As anglers, we all know weather patterns can affect our day on the river in many different ways. Whether you’re tracking moon phases, the wind conditions, or cloud coverage for any given day, the elements will always play a role in your fly fishing success. Does one weather pattern affect trout more than another? Depending on who you’re speaking to you’ll get a different answer to this question, but I’m here to tell you, yes it does! Let’s take a quick look at the effects of a change in barometric pressure on trout behavior.
By definition, barometric pressure is the pressure exerted by the weight of the atmosphere on its surface. It will be easier to understand what barometric pressure is if we first look at the difference between high barometric pressure and low pressure.
High barometric pressure is usually associated with fair weather and heavier air. It means there is more air pressure being pushed downward on the water surface. Low barometric pressure is typically associated with the coming of a storm, or the actual storm itself, and a light amount of air pressure being exerted on the river. The reason air pressure drops as a storm moves in is because as the front moves closer to the river the heavy air pressure that is currently being exerted on the river rises through the air column (this means pressure is dropping) and eventually moves into the clouds to help form rain and snow, hence the low pressure on the river.
So how does either high pressure or low barometric pressure affect your fishing day? Well, on their own neither is inherently bad. We’ve all had great days on the river when it’s a blue-bird day with not a cloud in sight when a nymph rig with plenty of weight is your best option—this is characteristic of a high barometric pressure day. But we’ve also certainly had exceptional fly fishing days in overcast weather as a storm is moving in– this would be a low barometric pressure day. As a matter of fact, I’ve had my best fly fishing in low barometric pressure days because this typically means there is a lot of cloud coverage which moves trout into shallower water, there is also a higher concentration of insect activity, and more aggressive trout feeding behavior. Trout are especially aggressive in cloud coverage with low pressure so think about skating an adult caddis across the surface or throwing on a streamer to be stripped through the water for that overly aggressive take.
The one weather situation you do not want to find yourself on the river is directly after the storm passes. This can be characterized as rain and clouds, which then move on to sun and clear skies. It’s also when the barometric pressure rises, meaning from light air pressure to heavy air pressure. The direct change from light air pressure to high and heavy pressure will push fish to the depths of the river and make them lethargic and very tough to catch. To relate it to humans it’s like when we’re flying on an airplane and cannot pop our ears—that’s the same pressure trout feel in the river when a change from light to heavy air pressure occurs. Do you find yourself hungry when you’re uncomfortable and can’t pop your ears? Absolutely not. You’ll find a noticeable change in a trout’s feeding behavior after the storm passes for two reasons: 1) its already gorged itself on food during the low pressure period, and 2) the new high pressure from the atmosphere is making the trout uncomfortable and lethargic so they will avoid your presentation. But like I mentioned earlier, this does not mean trout will not feed when there is a high barometric pressure reading, just simply that the temporary change from low pressure to high atmospheric pressure will turn the trout’s feeding behavior off. The lethargic behavior typically does not last more than 24 hours after the storm passes and thereafter it can be another heavy feeding period.
This brief article is not meant to discourage you from fishing inclement weather, but rather, add another tool to your knowledge bin for your next trip out on the river. Tight lines, my friends.