[bigoutdoors]

Under Pressure
A biologist’s guide to how barometric pressure affects fish and fishing
By: Dan Johnson






Have you ever caught fish like an all-star one day and bombed the next, even though you were using the same tactics and fishing the same area? You’re not alone. It’s a common and often frustrating fact of life on the water. But what makes a red-hot bite turn to ice?

Truth be told, many factors may contribute to the malaise. Fishing pressure, boat traffic and weather-related changes in water conditions can all cause forage species and predators to alter their locations and behavior, causing a productive fishing pattern to fall apart faster than you can say, “Should have been here yesterday.”

But change doesn’t have to be a bad thing, especially if you understand what’s going on in that mysterious, watery realm beneath the waves.

Pressure Player
NAFC member Roger Hugill—a seasoned fisheries biologist and tournament angler—says one of the least-understood factors affecting fish and fishing is barometric pressure. It may get a little ink or lip service now and then, but few people do more than scratch the surface of this important piece of the fish-behavior puzzle.

“Everything in the water either sinks, floats to the surface or suspends,” says Hugill. “Few anglers give it much thought, but a change in barometric pressure is to a small degree like a change in gravity.”

Plus, because objects weigh less in the water, the affect of a pressure change is far more pronounced beneath the surface than above.

Here’s how it works. In simple terms, barometric pressure—or atmospheric pressure—is the weight of the air pressing down upon us. Don’t laugh. It can be a heavy weight to shoulder.

A 1-inch-square column of air, measured from sea level to the top of the atmosphere, would weigh about 14.7 pounds. Multiply that by the surface of your favorite fishing hole and you’ve got serious pressure. So much for lighter than air.

Fish, anglers and other living creatures are built to handle this pressure, or we’d collapse like the Vikings in the playoffs. But changes in pressure can ignite major shifts in fish behavior. Understanding all the dynamics is the key to playing this wild card to your advantage.

During his 33-year career with Minnesota’s Department of Natural Resources Fisheries Section, Hugill has learned more than a few fascinating facts about fish. One is that they react to pressure changes more than most fishermen realize.

“Fish are extremely in tune with their environment,” he explains. “They have an incredible array of pressure-sensing systems—such as the lateral line—that key them in to changes in barometric pressure, which in turn could signal feeding opportunities or foretell the arrival of a major weather change.”

How gamefish react depends on what affect variations in pressure (and accompanying factors such as fluctuations in water temperature due to a warm or cold front) are having on their food supply and the world around them.

For example, a drop in pressure can cause tiny particles of sediment and other material to float off bottom or rise higher in the water column than they normally suspend—particularly when currents are involved—slightly reducing water clarity. But more importantly, it can affect tiny creatures such as zooplankton and phytoplankton—the building blocks of any respectable aquatic food chain.

“These organisms need to move up and down in the water column in response to changes in light intensity and other factors, so they have built-in mechanisms for maintaining buoyancy,” says Hugill.

Some have tiny air bladders. Others possess the ability to retain air as a means to regulate their position in the water. “They’re generally able to adjust to variations in barometric pressure, but a fast change can catch them off guard, making them slightly unstable.”

This can push algae, phytoplankton or zooplankton out of its comfort zone and make it more vulnerable to predators. In some cases, gamefish such as crappies may move in to feed on zooplankton, but often a parade of forage species ranging from bloaters to shiners and dace—depending on the fishery—may also show up to feast on destabilized prey. Larger predators follow to sample the baitfish buffet.

Most catchable-size fish aren’t phased by the change in pressure. If anything, they’re stoked by it. “The physical affect on bigger fish is less pronounced,” Hugill explains. “Bass, walleyes, pike and other larger fish are built to handle it, and the changes in pressure are small compared to their overall size, mass and ability to swim.”

Plus, these fish are used to adjusting to depth-related pressure changes as they travel up and down in the water column. If you’ve ever jumped into a lake or pool and had your ears “pop,” you know that pressure is greater the deeper you go.

“If a fish is neutrally buoyant three feet beneath the surface, then swims down to 10 feet it won’t suspend anymore; it will sink—so it has to adjust,” Hugill says. Compared to these depth-related pressure changes, a slight rise or fall in the barometer is easy for a bass or walleye to handle.”

All of this helps explain why a rising or falling barometer often signals good fishing. Now is the time to be on the water, fishing known feeding areas with aggressive tactics.

If you’re targeting largemouth bass on a weed-rimmed natural lake in summer, try raking the surface of feeding flats with fast-moving topwaters, or rip jerkbaits along deep and shallow weed edges.

Walleye anglers would do well to switch from snail’s pace techniques like dragging live bait behind walking sinkers to trolling high-action cranks or spinner rigs with beefy blades. In short, forget finesse—this is the time for action.

It’s worth noting that fish and other creatures living in shallow water are more susceptible to the affects of changes in atmospheric pressure than their deep-water counterparts. So a bluegill holding in five feet of water is more susceptible to changes than a lake trout 100 feet down.


The Down Side
Right now every experienced angler reading this is probably thinking, “If a rise or fall in barometric pressure heats up the fishing, why does the action go south when a low-pressure system—especially a dreaded cold front—passes through?”

That’s a good question.

The answer has more to do with the after-effects of the weather system than changes in pressure. “Falling barometric pressure makes tiny aquatic creatures unstable—which can make baitfish more active and trigger a flurry of activity in the entire food chain. It also alerts fish to the approach of a weather change,” says Hugill. “Bass, walleyes and other gamefish often react by feeding before the rain or storm arrives.

“But once the weather changes, its effects trump lingering variations in atmospheric pressure,” he continues. “For example, in the spring, when a cold front arrives and air temperatures drop, strong winds can push cold surface water into a warm bay where fish are active. The water temperature will drop and fish become less active.”

Indeed, fish may pull out of chilled shallows to sulk along the nearest drop-off or in the closest deep hole. After the front passes and the water begins to warm again, they will move back in and become more active.

“On the other hand,” he notes, “a cold front in August can trigger an increase in fish activity. I’ve seen sluggish walleyes on mid-lake reefs come alive when a cool front lowered the surface temperature by even a few degrees.”

Some fisheries are more vulnerable than others to rapid water temperature changes. “Rivers can really change a lot, up to 8 or 9 degrees a day,” Hugill points out. “Small feeder streams can change even more.”

Other factors besides water temperature can be involved, of course. Light intensity—due to wave action, cloud cover or water clarity—can have a huge impact on the feeding behavior of species like bass and walleyes.

And as many NAFC members know well, the amount of light passing through the water can also affect how baitfish and predators position themselves in relation to cover or structure. “When conditions are right (such as when the water is clear and warm), cloud cover can pull fish into the shallows,” Hugill says.

“In the end, the key to success is figuring out what the weather is doing to the area of a lake or river you’re fishing, then alter your fishing tactics accordingly. A drop in water temperature may call for smaller baits and slower presentations, or seeking out areas where the fish are less affected.”

Stable Pressure
We’ve talked much about the effects of change, but some of the most consistent fishing comes when the barometer is steady, especially for several days or more.

“An extended period of stable weather allows fish to find their comfort zone—a balance of the right water temperature, oxygen, light penetration and other factors—and fuels a nice, steady bite,” Hugill grins. “And that’s something we all like to see.”