Kent Coast Sea Fishing Compendium

Atmospheric Pressure

This page was last updated on

"Falling barometer, rising fish; rising barometer, falling fish …"

Admiralty tide tables give tidal predictions which are computed for average barometric pressure. Average sea-level pressure is 1,013.25 millibar (mbar or hPa) [1]. A difference of 34 mbar from that average can cause a difference in sea level height of about 30 cm. A low barometer will tend to raise sea level and a high barometer will tend to depress it. The water level does not, however, adjust itself immediately to a change of pressure and it responds to the average change in pressure over a considerable area.

[1] 101.325 kPa; 29.921 inches of mercury inHg; 760 millimetres (mmHg)

Changes in sea level due to barometric pressure alone seldom exceed 30 cm but, when mean sea level is raised or lowered by strong winds or by storm surges, the actual change in sea level is often much greater.

It is commonly believed by anglers that changes in barometric pressure strongly influence fish feeding behaviour. A rise or fall in barometric pressure, such as with an approaching cold front, usually means a shift in the weather pattern but 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. Barometric pressure is expressed 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). Fluctuations in barometric pressure indicate changes in weather. In general, low-pressure systems bring unstable conditions, often with precipitation and clouds, whilst a rising barometer indicates approaching high-pressure accompanied by stable weather and clear skies.

A normal value for barometric pressure is about 30 inches. Strong high pressure is about 30.7 inches. A powerful low, such as during a hurricane, can reach down to 28 inches or so. 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.

Accordingly, barometric pressure does not change quickly enough to influence fish feeding behaviour. However, it is certainly one of the factors 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 events below the surface.

Pressure in the ocean (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.

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 is 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. In the ocean, four main factors can change the hydrostatic pressure in the fish's world:

Firstly, a fish naturally changes 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. For example, going up or down just 3.28 feet will decrease or increase the pressure on a fish by 1/10 (0.1) 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.

Secondly, 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 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.

Thirdly, 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.

Fourthly, the weight of the air itself is an 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.

In summary, it is unlikely that barometric pressure alone can trigger the sudden bite that angling's common wisdom often asserts.

"Sea Fish" (1898) Frederick George Aflalo at page 8

Of the effect of thunder on sea-fishing, which has been much written about, I regret to have no interesting data to quote. My diaries are absolutely conflicting on the subject, for they show under these conditions almost as many bad days as good; the general opinion among the fishermen - and I give it without comment for what it is worth - is that the fish bite well during "thunder weather", especially pouts. On the other hand, I have had many blank days when the thunder only threatened without actually rolling.

"The Modern Sea Angler" (1979) Hugh Stoker at page 168

... scientists have discovered that a storm raging out in mid-Atlantic causes an earth tremor on a beach in England a day or so before the heavy storm swell arrives. Fish, with their delicate sensory organs, would be able to detect this preliminary earth tremor, and so receive warning that rough weather was on its way. This would cause some fish to move out into the safety of deeper water; whilst others such as bass, might be tempted to move closer inshore.

It is the latter "white water" fish which will interest the rod-and-line angler most at such times. A heavy swell breaking on the beach stirs up the sea bottom, and, emboldened by the murky water, the bass venture into the shallows to grow fat on the countless small sea-bed creatures unearthed by the pounding waves and swirling undercurrents.

"Trout, Salmon & the Evening Rise: The Barometric Breakthrough" (2006) Andrew Bett

However, game fish research shows that trout, sea trout and salmon are capable of detecting pressure change of as little as 0.5 mb. An increase in barometric pressure of just 1mb will indicate whether a hatch of flies is about to begin. If the barometer is rising, prepare for a hatch of flies (start fishing a hatching pupae, or an emerging insect) and be ahead of the game for the feeding game fish. Hatches of insect can be very short, and to have prior knowledge of what is happening under the water is a real advantage.

A steadily rising barometer is absolutely magic for salmon and sea trout fishing and the faster the barometer rises the more the salmon and sea trout seem to like it because they are responding to this conditioned reflex of feeding on rising aquatic insects as parr and smolts and aquatic insect hatches are also more prolific when the barometer rises sharply, most particularly, for instance, after a thunder storm. Thankfully salmon and sea trout can be caught on a stable barometer, and even the odd fresh run fish on a slowly falling barometer, but these are much less ready to come to the fly and are inclined to 'take short'.

Seeing the barometer falling therefore also tells you why the fish are becoming harder to catch, or even impossible, when the barometer is falling relatively quickly. In these conditions the fish simply 'go down', and sinking lines or sink tips with a great deal more perseverance is needed, hoping all the time that the barometer will start rising again. Eventually the barometer starts to give you better news and you fish again with re-invigorated enthusiasm.

The important thing is to fish hardest when the barometer is rising - these are the purple patches in your fishing week. The added knowledge, while you are fishing, about the actual behaviour of the fish is absolutely invaluable.