Pressure Defined
In a nutshell, barometric pressure – also called atmospheric or air pressure – is the weight of the air pressing down upon everything on the planet, including fish and anglers. Lest you think such a load is light as a feather, consider that a square-inch column of air rising from sea level to the top of our atmosphere weighs about 14.7 pounds.
If you’re higher than sea level, say on a fishing trip to Denver, Colorado, then you’d only shoulder about 85 percent of the burden. Nevertheless, air has mass. We don’t feel it because we’re used to the pressure, but air pressure still affects food chains above and beneath the waves. And if you multiply all of those little inch-by-inch squares on the surface of your favorite fishing hole, you can begin to appreciate the pressure fish are under. Of course, like us, fish are built to handle this pressure or they’d collapse like the Minnesota Vikings in the playoffs.
Along with altitude, high- and low-pressure weather systems also affect the barometer. There are also less-pronounced, twice-daily fluctuations due to heating from the sun, which peak around 10 a.m. and 10 p.m., and bottom out at 4 a.m. and 4 p.m.
Even slight changes in barometric pressure can cause big variations in fish behavior. One of the main reasons is that everything in the water sinks, suspends, or floats to the top. Changes in pressure act like minor changes in gravity, upsetting this delicate balancing act on a regular basis. Such rises and falls are compounded because objects weigh less underwater, which makes them more prone to ups and downs.
Opportunity Knocks
Fish are far more in tune with their environment than most anglers realize. Armed with an amazing array of pressure-sensing systems, such as the lateral line (a sensory organ found in dis that is used to detect movement and vibration in water), they’re easily able to pick up on slight changes in the atmosphere.
How game fish, such as largemouth bass, walleyes, or crappies react to these variations often hinges on what affect pressure changes – along with accompanying weather-related fluctuations in water temperature, light penetration, and other factors – have on their food supply and the world around them.
Think of it as a chain reaction. For example, even a slight drop in pressure can cause tiny particles of sediment and other material to float up off the bottom or rise higher in the water column than normal — especially if there’s increased current to help carry them. Such uprisings can slightly cloud the water, reducing visibility. But the effects of a pressure drop don’t end there.
Zooplankton and phytoplankton are on the building block end of the food web. Most can regulate their buoyancy via built-in mechanisms, like dainty little air bladders or by retaining air. But these tiny creatures can be caught off guard by quick changes in barometric pressure, pushing them out of their comfort zones and sparking a feeding spree among baitfish ranging from fatheads to shiners. It doesn’t take long for bigger fish such as crappies, bass, or northern pike to join the fray and feast on the schools of hungry minnows.
In fact, many game fish respond to pressure upticks and downturns by strapping on the feedbag. It makes sense because they’re designed to handle far greater depth-related pressure variations – which they routinely encounter when moving up and down the water column pursuing prey.
For example, a bass suspended 10 feet deep won’t be neutrally buoyant if it swims up to eyeball a frog at the surface – it will float. So it has to adjust on the fly or quickly return to deep water. If you’ve ever jumped into deep water and felt your ears pop, you can appreciate what predators deal with on a daily basis. This explains why small air pressure changes are of little physical significance to larger fish, other than signaling increased feeding opportunities and perhaps the approach of foul weather.
It also helps explain why a rising or falling barometer often coincides with fine fishing. To capitalize on a change, anglers should target prime feeding areas with aggressive presentations, and save the finesse for later.
On the walleye front, for example, you’re wise to stow the dainty jigs and live-bait rigs in favor of paddle-tail swimbaits, crankbaits and large-bladed spinner rigs. On the bass side, thrashing a topwater over feeding flats or ripping broad-shouldered jerkbaits along weedlines can trigger vicious strikes.
As you plan your tactics, keep in mind that fish in shallow water are more susceptible to air pressure changes than their deep-water kin. So, all other things being equal, a bass tucked into three feet of salad is more apt to feel the effects of a moving barometer than a trout lurking in the abyss.
Fair Weather
While much has been said about barometric pressure changes, it’s worth noting that some of the most reliable fishing action occurs when the pressure has been steady for several days or more.
Extended periods of fair weather allow fish to find the best blend of ideal water temperature, dissolved oxygen levels, light penetration, and other factors, and then fall into predictable feeding routines.
Buzz Killers
If the barometer has a dark side, it’s that a moving needle often foretells coming weather systems that adversely affect fishing. While pressure changes can trigger fish activity, low-pressure systems, like major cold fronts, can shut it down.
The reason fish get lockjaw has more to do with the after-effects of the weather change than the actual air pressure, however. For example, when a spring cold front cools down water temperatures in a shallow bay where panfish were feeding, the bite grinds to a halt. The fish may even evacuate, heading for deep water nearby.
Not all cold fronts are buzz killers, however. During late summer, a front that drops the surface temperature a few degrees can reinvigorate sluggish, dog-days walleyes, causing a bump in feeding activity.
In the end, the key to consistent catches is understanding how barometric pressure and other mitigating factors – both natural and human-related – affect fish behavior on your favorite lakes, and adjusting your fishing tactics to match.
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