Fishing Using Barometric Pressure
Learn the telltale weather signs that can predict the most favorable fishing conditions
by Bill Bernhardt
As you can imagine, because fish live in a liquid environment instead of a gaseous environment like we do, not only do they have sleek, streamlined, bodies to enable them to efficiently move through their dense medium, they must also have some means of suspending themselves at their desired depth in the water column. Otherwise, they would either sink to the bottom or be forced to swim continually in order to maintain lift on their fins as some shark species do.
It's this anatomical arrangement that's most susceptible to pressure changes, and starts our story of pressure's impact on our fish in the wild.
Most fish species have evolved an internal organ commonly known as a “swim bladder” that is similar to a Human’s bladder. But, instead of serving as a holding tank for liquid waste as Human’s bladder does, a fish’s swim bladder is designed to be inflated or deflated at will with oxygen obtained from the surrounding environment which, in turn, acts like a Personal Flotation Device that allows them to suspend themselves anywhere in the water column that they may choose.
While having such an internal organ is a wonderful adaptation to living in a liquid environment, it also causes fish to be vulnerable to changes in barometric pressure because, as the atmosphere above the water exerts more or less pressure on the surface of the water, so too does it increase or decrease the subsequent water pressure which, in turn, places more or less pressure on a fish’s swim bladder.
Barometric Pressure Mechanics
However, before we can understand how the barometric pressure either positively or negatively affects a fish’s feeding behavior, we first need to understand exactly what barometric pressure is. Therefore, although most people are not aware of it, air actually has mass and thus, it too is affected by the Earth’s gravity. Consequently, the amount of pressure that the Earth's atmosphere exerts on the Earth's surface as well as the surface of its bodies of water can actually be measured in units designated as Pounds per Square Inch, Pascalls, Millibars (equal to 100 kilopascals), or Inches of Mercury (aka Torr).
Depending on your elevation as well as the current weather conditions in your location, barometric pressure generally ranges from 28.5 Hg (Inches of Mercury) to 30.5 Hg. On days when the barometric pressure is low, the Earth's atmosphere is exerting relatively little pressure on the Earth's surface and, on days when it is high, it is exerting a significantly greater amount of pressure on the Earth's surface.
The same amount of pressure that is exerted on the Earth's surface is also exerted on the surface of any given body of water and thus, changes in barometric pressure also cause an increase or decrease in water pressure which, in turn, places more or less pressure respectively on a fish’s swim bladder.
Furthermore, a falling barometric pressure usually indicates the pending arrival of a storm while, a rising barometric pressure usually indicates clear skies ahead and both of these factors also affect the ambient light levels
Fish Behavior Impacts
Because both rising and falling barometric pressures create more or less atmospheric pressure respectively, it also increases or decreases the ambient water pressure at any given depth and thus, fish experience these changes in their environment as either increased or decreased pressure on their swim bladder.
While it is not possible for humans to truly understand exactly how an increase or decrease in barometric pressure feels to a fish, most biologists believe that an increase in pressure on their swim bladder causes the fish discomfort as well as causing them to lose their sense of balance and become lethargic which leads to a loss of appetite.
On the other hand, a rapid rise or fall in barometric pressure or, an extended period of either extreme high pressure or extreme low pressure can also make fish uncomfortable and thus, they tend to become disinclined to feed. In fact, a slight change of just 0.02 Hg up or down is enough to either positively or negatively affect fish's willingness to feed! Plus, as mentioned previously, a rising barometric pressure always indicates clearing skies while, a falling barometric pressure always indicates cloudy skies ahead and, it can also indicate an incoming warm or cold front. Therefore, not only is barometric pressure a good indication of the upcoming weather conditions, it also a good indication of a fish’s willingness to feed.
Rules of Thumb
It is helpful to use the following rules of thumb as a guide:
Most experienced fisherman are aware that fish tend to prefer stable, consistent, weather patterns regardless of whether it is sunny and warm, cold and raining, calm or windy. Either way, atmospheric stability is the key to good fishing. In fact, fish are most active and, their behavior is most predictable, during a stable weather pattern which means that they will engage in specific, genetically dictated, behaviors during different seasons and weather conditions. Therefore, the best fishing usually occurs after the third day of a stable weather period because, by then, the fish have had time to acclimate to the new weather conditions and thus, stabilize their feeding.
Rapidly Changing Weather
This type of weather causes fish to react rapidly by changing their feeding patterns as well as where they choose to hold in a given body of water and, even which senses they utilize to find prey. For example, fish are first and foremost sight feeders and thus, if heavy rains cause widespread turbidity due to runoff, they will be forced to rely more heavily on their lateral line and rudimentary hearing to locate their prey. However, if the turbidity is localized, such as at the mouths of creeks, streams, rivers fish may move to these locations where they can hold in the clear water at the edge of the discolored water to ambush their prey.
This weather is the bane of all fishermen. When cold fronts are approaching, masses of cold air move in to displace the warm air. As a result, the ambient air temperature can drop as much as 15° to 25° in as little as an hour! In addition, cold fronts also generate shifting and/or gusty winds along with a rapid rise in barometric pressure and this rapid rise often causes fish to completely cease feeding until they become acclimated to the new weather pattern. In fact, most biologists believe that an increase in pressure on a fish’s swim bladder causes the fish to feel uncomfortable as well as causing them to lose their sense of balance and become lethargic which, in turn, leads to a loss of appetite.
However, for a short period just before the arrival of a cold front, fish tend to feed avidly, although it should be noted that this window is small. Then, once the front arrives, they tend to seek cover till they once again become acclimated. Therefore, the first day after a cold front passes, fish in ponds, lakes, mangrove swamps, estuaries and sounds tend to hold deep in the available cover while pelagic fish species tend to seek deep water. Thus, when fishing during cold fronts, fishermen should to use smaller, slower moving, lures and they should fish them close to cover or in deeper water.
This type of weather is most favored by fishermen. Opposite from cold fronts, warm fronts cause masses of warm air move in to displace the cooler air which causes the ambient air temperature to rise. Also different from cold fronts, warm fronts move more slowly and often bring gentle rains instead of violent storms. But, even more importantly, warm fronts usually generate falling barometric pressures, cloudy skies, and mild winds which often cause fish to feed avidly.
Therefore, shallow water fish species tend to move out to the edges of cover to feed and are far more willing to chase prey while deep water fish species will move from deep water into transition zones between deep water and shallow water to feed. Therefore, when fishing during warm fronts, fishermen should use larger, faster moving, lures.
Table 1: Suggested Fishing Tactics by Pressure Trend
Fish seek shade or cover
Fish close to cover and in deeper water over a dark bottom using slow moving lures
Clearing or improving
Fish are slightly more inclined to feed
Fish with brighter lures close to cover.
Normal and Stable
Expect normal fishing behavior
Look for fish in their usual locations near food sources
Often the best fishing
Use somewhat larger lures
Cloudy. Rain starts.
Bait fish seek shelter in shallow water. Bigger fish come out to hunt
Fish with larger, faster moving, lures near the edges of cover or transitions zones
Fish tend to become less active during extended periods of low pressure
Use smaller lures and change them often until you find one that the fish will strike
The Bottom Line
It is imperative that fisherman learn to be aware of the barometric pressure and how it affects both the weather and a fish’s feeding behavior in order to best use that knowledge to their advantage.
However, it should also be noted that regardless of the present trends in barometric pressure, a fish’s feeding behavior is adversely affected most the first day a front arrives and, their willingness to feed drastically improves after three days even during extended periods of low or high barometric pressure.