Barometric pressure and trout fishing

Why Barometric Pressure Is Not a Useful Guide for Trout Fishing

Barometric pressure does not directly affect trout or their feeding patterns. But, barometric pressure does have an indirect influence because air pressure is one of the fundamental drivers of our climate, and changes in weather can influence when or how trout are more likely to feed. Barometric pressure can also change water levels, during periods of low-pressure springs are more active and water levels can rise.

In what seemed like a previous life, I use to study metrology and environmental physics so I still maintain an interest in all things weather. Like all fishermen, I like to look for patterns, how does the weather and other variables such as barometric pressure affect fishing?

Yes, the weather can have a massive impact on trout fishing. But, I am far from convinced that Barometric pressure itself matters. It all comes down to how pressure works, and at a fundamental level, there is no difference between pressure caused by air and water.

I will explain why. Water is eight hundred times denser than air. One foot of movement underwater is the equivalent of an additional 800 feet of atmosphere above our heads.

A trout swimming only a few feet deeper, or shallower will result in a difference of pressure orders of magnitude greater than what any changes of air pressure can cause. For every 33ft change in water depth the pressure increases by the equivalent of one atmosphere of pressure (14.7PSI).

The summit of Mount Everest (29,031.7 ft)4.84 PSI (9.81)
Mean Pressure at Sea level14.7 PSI (29.9)
Mean pressure 33.8ft below sea level29.4 PSI (59.8)
Example of very low air pressure (sea level)12.6 PSI (25.6)
Example of very high air pressure (sea level)15.2 PSI (30.9)
Example of extremes in pressure, and how the extra mass of water greatly ‘overwhelms’ any influence that changes in air pressure might cause.

So a trout leaping from the water to grab a damselfly will be under about 14.7PSI of pressure, if that trout then dives down just two feet to eat a nymph it will be experiencing about 16.5psi. Yes, Even a two feet difference in water depth is a greater difference in relevant pressure than comparing an extremely low pressure day to a very high pressure day.

Do not only take my word for it

To test my own biases, I decided to read a bunch of research papers which were looking at the effects of barometric pressure on fish feeding and movement patterns. I must have read over 20 papers and they all broadly come to the same conclusion. Barometric pressure was not a measurable factor when determining fish behaviour.

Over the years, I have had many discussions with avid fishermen many of which had their own theories about barometric pressure. Some seem to be very passionate about it and convinced in their own minds. When I listen to them in isolation, they can seem very convincing, but the problem is, I have met other anglers with widely differing views. Some say high pressure is good, while others say low. Even more, say that the trout behave differently in lake A than lake B and it is all based on the changes in air pressure.

So many contradictions and contrasting opinions is not just a consequence. These anglers are just simply looking for patterns and they are finding them thanks to confirmation bias. We are all probably guilty of it.

So no, I am not convinced that air pressure or barometric pressure affects trout fishing, but air pressure does represent changes in the weather and that does affect the fishing.

What is the counter argument? Why do some people believe barometric pressure affects fishing?

In this section, I will try and explain the ‘theory’ why some people believe barometric pressure has an influence on fishing. I will try my best to explain it without sounding dismissive.

Trout, like most fish, have a buoyancy organ called the swim bladder (also known as air bladder). Trout, like most bony fish use this organ to help regulate their buoyancy. Think of it as a form of ballast. It allows them to remain suspended in the water column without moving to maintain their position. The ‘gas’ in the air bladder is not atmospheric but contains a much greater concentration of oxygen. Fish fill their swim bladder from ‘gases, including oxygen which their gills absorb from the water.

Well, the theory goes as the barometric pressure changes the ‘air’ inside the swim bladder expands (or contracts) to such an amount that it puts pressure on the fish. When the air pressure drops, the air bladder expands causing discomfort putting the fish off their feed, and causing them to swim deeper to increase the surrounding pressure and equalize the pressure inside the bladder. Some people compare the discomfort fish experience to our own discomfort caused by changes in atmospheric pressure such as old injuries playing up when the pressure suddenly changes.

Since people even believe, that more minor pressure differences give the fish advance warning of impending weather changes. For example, when their swim bladder inflates, it indicates to the fish that less settled weather is about to arrive.

How Barometric Pressure Affects Fishing

Barometric pressureWeather and environmentTheory on how trout respond to changes in barometric pressure.
Higher Pressure (30.50)Clear skies, light wind and settled weatherTrout are reluctant to feed and seek cover. Fishing is slow but persistence can be rewarded.
Low Pressure (29.60)Cloudy, Rainy and generally unsettled weather. Springs more active.Fish move to deeper water and become reluctant to feed but a bit more active than high pressure
Fast Raising PressureImproving weather.Trout becoming more active, but shoals can be concentrated.
Fast Falling pressureUnsettled, Windy, developing cloud and fast-moving frontsFish start to feed, and activity picks up as the trout get in a meal before the bad weather hits
Medium / stable Pressure (29.70-30.40)Settled weatherTrout are reluctant to feed but might be tempted. Others claim that trout feed normally.
This table is an example of how many anglers believe barometric pressure affects trout fishing. Disclaimer, I personally do not believe such claims but included it for completeness.

Are insects and hatches influenced by barometric pressure?

There is some evidence to suggest that changes in barometric pressure do influence the behaviour of insects, but the behavioural changes are not consistent between species of insects and the interaction between air pressure and insects are still poorly understood.

There is evidence that many insects such as Caddisflies, and potentially Mayfly hatches are influenced by pressure changes. Most notably periods immediately prior to barometric pressure changes, and periods of depressions. But, for both groups of insects temperature remains the dominant factor that determines hatch activity.

Why does barometric pressure influence aquatic insects and not trout? Will, I can only speculate but it likely comes down to the rate of movement. Insects move slowly and when not hatching typically creep along the bottom. Because the nymph size barely moves, the pressure from the water remains constant so it is believable that they can detect changes in atmospheric barometric pressure.

Barometric pressure is a useful guide to predict the weather

While the trout might not be responding to the changes in barometric pressure directly, changes in pressure does change things that the trout can pick up on. High pressure, typically means calm weather and blue skies. In the summer they can be hot, in the winter they can be extremely cold. When it is sunny, trout prefer to seek shade. They might go deeper, or hide undercover.

But, calm conditions can trigger mayfly hatches. They hate the wind, in fact, most insects do. So the increase in food can be enough to tempt the trout out of hiding.

While low pressure, typically means high wind and storms. These conditions can make fishing challenging. We all know how hard it is to cast into the wind or to try and spot fish when the surface is choppy. But the trout do not care that we struggle to catch them.

They are still beneath the chop feeding. Storms and rain, weather events which low barometric pressure result in can at times cause the trout to feed. That is because rising water levels due to rains expose and wash downstream more food. It is an insect buffet for the trout.

How about the transitional periods?

The transitional periods when the air pressure is raising generally represent changes in the weather. If the pressure is rising, the clouds will depart and the winds will weaken.

Contrasty dropping pressure, generally speaking, will bring the wind which will blow in the clouds and because the wind is blowing from the polar regions it generally brings cold weather with it. Cold weather, depending on the time of the year can be a blessing or a curse.

In the middle of summer, when the water temperatures are hot. A drop in air temperature can cool the water, which will encourage the trout to feed. But in winter, when the water is already cold, a cold front is just as likely to discourage the trout even further.

Barometric Pressure can influence water levels!

This is something I have observed first hand many times. I grew up on a farm with a small creek, over the summer the pools become disconnected, the only inflows being springs.

During periods of high air pressure, the springs become weak and the water level will drop. But when the air pressure drops, and well before the storms roll in the springs erupt back into life and the levels of the ponds can raise several inches. Sometimes, the creek even starts to flow again.

In periods of drought, and low flow. Fish can become more active during periods of low pressure because the springs and more active and they help to cool the water. I do not know if this is of much relevance for trout fishing or not, because trout struggling to survive in such stressful conditions should probably not be caught.

Summary – Parometric pressure is not a good guide to determine the feeding behavior of trout

I will now summarize this topic. While many anglers believe barometer pressure strongly influences the feeding behaviour of trout, there is simply no evidence that this is the case. From a purely physics perspective Slight chances of air pressure are dwarfed by the pressure changes experienced by fish as they swim up or down in the water column.

Now, Barometric pressure is a good indicator to use to predict weather changes, and maybe even insect activity. These changes do influence trout feeding behaviour.

Disagree with me? feel free to share your thoughts and prove me wrong in the comments below.

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5 Comments

    1. It is complicated. I no longer check Solunar calendars because they are all too generalist to be useful. They could be right for species A but dead wrong for species B.

      I just glance at the moon phase, then go fishing anyway.

      Fish do respond to the moon, sun, and tides. But how they respond differs greatly based on species.

      For example, at night during a full moon many fish that hunt visually see better. So are more likely to feed when it is bright. Because they have been feeding at night, means they are more likely to rest during the day. It is all about knowing how the type of fish you are targeting responds to the moon.

      Rainbow trout are predominantly visual hunters, brown trout predominantly sense their prey via vibrations. Brown trout are happy feeding in the dark. So they are less affected by the moon.

      Rainbow trout, need some light to hunt. During the full moon, they can feed all night. That in turn can make them less active during the day. The opposite applies to the new moon.

      (Too further complicated matters, in some fisheries trout, are prey. So on a full moon, they might be hiding all night because the Bass, Pike or whatever have been harassing them. In such situations they might be pressured to feed during the day when they feel a bit safer)

      This is further complicated because some bait fish do time their spawning runs with the moon (or king tide if the area is tidal). This in turn can make the larger, more predatory fish more active. Same with insects, some insect’s hatch coincides with the phrase of the moon.

      It is all interconnected and much too species and site specific to ‘summarize’ in a simple calendar.

  1. Thanks for putting fact to fiction and sharing your knowledge.

    I fish with a friend who swears by pressure and lunar changes for optimum fishing. All the times we’ve had better than usual success outside of his “tables” I’ve asked him, “Why?”. He never has an answer.

    Fish have three things on their mind. Food, reproduction and a secure comfortable spot to do both. Follow those patterns and you’ll do well.

    I’m sending him a link to your article so he will have an answer the next time I ask. Lol

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