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 pressure 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 extremely high and 28.5 inHg as an extremely 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 proportion 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 extreme 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 population’s 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.
|Pressure Trend||Typical Weather||Fishing Trends||Suggested Tactics|
|High (30.5 in Hg)||Clear skies||Fish seek shade or cover||Use sub-surface flies such as nymphs or wet flies. Fish close to cover and in deeper water over dark bottom.|
|Rising||Skies clearing||Fish are slightly more inclined to feed||Fish with brighter flies close to cover. Try attractor nymphs or streamers.|
|Stable||Fair||Expect normal feeding behavior||Trout are more inclined to feed both on and below the surface.|
|Falling||Rain Imminent||Often the best time to fish||Try slightly larger flies. Also a good time to fish with larger 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, Sculpins and Crayfish. Also try larger nymphs such as Giant Sone Flies or Helgramites.|
|Low (28.5 in Hg)||Rain and High water||Trout tend to become less active during extended periods of low pressure||Try smaller nymphs and streamers. Change patterns repeatedly.|