Pretty much everyone knows that the weather affects fishing. Warm and cold fronts heat and cool the water, suppressing or encouraging feeding behavior depending on the season in which they arrive.
For instance, a warm front in spring, accompanied by a lot of sun, can make the water warmer than usual, turning fish on while the heat lasts. But that same warm front in high-summer can raise the water temperature to the point that fish become stressed, and they won’t feed under that kind of strain.
But experienced anglers know to watch atmospheric pressure--also called barometric pressure-- as that can dramatically impact their fishing.
What is barometric pressure? How does it affect fish? And how should you use it as a guide for the hottest fishing?
Want answers? Keep reading!
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Barometric pressure is a measure of the weight of the column of air above you. The earth’s gravity attracts everything with mass, including the oxygen, nitrogen, carbon dioxide, and other gases that make up our atmosphere.
The greater the weight of the column of air above you, the greater the atmospheric pressure. You’ve had first-hand experience with this phenomenon if you've ever felt your ears pop while driving down a mountain in your car or while descending from a high altitude on a plane.
In the United States, barometric pressure is measured in inches of mercury (in-Hg). By measuring the amount of pressure the atmosphere exerts on a column of mercury, scientists and meteorologists can assess this important weather marker.
As Stephen Baig, an oceanographer with The National Oceanic and Atmospheric Administration, explains, "Imagine a U-shaped tube… At one end is liquid mercury, whereas the other end is open to the atmosphere. When the air pressure rises, it pushes the mercury higher. When the air pressure drops, so does the mercury level."
At sea-level, under normal circumstances, barometric pressure pushes that mercury to a height of 29.9212 inches. Any number above that is high, and conversely, any number below that is low.
High barometric pressures tend to coincide with sunny, clear skies, whereas low barometric pressures typically indicate storms, clouds, and the approach of both warm and cold fronts. Very low pressures are associated with hurricanes and other powerful storms.
And according to Sciencing, “If barometric pressure rises or falls more than 0.18 in-Hg in less than three hours, barometric pressure is said to be changing rapidly. A change of 0.003 to 0.04 in-Hg in less than three hours indicates a slow change in barometric pressure. A change of less than 0.003 in-Hg in less than three hours is considered to be holding steady.”
There are three primary ways barometric pressure affects fish.
Lateral lines - Fish have a long, sensitive organ called a lateral line that runs the length of their bodies. They use it to detect vibrations in the water, alerting them to the presence of injured fish and other food sources. It’s precisely this lateral line that fluttering, rattling, and vibrating lures excite, attracting hungry fish.
That sensitive lateral line feels even tiny changes in atmospheric pressure, too. That’s because as the weight of the air above the water changes, so does the pressure of the water against their bodies.
In effect, fish have a natural barometer measuring atmospheric changes.
Swim bladders - Many bony fish--species like largemouth bass, crappie, pike, muskie, redfish, trout, and tuna--have an air-filled sack in their abdomen. This swim bladder allows them to regulate their buoyancy, enabling them to control their depth without swimming.But shifts in barometric pressure cause accompanying changes in water pressure. And as that rises or falls, more or less pressure is exerted on the fishes’ swim bladders, forcing them to adjust--and possibly causing them discomfort as well.
Just as your ears can struggle to adjust to the increased pressure as you dive deeper into the water, fish probably feel some pain as pressure increases rapidly.
"Fish that have small air bladders, such as kings, Spanish mackerel, wahoo and dolphin, aren't as affected by barometric changes as those with large bladders, such as trout, redfish, tarpon, grouper and snapper," explains Spud Woodward, Assistant Director for the Georgia Department of Natural Resources Coastal Resources Division.
But Ralph Manns, a fishery scientist who has studied the effect of barometric pressure on bass in Texas, cautions that fish with sensitive swim bladders can adjust simply by moving a few inches higher or lower in the water column.
Plankton and other tiny food sources - Barometric pressure directly affects water pressure at depth, and as it changes, tiny food items like plankton will be moved in the water column. As these pressure-induced migrations occur, the fish that feed on them will move, too.That can drive some species, like crappie, higher in the water column or send them back into the depths. It can also encourage or discourage feeding. And don’t forget that this will impact plenty of prey species as well.
That’s where things get a lot less clear.
Scientists like Manns are skeptical. “We observed no obvious relationship between pressure readings or the nature of pressure changes and the behavior of largemouth and Guadalupe bass in Lake Travis, Texas… when the possibility that air pressure alone controls fish behavior is considered,--distinct limitations appear.”
That doesn’t mean that barometric pressure isn’t having an impact, but rather that its effect can’t be separated out from other changes in weather, precipitation, wind, and solunar cycles.
As he concludes, “We await any scientific information or interpretation that better explains the relationship between gamefish behavior and changes in air pressure, when isolated from the confounding effects of weather conditions. Until a biologically reasonable mechanism is proposed, we think it's more reasonable and likely more accurate to consider weather and sky conditions rather than barometric pressure in explaining fish activity and inactivity.”
So what does that mean for you?
Barometric pressure readings signal something about weather patterns, and the complex associations between pressure, sunlight, wind, precipitation, and humidity seem to affect fish behavior--albeit in ways we don’t entirely understand.
But drawing on the experience of long-time anglers, we can get a general sense of how atmospheric pressure relates to fishing.
Steady, high pressure tends to accompany clear, sunny skies, making for great days to head out on the water.
That’s good news, too, because prolonged periods of high pressure mean that fish and prey items can be found high in the water column and will feed actively. Expect vigorous feeding 48 to 72 hours after a high-pressure system settles in, and the fishing will continue to be hot until a change happens.
This is the hottest barometric period for fishing.
Fish seem to sense an impending drop in pressure, and to prepare for a period of inactivity as they wait it out in deeper water, they’ll feed intensely.
Savvy anglers know that a rapidly falling barometer means to hit the water--and hit it hard!
Terry Sullivan, a charter captain in New Jersey, is familiar with this pattern. "I've seen striped bass go on a wild feed right before the barometer began to drop," he says. "During summer, we get an upwelling effect ahead of a front. Right before our southeast wind shifts more southerly and begins to blow, which precedes the front, it triggers a hot bite locally. The fish sense that a change in weather is about to occur and feed heavily right before the front. Once the wind goes hard south, they shut down. I guess they know they won't be eating for a few days, so they have to gorge themselves."
Is this just a question of atmospheric pressure? Not quite--but I’d take it seriously, nonetheless!
Even Manns thinks there’s something to this. “...Frontal passages and associated conditions, including overcast skies, wind, rain, and temperature changes, often seemed to turn bass on. Apparently, heavy cloud cover and low-light conditions affected bass activity, not air pressure changes alone.”
We wish there was a clear, science-supported answer to lay bare here, but that’s just not the case.
We do know that barometric pressure and its associated weather changes affect fish behavior--we’re just not certain about the mechanisms in play. What we can say is that steady, high pressure and rapidly falling pressure seem to turn fish on, and the experience of professional anglers and scientists supports this claim.
If you have a question or something to add, please leave a comment below. We’d love to hear from you!