Barometric Pressure

    It's curious as to how changing environmental conditions affect things around us and us being a fishermen we can use these changes to help us figure out fish.  Let's look at how our fishing eco-system basically works.  We do know that certain zooplankton follow a diurnal pattern and rise and sink vertically in the water column with low and bright light conditions. Bright sun will cause them to sink in the water column and darkness will bring them to the surface. Now a front coming though will bring with it increased clouds and changing winds. Winds whip up the surface waters and effect the light penetration as much as increased clouds. Winds might also change direction with the oncoming front and we should remember that the zooplankton and phytoplankton drift with the winds and currents. Changing wind direction and intensity will cause the food source of the small minnows to change position in the lake. The minnows may follow their food and then the bass will follow the minnows. 

    A maximum change in atmospheric pressure, which might occur during a hurricane, could drop the pressure from 30.50 to 28.50. This is equivalent to an underwater depth change of just 2.2 feet.  Normal barometric shifts are small, typically less than .20 inches per hour. This is equivalent to a 2 1/2-inch depth change per hour by a bass. Fish adjusting a few inches at a time can fully compensate for changes in barometric pressure, leading us to believe that barometric pressure alone isn't likely a strong motivator in bass activity. 

    It's been argued that a small change in AIR PRESSURE of only 3/10 of an Inch of Mercury would have little effect on a bass underwater.  Especially if that bass were down 10 ft or so.  The bass will not even sense the small change in air pressure above.  The increase pressure of the air will push down on the water's surface and spread out in the water in all directions.  Now the changing air pressure will increase the exchange of Oxygen between the air and the water by adding more mixing due to increased wind and wave action.  Maybe the increased air pressure affects the minnows in very shallow water 6" or less.  That could make the minnows uncomfortable or drive them out into deeper water where the bass are waiting to feed. 

    A bass going from 1 ft of water to 33 ft will experience a change in water pressure of 1 atmosphere or 29" of mercury pressure.  Now the other changes that accompany the dropping BP will directly affect the fish.  A bass can easily move from very shallow water 2 ft or less to 20 ft deep water and change depth rather quickly with no visible ill effects.  Many of you have seen a bass quickly scurry out to deeper water when you scare them along the bank.  Bright sunlight has an instant and direct effect on the fish. 

  The Prey Factor:

    Observations suggest that changes in bass behavior are based primarily on the changing location and position of prey.  Here’s their scenario: When the Barometer rises, zooplankton lose buoyancy and settle into cover and closer to the substrate.  Their position makes it harder for shad, small bluegills, and other prey fish to feed on them.  As a result, baitfishes take cover and wait for easier pickings.  It seems that plankton are not worth chasing in those conditions.  Bass also seem to turn down their activity level in response to the scarcity of easily caught baitfish.

    It makes so much sense that it's not actually the pressure itself, but the effect of the pressure on other things that have the trickle down effect to the bass. Sunlight penetration is often associated with the passing of a cold front and affects the plankton, which then has the trickle down effect.  Low pressure makes plankton more buoyant, hence pushing it to the surface, that's why when a front is approaching and the pressure is low, and there is cloud cover, there is so much top water activity.