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How Barometric Pressure Relates to Fish Feeding in Lakes and Rivers
As fly anglers, we have long observed that fish activity fluctuates based on weather conditions, and one of the most significant yet least understood factors influencing fish behaviour is barometric pressure.
Barometric pressure, also known as atmospheric pressure, refers to the force exerted by the atmosphere on the Earth’s surface. It varies with altitude and weather systems, directly affecting many natural processes in both terrestrial and aquatic environments. When it comes to fishing in lakes and rivers, barometric pressure can be a key determinant of when fish are likely to be feeding, impacting catch rates and fishing strategies. This article dives deep into the relationship between barometric pressure and fish feeding behaviour, explaining how changes in pressure affect fish physiology, movement, and feeding patterns.
Understanding Barometric Pressure
Barometric pressure is the weight of the air in the atmosphere pressing down on the Earth. It is measured in units of millibars (mb) or inches of mercury (Hg), and at sea level, the average barometric pressure is around 1013.25 mb or 29.92 inches Hg. Barometric pressure is not static; it fluctuates due to various weather patterns. These fluctuations are linked to weather changes, such as approaching storms, clear skies, or shifts between high- and low-pressure systems.
The relationship between barometric pressure and weather conditions is a key factor for fly anglers. High pressure is generally associated with clear, calm weather, while low pressure is often linked with stormy or unsettled weather. However, it is not just the weather itself but the changes in pressure that have a profound impact on fish behaviour.
Barometric Pressure and Fish Physiology
To understand how barometric pressure influences fish feeding behaviour, it’s important to first look at how pressure changes affect the internal physiology of fish. Fish are highly sensitive to changes in their environment, including pressure. Many fish possess a swim bladder, a gas-filled organ that helps them regulate their buoyancy. The swim bladder expands or contracts in response to changes in atmospheric pressure, allowing fish to move easily through different depths of water.
During periods of high barometric pressure, the increased weight of the atmosphere can cause the swim bladder to compress slightly, allowing the fish to remain at comfortable depths without exerting much energy. However, when the pressure drops, as it does when a storm or low-pressure system approaches, the swim bladder expands, which can cause discomfort and require the fish to adjust their depth more frequently. This discomfort leads to changes in fish behaviour, including feeding patterns.
High Pressure and Fish Behaviour
When the barometric pressure is high and stable, weather conditions are typically clear, with calm winds and sunny skies. Fish tend to be less active during these times because they are comfortable in their current depth zones and do not need to expend energy adjusting to pressure changes. While fish may still feed during high-pressure periods, they are generally more lethargic and less likely to chase after food.
In lakes, fish tend to stay in deeper waters where temperatures are cooler, and they will only occasionally move into shallower areas to feed. Anglers may find that during these high-pressure periods, fish are more selective and require a specific presentation to trigger a bite, i.e. something they do not have to chase and is presented directly in front of them. In rivers, the flowing water can still encourage some movement, but fish tend to remain closer to cover, where they feel secure and exert less energy.
While high-pressure periods may not be ideal for catching actively feeding fish, it is not impossible to have success. Anglers who fish in deeper areas or focus on structure like submerged logs, rocks, or weed beds may still find fish that are opportunistically feeding. Slower presentations with jigs or light lures may work better in these conditions.
Low Pressure and Fish Feeding Frenzy
Low pressure, on the other hand, tends to create much more favourable conditions for fish feeding activity. When barometric pressure drops, fish become more active, moving throughout the water column and feeding aggressively. This behaviour is driven in part by the fact that fish anticipate the arrival of a storm or other adverse weather conditions, which can disrupt their normal feeding patterns. In anticipation of these changes, fish take advantage of the more stable conditions before the storm hits to feed heavily and store energy.
Low-pressure systems are usually associated with cloudy skies, stronger winds, and increased humidity—all factors that can improve fishing conditions in lakes and rivers. Cloud cover reduces the amount of sunlight penetrating the water, making fish feel safer in shallower waters where they can feed more freely without worrying about predators. Wind can also help break up the surface of the water, reducing visibility and allowing fish to move more freely without being easily spotted.
Anglers often report that just before a storm or significant drop in pressure, fish feeding activity spikes. Fish, including species like pike are known to feed aggressively during these periods, and they may be more likely to strike at larger lures that they might otherwise ignore. This is an excellent time for anglers to be on the water, as the increased fish activity can lead to higher catch rates.
Rapid Pressure Changes and Their Impact
While stable high or low-pressure systems have predictable effects on fish behaviour, rapid changes in barometric pressure can cause more dramatic shifts in feeding patterns. When the pressure changes suddenly—either rising or falling—fish may become disoriented or uncomfortable, leading to a period of inactivity. This is especially true for species that rely heavily on their swim bladders to maintain buoyancy, as rapid pressure changes require them to constantly adjust the amount of gas in their bladders.
After a sudden drop in pressure, fish may need time to adjust to the new conditions, during which they may stop feeding altogether. This phenomenon is often referred to as the “post-front blues,” where fishing becomes notably slow following the passing of a low-pressure system. Fish tend to move to deeper waters where pressure changes are less noticeable and may not feed for several hours or even days.
Conversely, a rapid increase in pressure can also cause fish to become lethargic and less interested in feeding. High-pressure systems that move in quickly after a storm can create conditions where fish retreat to deeper, cooler water and become less responsive to any fly pattern presented to them. Anglers should be aware of these rapid pressure shifts and adjust their fishing tactics accordingly, i.e. time to stop for a cup of coffee!
Seasonal Variations in Barometric Pressure and Fish Feeding
Barometric pressure fluctuations affect fish behaviour year-round, but their impact can vary depending on the season. In spring and autumn, when weather systems are more volatile, pressure changes are more frequent and pronounced. These periods often coincide with increased fish activity, as fish are either coming out of their winter dormancy or preparing for the colder months ahead.
In the spring, rising temperatures and fluctuating pressure systems encourage fish to move into shallower waters to feed and spawn. Low-pressure systems during this time can trigger feeding frenzies, as fish take advantage of the warmer water and abundant food supply. Anglers can expect more active fish during these transitional periods, particularly before and after storms.
Autumn is another prime time for fishing, as fish begin to bulk up for the winter months. During this period, low-pressure systems can again trigger aggressive feeding behaviour, especially as fish prepare for the colder water temperatures. In both spring and autumn, fishing just before a front or a period of low pressure can lead to some of the best fishing days of the year.
In summer, when high-pressure systems are more common, fish tend to retreat to deeper, cooler waters where they can avoid the heat and conserve energy. Fishing can be more challenging during these periods, as fish are less likely to feed actively. However, during the hot summer months, a sudden drop in pressure due to a storm can create a brief window of increased fish activity, offering anglers a chance to capitalize on the conditions.
Winter brings its own set of challenges for anglers, as cold temperatures and ice cover can limit access to fish. However, barometric pressure still plays a role in winter fishing, particularly for species like trout that remain active under the ice. Low-pressure systems in winter can create favourable conditions for ice fishing, as fish may move into shallower waters to feed more actively.
Using Barometric Pressure to Your Advantage
Understanding the relationship between barometric pressure and fish feeding behaviour can give fly anglers a significant edge when planning fishing trips. By monitoring changes in pressure, we can predict when fish are most likely to be active and adjust our strategies accordingly. There are several tools available, including weather apps and barometers, that can help us track pressure changes in real time.
- Fishing During Stable High Pressure: When barometric pressure is high and stable, fish are likely to be more lethargic and less willing to chase after a fly. In these conditions, anglers should focus on slow, methodical presentations – i.e. dry fly. Fishing deeper water with nymphs or targeting structure where fish are holding can increase the chances of success.
- Fishing During Falling Pressure: As pressure begins to drop ahead of a storm, fish activity typically increases. This is the time to be on the water, as fish are more likely to feed aggressively. Anglers should consider using fast-moving lures, to take advantage of the heightened fish activity. Shallow waters and areas with cover are also good spots to target during falling pressure.
- Fishing During Low Pressure: When the barometric pressure is low, fish are at their most active, especially in the hours just before a storm. This is an excellent time to use a variety of lures and imitative fly patterns, as fish are less selective and more likely to strike at anything that resembles food. Anglers should be prepared to move around the water body, as fish may be feeding at different depths.
- Post-Front Fishing: After a storm passes and the pressure begins to rise, fishing can become very challenging. Fish may become inactive and retreat to deeper waters, where they feel more comfortable and are less likely to feed. In these conditions, it’s best to use subtle presentations and focus on areas of the water body that offer cover or structure.
Barometric pressure is a powerful force that influences fish behaviour in lakes and rivers. By understanding how pressure changes affect fish physiology and feeding patterns, anglers can make more informed decisions about when and where to fish. Whether fishing during periods of high pressure or taking advantage of low-pressure feeding frenzies, paying attention to barometric pressure can improve catch rates and lead to more successful fishing trips.
