Barometric pressure is the weight exerted by atmospheric gases at a particular point. As the pressure rises and falls, it brings a series of weather patterns and affects fish in complex ways that are difficult to predict. The changing air pressure affects the swim bladders of the fish, which affects their feeding habits. Deep-water fish, already under a large amount of water pressure, are less affected by the comparatively minor shifts in atmosphere than shallow fish.
High, Steady Barometric Pressure
High barometric pressure brings bright, cloudless days, which are conducive to spending time on the water. But the fish often won't bite.
As the pressure starts to fall and the weather turns, the fish become more active. Falling or fluctuating barometric pressure is usually an effective time to fish. It doesn't have to be stormy for good fishing. Falling barometric pressure brings cloudy skies, but it doesn't necessarily bring a storm, particularly if the fall is gradual.
Steady Low Pressure
Fishing during a low-pressure system or even in a storm can be productive at first (despite the obvious dangers) but it tends to become less so as the low-pressure weather continues. Long-term low-pressure systems will eventually yield lethargic fish that are no longer biting.
As the weather starts to clear and the barometer rises, the fish start to bite. A clearing system is an optimistic time for fishermen. The clouds withdraw, the sun comes out and the fish usually feed.