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Home Barometric Pressure


How Barometric Pressure Affects Trout Feeding Behavior


Illustration of a swim bladder

 The Swim Bladder





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     When it comes to fishing, most people don't give the weather a thought other than for their own comfort. But, the same weather that makes us miserable also makes trout uncomfortable and therefore, knowing both the recent and present Barometric Pressure in your locale can help you to pick a good day to go fishing. The reason this information is helpful has to do with the relationship between the atmospheric pressure and the trout's Swim Bladder (buoyancy compensator). FYI, the amount of pressure that the Earth’s atmosphere exerts as it presses down on the Earth’s surface is measured in units called either "millibars" (mb) or "inches of Mercury" (inHg) and changes in this atmospheric pressure (also called barometric pressure) cause the ambient pressue on a trout's swim bladder to increase or decrease accordingly which can make the trout uncomfortable. So, as a general guideline for the continental United States, think of 30 inHg (1016 mb) to be a normal level with 30.5 inHg as an extreme high and 28.5 inHg as an extreme low. Thus, because a higher or lower barometric pressure exerts more or less pressure respectively upon the surface of the water, the water pressure also changes in direct proprotion to the changes in the atmospheric pressure and therefore, fish feel these changes in their swim bladder. Consequently, a rapid rise or fall in the barometric pressure or an extended period of either extreame high or extreme low barometric pressure can make a huge difference in the quality of the fishing on any given day. Consequently, a slight change +/- of just 0.02 inHg is enough to positively or adversely affect a trout populations feeding habits. Therefore, it is very important for a fly angler to be aware of the Barometric Pressure in their locale and how it affects both the weather and the fish’s habits and to use that knowledge to their advantage.


Chart Showing The Effects of Barometric Pressure on Trout


Pressure Trend Typical Weather Fishing Trends Suggested Tactics


(30.5 inHg)

Clear Skies Fish seek shade or cover

Use sub-surface flies such as nympsh or wet flies. Fish close to cover and in deeper water over a dark bottom.

Rising Skies clearing Fish are slightly more inclined to feed Fish with brighter flies close to cover. Try Attractor nymphs or streamers.



Expect normal feeding behavior Trout are more inclined to feed both on and below the surface.

Rain Immenent

Often the best time to fish Try slightly larger flies. Also a good time to with large nymphs and streamers

Slightly Lower

Rain starts

Bait fish seek shelter in the shallows.

Bigger fish come out to hunt.

Try larger streamer patterns such as Dace, Sclupins, and Crayfish.

Also try larger nymphs such as Giant Stone Flies or Helgramites


(28.5 inHg)

Rain and high water

Trout tend to become less active during extened periods of low pressure

                  Try smaller nymphs and streamers.

                        Change patterns repeatedly.




     So, the easiest way to remember how changes in barometric pressure affect your ability to catch fish is like this: a steady barometric pressure around 30 inHg is good while either an extremely high or an extremely low barometric pressure is bad. However, a falling barometric pressure is better than a rising barometric pressure.




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