Ask any angler and you are bound to get a range of answers as diverse as the lures in your tackle box. However, some experts do agree that fish behavior is influenced by changes in barometric pressure. While barometric pressure may not directly affect the fishing conditions, it is a major factor that can influence what a fish may be doing at any given time. Having a better understanding of this variable along with others such as: time of year, water temperature, etc., will improve your ability to understanding fish and increases your likelihood of catching them.
So, what exactly is this invisible variable that so many anglers seem to swear by? Barometric pressure is a measurement of the amount of force the earth's atmosphere is exerting downward at any given time. Though air is, well, "lighter than air", it still has some mass associated with it. The matrix of gas atoms, water vapor, and other aerosolized particles are all weighing down on the earth's surface, to a slight degree.
To get a grasp on the understanding, take for example this. Standing high on a mountain top you will have less air above you than at sea level. Therefore, at higher elevations, there will generally be lower barometric pressure than at sea level.
A greater influence of fluctuations in barometric pressure, (the short-term fluctuations that can influence fish behavior) are caused from local weather patterns which involve pressure ridges of air moving across the landscape.
Barometric pressure can be measured in several different units of measure. However, the most common official measurement you will encounter is mmHg.
These abbreviation stands for "millimeters of mercury" and "inches of mercury" and is derived from one of the original and simple ways that barometric pressure was measured.
This was done by reading the movement of mercury up and down in a u-shaped glass tube. One end was sealed, the other open, and mercury filled the "u" in the bottom of the bend.
The atmosphere would influence the open end and drive the mercury up or down and allowed a standardized way to measure barometric pressure. Nowadays, fancy equipment has mostly replaced these mercury and glass tools.
So, as we briefly touched on, one of the primary influences of daily barometric fluctuations is weather and climate moving over the landscape. In a very general scope, storm fronts involve low pressure; bluebird sunny days typically represent high pressure.
So, what is a baseline "normal" pressure. Generally speaking, somewhere around 29-30 inches Mercury is a rough "baseline", though remember this is also dependent on your elevation. Higher elevation will have a lower baseline barometric pressure than lower elevations. It is better to watch your local weather patterns to understand your local baseline conditions.
When a storm system begins to move in, barometric pressure begins to drop. In the midst of a storm, the barometric pressure readings are often low, around 26-29 ,generally. After a storm has passed, barometric pressure will begin to rise as calmer and clearer weather takes over.
High pressure is generally associated with very calm, clear, and warm weather. These high pressure systems can keep storms and low pressure at bay for days on end. It is hard to physically distinguish "normal" from "high pressure" without ready an instrument. Though generally, "high pressure" can be considered 30 inches of mercury and up.
The reason so many anglers swear by fishing by the barometer, is that their is actually some evidence to back it up on a physiological level for the fish. Nearly all fish have a swim bladder organ, which is an inflated air sac that helps the fish to maintain neutral buoyancy in the water. This air sac, particularly in small-bodied fishes, is very sensitive to changes in atmospheric pressure (which weighs on the water and in turn affects organisms under the water as well).
When the barometric pressure drops, the swim bladder inflates slightly from the reduced pressure around of it. Conversely, when the barometric pressure rises, the swim bladder shrinks ever so slightly underneath the increased pressure.
So, what does this mean for the fish? Think of an inflated swim bladder like a bloated belly, it doesn't feel too good. When the pressure drops, fish can get uncomfortable from their swim bladder inflation and seek out a place where they don't feel this anymore. By swimming deeper into the water, they can experience higher pressure from all the weight of the water and it will help to reduce the size of their swim bladder to normal.
Now on a normal pressure or high pressure day, fish may not feel this discomfort from an inflated swim bladder. Therefore, they feel comfortable utilizing a larger variety of the water column and are more apt to be active and feeding.
So, how do you use all this information to better your fishing? Many anglers will tell you to "fish on the drop". They mean to fish just before an incoming storm front, when the barometric pressure is just starting to drop. There is some validity to this. The swim bladder is highly sensitive to these slight pressure changes, particularly in small fish where they have a swim bladder that is proportionally large to their body size. The fish can sense the incoming low pressure and know they will have to seek refuge deeper.
This means that they become very active during this time trying to feed as much as possible or moving around to find deep cover. This can mean sportfish that are hungry in preparation to hunker down, or simply sportfish that are taking advantage of the frantic small fish which are affected more by the pressure change.
Once the pressure drops low in the middle of the storm, you can best assume that the fishing may not be stellar. Much of the fish may be waiting out the low pressure in deeper water, not wanting to feed heavily. This is why many old timers will say not to bother fishing in the middle of a storm.
On the other end of the spectrum, the normal to high pressure days, fish activity can be much more normal and leisurely. During these times, the fish are not likely feeling much swim bladder discomfort. Therefore, they are more apt to be cruising around the water body, exploring varying depths, and feeding at a normal rate. The fishing conditions usually improve several days after a low pressure system has left, as the first day or two the fish may be acclimating.
To get a better understanding of your local weather conditions, normal barometric pressure, and how pressure systems move in and out; start watching on your weather app or website.
Many of the weather services offer an easy to read barometer and more ideally, a barometric graph showing past days and upcoming forecasts. Take a glance at it every now and then, particularly around storm systems. This will give you an idea of when you might expect to be fishing before and after a typical storm front.
It is just one more variable to help you understand fish and one more tool in your bag of fishing tricks. No one variable is a do or die for how the fishing is going to be on any given day. However, having a broad understanding of the things that make fish behave the way they do is one step to becoming a much better angler.
If you want to start tracking the barometric pressure while you're out fishing, I recommend you take a look at the Trac Outdoor T3002 Fishing Barometer.
This article was originally found on the Skilled Angler web site. The site contains considerable information about many aspects of fishing. Take a minute and check out the site.