Wednesday, April 18, 2007

Barometric Pressure

This is part 1 of a series of articles on the theories of the different factors which produce good and bad fishing conditions. The theories aren't necessarily mine, although I've attempted to provide a thoughtful summary of information I have researched, with some modification based on personal experience.

I'm not entirely convinced that you should solely base your decision on whether to go fishing, on the measurement of the positive or negative factors based on theory. Although a number of the positive factors may indicate that the fishing will be good at your regular spot, I think to a large degree that the negative factors mean that you have to find and catch the fish in a different fashion than you would normally.

Barometric Pressure

The effect of barometric pressure on fishing feeding activity is one of the more interesting theories. In short, the theory proposes that a dropping air pressure brings on feeding activity, rising pressure turns the fish off feeding, high pressure results in the fish moving to shallower water, and low pressure results in fish moving to deeper water. This article will provide an overview on barometric pressure, summarize various theories about why atmospheric pressure affects fishing activity, and then summarize in tabular fashion the generally accepted fish behaviours with various barometric patterns.

Barometric pressure is the measure of the weight of the atmosphere above us. A barometer is used to measure air pressure. The earliest barometer consisted of a glass vacuum tube inserted into a container of mercury which was exposed to the pressure of the air. Increased air pressure would force the mercury up the tube in a height proportunal to the pressure. The height was measured in inches (inHg) or in millibars (1 inch = 33.864 millibars). Although new types of barometers are now used, these measures are still in place. In general, 30 inHg or 1016 millibars is considered to be normal air pressure. In normal weather, 30.5 is considered extreme high, and 28.5 is considered extreme low. The measures are taken at sea level, a higher elevation has less atmosphere above it, so a correction factor against the normal measure is needed depending on altitude.

It is believed that the effect of barometric pressure is greater in shallow water than deep, probably due to the pressure of the weight of the water in deep water being so high, that the air pressure above it is not relatively significant.

The principal theory, is that the effect of changing pressure on the swim bladders of fish makes them uncomfortable or dis-oriented. In this theory, the fish will move to feel well, or they feel bloated or full. With a lowering barometer, it is believed these fish move into deep water seeking higher water pressure and ride out the low pressure around structures. The theory suggests that just prior to change from a high to a low, fish will bite like crazy until the low hits and then stop. The difficulty with this theory, is that water is 900 times more dense than air, and generates signicantly more pressure than air. In fact,a 3 foot wave will produce a variation of pressure more significant than can be expected from a change in atmospheric pressure through a dramatic change, and the wave effect is happening every few seconds, rather than the hours or days that the atmospheric changes takes to occur.

Frankly, given that the pressure of water depth is such a significantly greater factor than the pressure of atmosphere, it seems likely that the weather conditions created by changes in barometric pressure, such as clouds, rain and wind, have more effect on fishing than the barometric pressure alone. So in the opinion of the author, barometric change is a good indicator of fishing change, but it's not because of the pressure change by itself, as much as what other weather conditions are likely to occur because of the pressure change.

The following table attempts to summarize the barometric pressure, and observations on fish activity and fishing techniques.

Pressure Trend

Typical Weather

Fish Behaviour

Suggested Fishing Tactics


Clear skies

Fish seek cover, look for logs, weeds in shallows. If water too warm, will stop biting.

Fish structure close to surface, with shallow crankbaits, poppers, etc..


Clearing or improving

Fish start to move out of deeper water. After a day or so, go to normal feeding.

Fish with brighter lures and near cover, moving from deeper water to shallower water.

Normal and stable


Normal activity.

Experiment with your favorite baits and lures.



Most active feeding.

Range of different methods. Surface and shallow running lures may work well.

Slightly lower

Usually cloudy

Fish seek deeper water, with water temp maybe also slowing them down. May need to settle before feeding again.

Use deep running lures at a moderate speed.


Rainy and stormy

Fish move to deeper structures, may not feed.

Fish deep structures, vary your methods.

Of course, the longer a period of high feeding activity, the more likely the fish are to stop feeding. And the longer the period of inactivity, the more likely the fish are to start feeding.

Draw your own conclusions on the effect of a changing barometric pressure on fishing activity. Whether pressure changes by themselves cause feeding changes may be in question, but the patterns seem to be there irregardless.

By David Girdwood
Manager, Thefishingnut

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