wish to join that team of elite anglers that strategically plans for a successful day of fishing.

Pressure Situations

Barometric pressure is nothing more than a measurement that tells us the weight of the air over a certain point. It is instrumental in weather observations since its fluctuations indicate the movement of weather fronts and systems. Barometers consisting of liquid mercury are commonly used to measure surface air pressure. A mercury barometer is a glass tube with placed upside down in a container of mercury. Any change in pressure causes the mercury in the tube to either rise or fall.

For example, when the air pressure rises, it pushes the mercury higher. When the air pressure drops, so does the mercury level. The mercury is measured in millibars (mbar) or inches of mercury. A normal barometer reading is 30 inches at sea level. If a relatively strong high-pressure system moved into an area, the barometer would read between 30.50 and 30.70 inches. Conversely, if a strong low-pressure system were to move in, the barometer could dip as low as 27.25 inches of mercury.

The greater the difference in air pressures between the two competing systems, the greater the wind. Just the same, the greater the surface area covered by low pressure, the greater the size of the area of wind. Under high pressure, weather is typically fair, with sunny skies. Conversely, low pressure means precipitation, wind and inclement conditions prevail. A barometer reading of 28 inches or lower is generally associated with strong gusty winds from out of the east and northeast. When the pressure drops below 28 inches, you’re looking at anything from a classic nor’easter to a potential hurricane.

Strong fronts often bring severe weather with them and the fishing will often fire up right before or right after the front passes. Brian Dimento Photo

Aggression & Bellyaches

There are no definitive answers for how a rising or falling barometer affects fish. In fact, this is one influence that marine biologists and ichthyologists have studied least. However, most marine biologists and ichthyologists agree that fish sense pressure changes through their air bladders. Most also believe that fish without air bladders sense pressure change through their lateral lines. The lateral line system found in many fishes is the sensory unit of the species, which is sensitive to differences in water pressure. Even a minor barometric pressure change affects a fish’s swim bladder.

The general theories are that when the barometric pressure rises, it exerts pressure upon the bladder, thus affecting their behavior and appetite. When the barometric pressure drops, there is less pressure on the fish’s bladder, causing it to expand. When their bladders expand, fish feel the discomfort and don’t concern themselves with feeding as much. When the barometric pressure is steady and holding at around 30 inches for at least 24 to 48 hours, either after a high or low-pressure system has passed, the fish become stabilized and begin feeding aggressively. During this period, the weather is usually fair. Winds are light and may remain that way for several days or until the next low pressure system moves into the area.

Just before a low pressure system is about to move into an area, the fish can sense that the barometer is about to drop. Since they know that it won’t be long before their bellies begin to ache, they will often go on an intense feeding binge until the pressure drops.

That Gassy Feeling

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. Fish can tolerate hydrostatic pressure because they have a swim bladder containing a volume of gas, which they adjust to equal their environment. This gas enables most fish to comfortably make small and quick up or down movements in the water column.

Hydrostatic pressure in the ocean allows a fish to naturally change pressure around itself by making movements associated with feeding, swimming about, avoiding predators or trying to lose a hook. A small move can result in a relatively large pressure variation. 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.

Tides can alter hydrostatic pressure. Even a small 3-foot rise in the 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 6 hours 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 significant drop during a storm.

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 wave of about 4 to 6 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 influences hydrostatic pressure, but its effect is quite gradual. This gives fish considerable time to make any necessary adjustments.

Usually approaching storms align with a drop in the barometer, causing fish to feed. Sometimes an angler will experience their best trips during this window.

Fluctuational Influence

How much do fish respond to day-to-day fluctuations? Since fish sense pressure changes with their swim bladders, it must be noted that different fish species have different size bladders. Therefore, each species will sense barometric pressure differently, which may be why some fish will shut down when others may bite under the same conditions. Even slight changes in barometric pressure can cause considerable variations in fish behavior.

Consider that a typical value for barometric pressure is about 30 inches. Strong high pressure is approximately 30.70 inches. A powerful low can reach down to 28 inches or less. The difference between these two extremes (2.7 inches of barometric pressure) equals 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 turn the bite on or off magically. 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 are insignificant compared to what’s going on below the surface.

Paying close attention to the barometer can give away clues as to when a bite could be best.

Food For Thought

Fish are far more in tune with their environment than most anglers realize. Armed with a fantastic array of pressure-sensing systems, such as the lateral line (a sensory organ found in fish used to detect movement and vibration in water), they’re easily able to notice slight changes in the atmosphere. It also helps explain why a rising or falling barometer often coincides with fine fishing. Anglers should target prime feeding areas with aggressive presentations and save the finesse for later to capitalize on a change.

While much has been said about barometric pressure changes, it’s worth noting that some of the most reliable fishing action occurs when the pressure has been steady for several days or more. Extended periods of fair weather allow fish to find the best blend of ideal water temperature, favored living quarters, water quality and some other factors. Keeping a journal or fishing record will aid significantly in predictable feeding routines for years to come.

If the barometer has a dark side, a moving needle often foretells coming weather systems that adversely affect fishing. While pressure changes can trigger fish activity, low-pressure systems can shut it down like major cold fronts. Other reasons fish get lockjaw has more to do with the after-effects of the weather change than the actual air pressure. For example, the bite usually halts when a spring cold front cools down water temperatures in a shallow bay where bottom dwellers and gamefish are feeding. The fish may even evacuate the shallows and head for deep water nearby.

 Not all cold fronts are killjoys; however, a late summer cold front that drops the surface temperature a few degrees can reinvigorate sluggish fish during the dog days. Truth be told, plenty of other factors can cause a sweet bite to turn sour or, on the other side, trigger a feeding frenzy of epic proportions. Factors such as fishing pressure, recreational boat traffic and changes in water conditions can all affect the behavior and location of forage fish and larger predators.

In the end, the key to consistent catches is perseverance and insight into how barometric pressure and other mitigating factors affect fish behavior.


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