Successful fishing depends on a variety of environmental factors. Most anglers will know how to consider temperature, currents, light levels and how to read the water for the best place to fish. However, often overlooked is how barometric pressure affects fishing.
Barometric pressure affects fishing through the physiological changes it causes in the fish. Pressure changes have an impact on the swim bladders in fish. The swim bladder senses changes to the atmospheric pressure on the water and causes fish to react by swimming higher or deeper in the water.
It can be tough to work out why you can catch fish in the same section of water one day and not the next. Often, it’s to do with a change in the barometric pressure. Below, we’ll look at the effect of barometric pressure on fishing in more detail.
What Is Barometric Pressure?
Barometric pressure relates to the weight of the atmosphere as it presses down on the earth. In most places and climates, it is fairly constant, with only mild fluctuations. The most consistent differences in barometric pressure relate to differences in altitude. In more mountainous regions of higher altitude, air pressure is considerably lower compared to areas at sea level.
Pressure fluctuations are indicators of changing weather systems. High pressure is associated with calmer days and clear skies, largely free from any threat of rain. Low pressure tends to be a forewarning of the sort of days most of us find pretty miserable. Those cloudy, windy, rainy days of unsettled weather we usually stay in to avoid are a result of low pressure.
Measuring Barometric Pressure
Barometric pressure is normally measured in either millibars (mb) or inches of mercury (inHg). At sea level the standard level of pressure is 1013.25 mb, or 29.92 inHg if using a mercury scale. Therefore, any reading higher than these levels is termed high pressure and anything below is low pressure.
The Effect Of Barometric Pressure On Fish
Fish have organs called swim bladders to provide buoyancy, and it is these organs that detect any atmospheric pressure changes. Swim bladders are air-filled sacs that are important in allowing fish to stay at a certain depth without having to expend energy swimming. The swim bladder inflates or deflates to allow fish to ascend and descend in the water.
Changes in barometric pressure pushing down on the water are detected by the swim bladders. This manifests itself in a level of discomfort within the bladder, which can also affect the balance of the fish. Falling or rising pressure sees the swim bladder inflate or deflate to offset the change and will result in the fish moving to different depths in the water.
Low pressure associated with a weather front is sensed by fish, as the pressure on the swim bladder becomes reduced. The swim bladder will inflate a little to help counter the reduced pressure it is experiencing. This becomes uncomfortable for a fish, akin to humans having an uncomfortably bloated stomach.
The fish responds by searching out waters where the swim bladder will feel balanced again, and where they will feel more comfortable. To relieve discomfort from low pressure, fish will usually head to lower depths where the pressure will be greater, deflating the swim bladder.
Low pressure does not tend to last for too long, and relief arrives for the fish with a return to stable or higher pressure. However, the return of better weather is not an instant cure, as fish can take a couple of days to re-adjust to the rising pressure. Gradually the fish will return closer to the surface and continue their regular feeding habits.
High barometric pressure has the opposite effect, and it causes the swim bladder to deflate slightly. This can cause the fish to swim closer to the surface, where the pressure will be lower, and the swim bladder balances out again.
Fishing The Barometric Conditions
Experienced anglers appreciate how the changing weather seems to impact whether they have a successful day’s fishing or not. Even when all environmental conditions seem the same, sometimes two concurrent days fishing can seem like chalk and cheese. This is where a change in barometric pressure could be playing a role.
If changes in air pressure are causing fish to move to different water depths, then knowing this should put you at an advantage when deciding where to fish. When low pressure forces fish to lower depths you need to fish closer to the bottom of the water. However, as is often the case with fishing, it can be a little bit more complex than this.
Fish On The Drop
When fish relocate to lower depths, they become reluctant to feed, as they are more concerned in relieving the discomfort in the swim bladder than eating. They will also be conserving energy and less interested in chasing after food. The ideal fishing time comes just before the low pressure takes hold.
As the fish begin to sense the falling pressure, they will have a feeding frenzy prior to dropping to lower depths. This is to counteract the fact they understand that eating when the barometric pressure of a weather front is at its lowest point will be a challenge. “Fish on the drop” is a common term given to the act of fishing just before a storm, when the fish are very active.
Once the weather front passes and air pressure starts to rise, the fish will look to move back up closer to the surface again. However, it still may take a day or two for them to return to their normal feeding habits. They will need to re-acclimatize to the increased pressure, and the colder the water temperature the longer they will take to adjust.
Tips On Lures
As well as taking barometric pressure into consideration when finding the fish, it will also influence the choice of the kit you use. When low pressure arrives and the fish are nearer the bottom of the water, you may want to use a slower lure or smaller fly. This can help interest a fish looking to conserve energy by tempting it with a smaller, slower moving bait.
Live bait dangled right in front of fish just off the bottom of the channel can be the best approach. However, this could be switched around during the feeding frenzy of falling pressure. Artificial baits on fast lures are perfectly fine for fish happily looking for any food they can get prior to swimming to lower depths.
Are All Fish Affected?
Different fish species have different sizes of swim bladders, and the size affects the influence barometric pressure has on the fish. Fish with larger swim bladders are most affected by pressure changes. A large swim bladder will inflate more when there is less pressure exerted on it, and this translates to more discomfort for the fish.
If the swim bladder is small, then the body of the fish already has a density similar to the water it is swimming through. Therefore, any changes to barometric pressure tend not to affect fish with small swim bladders as much as fish with large ones. However, if these fish are predators, then they will still follow their prey to lower water depths.
Examples of fish which head for lower depths during periods of low pressure:
The Ideal Barometric Pressure
The barometric pressure for normal fishing conditions is between 29.70 and 30.40 inHg. The mercury measurement is usually the best reference as it is the one most household barometers measure. These are the conditions when you would find fish feeding in their normal way for a particular stretch of water. This is always an ideal time to try new kit and techniques.
However, it may be argued that the ideal barometric pressure is when the readings start to fall just below the baseline level. As we have discussed, this is when fish look to feed heavily, aware of the falling pressure and the need to move to deeper waters where feeding will be harder. Experienced anglers will recognize these conditions and the opportunities they bring.
Weather apps on your phone are handy when fishing, as most contain barometric pressure readings. However, if you have no immediate way to look up pressure measurements, then the old-fashioned approach can still work well. When you start to see the clouds roll in, you knowthere is a good chance the atmospheric pressure is about to drop,and the fish are about tofeed.
Barometric pressure can affect the depth where you find fish. Low pressure will see fish move toward the bottom of the water, where they will feel more comfortable. Here they can equalize the pressure within their swim bladders until higher pressure returns. They are less inclined to feed during spells of low pressure but can feed heavily when the pressure first starts to fall.
Therefore, barometric pressure is an important environmental factor to consider when fishing. Similar to water temperature, wind, currents and light levels, barometric pressure can affect fish feeding patterns. By being aware of which changes in barometric pressure lead to fish moving to different depths, you can better prepare for your next fishing trip.