Thursday, December 20, 2012

Watch Your inHg

Hey Everyone,

If you're a dedicated trout bum like myself you watch the weather a lot. I don't mean to make sure you wont be rained on or to see if you'll be able to feel your fingers that morning. If you know me then you know I don't care about a little rain and a cold day to me means no body else on the water. I watch the weather to see what it is doing. See knowing what the weather has been doing and what it is going to do tells me a lot about what the fishing is going to be like for the day. See as the weather changes so does the barometric pressure. How does this affect the fishing? A trout has a small bladder called a swim bladder which acts as a buoyancy compensator. As the barometric pressure changes trout can feel those changes on their swim bladder. Think of a time that you ate way too much and felt like crap all day. The average measurement for atmospheric pressure is around 30 inHg (inches of Mercury) with 30.5 being an extreme high and 28.5 being an extreme low. Consequently, a rapid rise or fall or and extended period of extreme high or low can make a difference in the quality of fishing on any given day. A slight change of just +/- 0.02 inHg is enough to affect a trouts eating habits. Having flash backs of high school biology? Here is a easy chart that I think will help.

Pressure Trend Typical Weather Fishing Trends Suggested Tactics
(30.5 inHg)
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 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 Imminent
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, Sculpins, 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 extended periods of low pressure
Try smaller nymphs and streamers.

I hope this helps.

See you on the trail,




  1. Right on lance. 29.92 is normal sea level pressure and it goes up from there. Pressure definitely plays a factor. I got a barometer just for that purpose. Nice post.

  2. I love these posts Lance. They are very interesting and give info I've always wanted to know more about. I like your chart with suggested tactics. I'm working on something I hope will be just as usefull. There isn't much information about fishing around the triangle. I working on a chart to show what species start biting during certain times of the year. That why no matter when it is you have something to fish for. I'm also working on what flies to use and tactics. I'm just not sure how much fishing location info I want to devulge.