Myth-busting: The Barometric Pressure Debate




For as long as Due West Anglers has been writing articles, there have been two white-whale topics nagging us: the influences of moon phase and barometric pressure on fishing success. I've taken deep dives into literature on each topic, learned nothing conclusive, and tabled each topic repeatedly. But the time is finally upon us, and we're goin' myth-busting. Let's look at the Barometric Pressure debate and dive into one of the most enigmatic topics of the modern fishing community. I'll try to give a fair representation to both sides of the argument, but the emphasis will be on which side is supported by facts.


Note: Most of this discussion uses trout as an example species, but this isn't a trout-specific conversation. I tried to interject information about other game fish whenever applicable.

What is Barometric Pressure?

Atmospheric pressure (also known as barometric pressure) is caused from the downward force of air in the atmosphere. Generally, pressure is minimal in the higher atmosphere, and greatest at the Earth's surface. But even at Earth's surface, pockets of high and low pressures can be found, and are caused by heat convection and Earth's rotation. Air wants to flow from areas of high pressure to low pressure. Rising air leads to low pressure at the surface, while sinking air leads to high pressure. These movements of air due to pressure changes create weather. With high pressure cells, descending air suppresses weather development. Usually calm, clear, and sunny conditions are associated with high pressure systems. During low pressure, wind circulates upwards. As air rises, it cools and the moisture held in the air condenses, creating clouds, causing wind and sometimes precipitation. Many anglers believe changes in pressure influence fish behavior, mainly in their feeding aggression. Therefore, theories have been developed about which types of pressure are best for getting a fish to bite.


The Pro Argument

Barometric Pressure Influences Fish Behavior and Feeding

The basic argument for how barometric pressure influences fishing is based on how changes in barometric pressure affect fish physiology, specifically, its swim bladder. Many game fish, including trout, bass, walleye, bonefish, and many others, regulate their buoyancy with an organ similar to an inflated lung, which they manipulate to adjust their water depth. If a fish wants to hold higher in the water column, they may add more gas (typically oxygen if it's filtered from the blood, or gulped air from the surface) to their swim bladder to elevate themselves so they can maintain depth without needing to expend more energy. Likewise, they can expel gas from their swim bladder to sink further in the water column.


Boots Allen discusses effects of barometric pressure in his book "Finding Trout in All Conditions: A Guide to Understanding Nature’s Forces for Better Production on the Water." In his book, Allen claims that fish, like trout and bonefish, have relatively large swim bladders in proportion to their body size. Since a fish's buoyancy is influenced by their swim bladder, external pressure changes (barometric pressure changes) can impact the swim bladder in a way that makes a fish uncomfortable. For example, if pressure drops quickly, the swim bladder suddenly becomes over-inflated and forces the fish to adjust their swim bladder by dumping gas. Allen argues that these barometric pressure changes and the resulting pressure change in a fish's swim bladder can be uncomfortable. For fish with larger swim bladders, the shift puts greater pressure on their surrounding internal organs. This is not to say that the fish is suddenly in peril, but as the fish adjusts to alleviate the pressure, feeding activity drops off. Allen goes on to suggest that when storms approach, trout anticipate the change and feed heavily prior to drops in pressure. He concludes that times of low or falling pressure offer greater opportunities for fishing than during high pressure systems. During high pressure spells, trout are forced deeper in the water column. As a result, fish are less likely to actively feed, chase streamers, or rise for dry flies. Allen's findings are supported by meticulous field-notes and years of records that he uses to corroborate his findings.


Many others agree about the effects of barometric pressure, but feel that the relationship differs from what Allen has described. Keep in mind these different perspectives range between river and lake fishing, and at differing altitudes. Several other sources also prefer falling pressure, but suggest that low pressure cells are most impactful at turning off the bite (Fishing Instinct, 2022; BC Fishn, 2017). Others suggest stable pressure, not drastically rising or falling pressures, as the best time for positive results (FlyCast USA, 2021, Trout ProStore, 2009). Yet, others tout the opposite to be true (FlyGuys, 2011, Fly Fisher Pro, 2022). There are even sources suggesting high pressure is best for fishing, because the increased pressure squeezes the swim bladder tightly and relieves pressure otherwise felt (Slaney Fishing, 2021). Others go one step further, stating that fish in shallow water are more impacted to pressure changes than fish holding in deeper water (In-Fisherman, 2021).


The shear amount of material available regarding barometric effects should highlight how thoughtful the fishing community has been about detailing this phenomenon. Though authors don't clarify their angling settings— lakes vs. stream, or elevations where they've drawn their conclusions (both distinctions play important roles in how barometric pressure manifests)— many draw absolute conclusions. An expansive number of anglers and writers have arrived at a singular agreement: barometric pressure has a role to play in angling success. They would argue that this variable cannot be ignored.


But barometric pressure changes coincide with many other potential variables, including cloud cover, temperature swings, and precipitation to name a few. There is another guild of anglers who dispute the importance of barometric pressure as its own variable.





The Against Argument

Barometric Pressure Is A Red Herring

It's relatively easy to find "sources" explaining how barometric pressure influences fish. Keep in mind, many anglers are hunting for answers, including saltwater anglers, bass anglers, and trout anglers. Results vary widely as different fish species may have different responses to changing pressure. But a key element that all 'Pro' arguments rely on is the assumption that a fish has to withstand some level of discomfort during swim bladder recalibrations when pressure changes.


Certain physiological traits can help clarify these effects. Can fish quickly adjust the pressure in their swim bladders? For fish like walleye and bass, rapid pressure changes (for example catching a fish deep and pulling it up to the surface) has detrimental consequences for the fish (Outdoor Canada, 2014). These fish are phyoclistous, meaning that their swim bladders are not connected to their alimentary canal (digestive tract), which effectively means that they must diffuse oxygen gas from their blood to fill their swim bladders. There's no doubt that being quickly pulled up from the depths has detrimental effects (similar to 'the bends' experienced by scuba divers who resurface too quickly), but subtle barometric pressure changes may require diffusion time to fill or empty a swim bladder.


Alternatively, and quite possibly more easily, a fish could reposition their depth to compensate for any pressure differences. This differs from trout and other physostome fish, where the swim bladder is directly connected to their digestive tracts. Meaning, trout can burp to rapidly ameliorate uncomfortable pressure from within their swim bladder (Trout Unlimited, 2021). This is exemplified by lake trout anglers who pull up Mackinaw from deep water. Often, trout are seen bubbling as they surface, which is the fish's attempts to adjust their swim bladder to a rapid change in pressure as they approach the surface. If a trout can rapidly adjust to changes in pressure from 30+ feet deep, how would minor barometric pressure changes disturb a trout? Unfortunately, no above source (from the pro camp) seems to mention the differences between phyoclists vs physostomes, and failed to do their trout anatomy and physiology homework.


Next, force applied from the pressure of the atmosphere doesn't influence the underwater world the same way that it does over dry land. After all, water is more dense than air (at sea level, ambient air is about 784 times less dense than water). Also, let's remember that the pressure of water exerted on a fish is something that they have evolved to accommodate at every life stage.


Let's compare with some actual numbers. At sea level, the pressure of the atmosphere can be measured at 14.7 pounds per square inch (psi). At higher elevations, this pressure is lessened since the air is thinner. In Denver, at approximately a mile high, the atmosphere has a pressure of about 12 psi. Therefore, it takes an additional 5280 vertical feet to remove 2.7 psi of pressure. Now from Denver, find the nearest stream and drop a probe 3.25 feet deep underwater. The pressure increases to 1.425 psi. Now drop the same probe to 6.5 feet. The pressure increases to 2.85 psi. That pressure gain (2.85 psi) in those 6.5 feet is now comparable to the difference from sea level to 1 mile high on land. Imagine a fish rising from 6.5 feet deep in a water column, to the surface, to feed on an insect. The fish would experience a change in 2.85 total psi as part of a routine feeding excursion (Engineering Toolbox, 2010; Baxter, 2021).


We can also read barometers in pounds per square inch (psi). The entire scale of a barometer ranges from 13.75 psi (traditionally read as 28.00 inches Hg) to 15.34 psi (traditionally read as 31.25 inches Hg). The entire barometric pressure range is around 1.59 psi (Engineering Toolbox, 2010-2). When we compare this range against the pressure change a fish experiences when rising to the surface from 6.5 feet deep, the fish will experience 1.8 times more pressure than if the barometer rapidly swings from the lowest possible reading to the highest possible reading. Therefore, predictable weather-associated changes in barometric pressure simply don't add the additional pressure many think.


But what about a phyoclistous bass living in a lake? Without the ability to rapidly adjust their swim bladder, pressure changes should surely influence their comfort, right? Physics says otherwise. Water is considered uncompressible, meaning without extreme force exerted upon it, water's density and the pressure experienced underwater remain undisturbed. The subtle world of barometric pressure changes don't exert enough force to overcome the physical properties of water (Satinover, 2017).


Another hypothetical for your consideration: What about a saltwater fish swimming along a beach as routine waves move overhead? That fish would experience an increase in pressure for each wave crashing, and then a release of pressure between waves (Ross, 2018-2). I don't think many anglers would appreciate the pressure changes that this fish experiences as it swims along a beach, but when a storm appears, suddenly atmospheric pressure becomes important?


There are other issues at play that confuse the argument in support of an influence of barometric pressure. Barometric pressure is one of many variables that (may?) have an influence, but has been mistakenly interpreted independently from the other important factors and given a greater importance (Ross, 2018). Though related, this angler doesn't believe barometric pressure to be as predictive as overcast skies and cloud cover when it comes to influencing a trout's behavior. Often, falling pressure is associated with increase feeding… but precipitation, and cloud cover are also associated with that same falling pressure. Moist conditions are often considered favorable for insect emergence, as the added moisture increases an emerging insect's ability to shed it's nymphal husk, and also corresponds with light conditions widely regarded as more favorable for trout feeding (compared to bright and sunny conditions). Short-lived mayfly adults may have extended lives during moist conditions too, prolonged from eventual dehydration which introduces extended trout feeding opportunities (Allen, 2016).


One final consideration: very little scientific research has specifically targeted the question at hand, "How does barometric pressure affect feeding behaviors in fish?" Only one study has been designed to assess this question. The author examined the barometric effects of feeding activity in yellow perch (a phyoclistous fish) and found no significant relationship between barometric pressure and feeding behavior (VanderWeyst, 2014).





Conclusions

There are so many factors at play in this conversation, we definitely didn't discuss every line of argument. We simply highlighted the most convincing evidence. Based on our findings, the barometer isn't something you need to track. That being said, if there are more compelling arguments that become available, we'd love to continue this discussion. After all, this is how the scientific method uncovers important answers. We ask questions, form hypotheses, test said hypotheses, analyze the results and discuss whether the findings corroborate or dispel our hypotheses. Then we call on other knowledgeable folks (such as yourselves) to review the evidence.


I hope you enjoyed diving into this puzzling topic as much as we did. Let us know what you think by leaving a comment below, or DM us on Twitter or Instagram (@DueWestAnglers). Stay tuned for the lunar version down the road. As always, sources are listed below.







Sources:

  1. Allen, B. 2016. Finding Trout in All Conditions: A Guide to Understanding Nature’s Forces for Better Production on the Water. WestWinds Press.

  2. Fishing Instinct. 2022. The Best Barometric Pressure for Trout. https://fishinginstinct.com/best-barometric-pressure-for-trout/#:~:text=The%20best%20barometric%20pressure%20while,is%20recorded%20as%2029.60%20atm.

  3. BC Fishn. 2017. How Barometric Pressure Affects Fishing https://www.bcfishn.com/barometricpressure/

  4. FlyCast USA. 2021. How Barometric Pressure Affects Trout Feeding & Behavior. https://www.fishingthegoodfight.org/post/how-barometric-pressure-affects-trout-feeding-behavior

  5. Trout ProStore. 2009. How Weather Affects Trout Fishing. https://www.troutprostore.com/class/how_weather_affects_trout_fishing

  6. Fly Guys. 2011. Barometric Pressure & Fishing. https://flyguys.net/fishing-information/still-water-fly-fishing/affect-of-barometric-pressure-on-fishing

  7. Fly Fisher Pro. 2022. Barometric Pressure and Fishing. https://flyfisherpro.com/tips/barometric-pressure-and-fishing/

  8. Slaney Fishing. 2021. Barometric Pressure – A fishy myth or a an untapped resource? https://slaneyfishing.com/barometric-pressure-fishy-myth-untapped-resource/

  9. In-Fisherman. 2021. Understanding Barometric Pressure. https://www.in-fisherman.com/editorial/understanding-barometric-pressure/154358

  10. Outdoor Canada. 2014. Are Lake Trout Releasable? https://www.outdoorcanada.ca/are-lake-trout-releasable/

  11. Trout Unlimited. 2021. Burp! Please excuse my swim bladder. https://www.tu.org/magazine/community/youth/trout-in-the-classroom-youth/burp-please-excuse-my-swim-bladder/

  12. Midcurrent. 2014. The Pressure Myth. https://midcurrent.com/science/the-pressure-myth/

  13. Engineering Toolbox. 2010. Hydrostatic Pressure vs Depth. https://www.engineeringtoolbox.com/hydrostatic-pressure-water-d_1632.html

  14. Engineering Toolbox. 2010-2. Barometric Pressure - Pressure in psi, psf and kPa. https://www.engineeringtoolbox.com/barometric-pressure-psi-psf-d_1618.html

  15. Satinover, S. 2017. Ask A Scientist: How do changes in atmospheric pressure affect aquatic life? https://www.utdailybeacon.com/opinion/ask-a-scientist-how-do-changes-in-atmospheric-pressure-affect/article_fe8a282a-b8d2-11e7-9a4f-f7aeb500cf21.html

  16. VanderWeyst, D. 2014. The Effect of Barometric Pressure on Feeding Activity of Yellow Perch. Bemidji State University. https://www.bemidjistate.edu/directory/wp-content/uploads/sites/16/2014/10/2014-VanderWeyst-D_-The-effect-of-barometric-pressure-on-feeding-activity-of-yellow-perch.pdf

  17. Huk Gear. 2021. Barometric Pressure and Fishing. How Weather Affects Fishing. https://www.hukgear.com/blogs/news/barometric-pressure-fishing-weather-affects-fish

  18. Ross, D. 2018. The Truth About Barometric Pressure. https://www.thefisherman.com/article/the-truth-about-barometric-pressure/#close-modal

  19. Ross, D. 2018-2. Effects of storms, barometric pressure...if any...on fishing. https://www.carolinacoastonline.com/news_times/sports/article_046f75a0-699b-11e8-8ff7-bf1cb4994b7f.html

  20. Baxter, R. 2021. Does atmospheric pressure add to water pressure when calculating underwater pressure? https://www.quora.com/Does-atmospheric-pressure-add-to-water-pressure-when-calculating-underwater-pressure