Barometric pressure and fishing: what is known

What pressure is and how it is measured

Atmospheric pressure is the weight of the air above you pressing down on the surface. It is measured with a barometer, which is why it is also called barometric pressure. The usual units are hectopascals (hPa) and millibars (mb), which are the same size, so 1,013 hPa is 1,013 mb. In the United States, inches of mercury (inHg) are common. One inch of mercury is about 33.9 hPa.

Average pressure at sea level is about 1,013 hPa, or 29.92 inHg. Day to day it mostly stays within a few tens of hectopascals of that figure, lower in deep storms. Pressure also falls with height, so forecasts and weather maps quote a value adjusted to sea level to make places comparable. A raw reading from a sensor above sea level will be lower than the forecast figure.

What rising and falling pressure indicate

Areas of low pressure are associated with rising air, cloud, rain and wind. Areas of high pressure are associated with sinking air and more settled weather, often with lighter winds and clearer skies, although highs can also bring fog or persistent low cloud. The single reading matters less than the trend, which meteorologists call the pressure tendency.

Steadily falling pressure usually means a low or a weather front is approaching: thickening cloud, a strengthening wind and often rain.

A rapid fall is a traditional warning of strong winds, and should be read first as a safety signal.

Rising pressure after a front usually brings clearing skies and often cooler, drier air, sometimes with a marked shift in wind direction and a gusty spell before things settle.

Slow changes or steady pressure suggest the current weather will persist for a while.

These rules work best in the middle latitudes, where weather systems pass through regularly. In the tropics pressure varies much less from day to day, and the same rules of thumb are less useful.

What anglers believe

The common account runs as follows. As pressure falls ahead of a front, fish feed actively. Once the front has passed and pressure rises quickly under bright, clear skies, fishing becomes hard for a day or so. Long spells of steady pressure give ordinary, predictable fishing.

This belief is widespread in both freshwater and saltwater fishing, and the experience behind it deserves respect. The usual explanation offered is that fish sense the pressure change through the swim bladder, the gas-filled organ that many species use to control buoyancy.

What the evidence supports

The first difficulty with the explanation is one of scale. Water is heavy, and pressure underwater rises by about one atmosphere, roughly 1,000 hPa, for every 10 metres (33 feet) of depth. A fish that moves up or down by just 1 metre experiences a change of about 100 hPa. A passing weather system might change the air pressure by 10 to 30 hPa over a day. Fish therefore go through larger pressure changes in seconds, simply by changing depth, than the weather imposes over many hours.

That does not prove that fish cannot detect a slow change in air pressure, but it does mean a simple mechanical account is not obviously right. The second difficulty is the research itself. Studies that have compared catch rates or the movements of tagged fish with barometric pressure have not produced a consistent answer: some report a relationship, others find none, and results differ between species and waters. It is also hard to design a study that isolates pressure, because pressure never changes on its own.

Other explanations for the same pattern

Almost everything that accompanies a pressure change is a plausible influence on fish in its own right.

Light. Falling pressure generally brings cloud. Many predatory fish feed more boldly in low light, as they do at dawn and dusk, and the bright, clear skies that follow a front have the opposite effect.

Wind. A rising wind ruffles the surface, reduces light penetration, stirs up shallow water and pushes surface water and drifting food towards downwind shores.

Temperature. A cold front can lower air temperature sharply and cool shallow water. Fish are cold-blooded, and their activity follows water temperature.

Rain and runoff. Rain changes river flow, water colour and, in estuaries, salinity.

Water clarity and calm. Still, clear, bright conditions after a front make fish in shallow water more wary and make lines and anglers easier to see.

Any of these could produce a good bite before a front and a slow one after it without pressure playing a direct part. Light and temperature have their own articles, on low-light fishing and on water temperature and fish activity.

How to use pressure in practice

Watch the trend over the last several hours and the next day, not the single figure.

Treat a period of falling pressure ahead of a weather change as a reasonable time to fish, provided the wind and sea state are safe for what you are doing.

Treat a rapid fall as a warning about wind before it is a fishing tip.

Keep a simple log of your own catches against the weather. On your own water it will be worth more than any general rule.

The consequences of a pressure change show up in the forecast as wind, cloud, rain and temperature, which CatchWindow location pages show alongside tides and a week-ahead view. Those are the conditions that fish and anglers actually experience.