How Barometric Pressure Affects Trout Feeding & Behavior

Written By: FlyCastUSA.com

As anglers, we’ve all had days when you catch so many trout that you lose count and days when no matter what you try, you can’t seem to get their attention. This phenomenon sometimes feels unexplainable but we’ve learned over the years that paying close attention to weather conditions and barometric pressure can help you foresee and plan for those difficult days.

Caddis Larva were the ticket on this low pressure day.

Through our on-water experience and research, we believe that barometric pressure (aka air pressure or atmospheric pressure) has a big impact on trout behavior and ultimately, fishing productivity. Like sky conditions, barometric pressure can directly impact a trout’s holding position and behavior. Before we break down the behavioral changes associated with the various levels of barometric pressure, let’s discuss what it is.

Simply put, barometric pressure is the weight or pressure applied to the earth’s surface by the atmosphere. As humans, we often notice a change in barometric pressure when our ears plug or pop while flying in an airplane or driving through areas with rapid changes in elevation. Less obvious, is how barometric pressure fluctuates on a daily basis. While we may not feel this change, trout are very aware. Trout have large air bladders that are sensitive to changes in pressure. Pressure on their bladder will encourage them to adjust depths in the river to relieve or neutralize the pressure. Another key component to barometric pressure is its correlation with weather. In general, high pressure is associated with mild conditions and sunny skies while low pressure is associated with storms and clouds. Understanding the impact barometric pressure has on trout air bladders and the corresponding weather conditions, we can predict trout behavior and determine the ideal fishing strategy.

In general, there are three levels of barometric pressure (high, neutral and low) to keep an eye on as it relates to trout behavior and holding position. And while there is a fair amount of overlap in terms of the impact associated with weather and air pressure, the following breakdown will provide you with the basics and what it means from a fishing perspective.

Low Pressure (< 29.90 inHG): When low pressure moves in, trout air bladders expand and create discomfort. To counter this, trout will move to the deepest water column to neutralize their bladder. Think back to our example of your ears plugging at higher elevations. No one likes this feeling and ideally, assuming we have the option, we would seek out lower elevation in order to relieve the pressure that has built up in our ears. Additionally, a drop in pressure will also alert trout that a storm is moving in. To prepare for this discomfort and harsher conditions, trout will aggressively feed.

Fishing Strategy: Heavy nymph rigs and streamers are ideal setups as low pressure moves in. When nymphing, a slow and deep drift is ideal. Similar to high pressure, trout will be more selective, but rather than shiny patterns, large offerings followed by a small larva/emerger pattern is ideal. As clouds roll in with the storm, trout will feel a sense of protection and be more willing to move around the water to chase meat. Streamers are a great way to take advantage of this and their aggressive attitude.

Fooled in Fast Riffles on a High Pressure Day

High Pressure (> 30.20 inHG): While we do receive both high and low pressure storms, high pressure is more often associated with mild temperatures and blue bird skies. As anglers, when we see high pressure in the forecast, we expect trout to hold at varying depths and gravitate towards structure. Trout will adjust their depth until they find the optimal amount of pressure to neutralize their bladder. However, as we mentioned earlier, high air pressure often correlates with sun exposure which can contradict this holding position theory. In this case, trout will seek protection from airborne predators and hold in areas with structure and/or in the depths of the river, despite bladder comfort. That being said, riffles, faster runs and pockets offer decent coverage, making those sections viable holding positions as well.

Fishing Strategy: Nymphing is typically our go-to strategy when it comes to high pressure, sunny days. Using the sun to your advantage and sight fishing is important because it tells you which water column trout are holding. A heavy nymph rig is usually our default setting until we observe trout suspended in the water. At that point, we will decrease our weight to drift our flies in a higher water column. Fly selection is also important. Considering trout are preoccupied with trying to find their ideal depth, they aren’t as concerned about feeding and will be more selective. We’ve found that small and shiny patterns are the way to go in this situation. Lead with an attractor such as a Rainbow Warrior and trail a smaller pattern with a little flash below it that’s consistent with current hatches. During the spring, a Sparkle Wing RS2 or Flashback Pheasant Tail are two great options.

In general, trout like consistency. Consistent flows and weather allow them to normalize to their habitat and get into a routine. It’s drastic changes to the weather, flows and barometric pressure that really alter trout behavior and present tough days on the river. When you see that the forecast calls for a spike or drop in pressure, be prepared to make the changes we discussed above.