It's fairly well known that fish respond to barometric pressure changes. For example a drop in pressure tends to make them become aggressive and seek shallow water, a further drop and they will become less active and seek deeper water.

 

Can anybody tell me how and why they do this?

Well, I don't know enough about fish (or fishing) to say why (maybe a small drop stimulates airborne insects which can be caught at the water surface?), but how is either by means of their lateral line system, a system of fluid-filled tubes along the animal's sides to detect vibrations in the water and also pressure differences, or by means of sensory receptors in the swim bladder.

 

Mokele

Mokele is right on the how of this question. The lateral line organs (and in sharks, ampullae of Lorenzini - small pores that detect hydrostatic changes and electrical charges int he water - as well) are how fish sense changes in pressure and low-level vibrations, like a thrashing, injured animal. The why of this is most likely that they are sensing weather patterns - like the severe drop in pressure would be associated with an approaching tropical storm down here in Florida, and the fish would seek deeper water to stay out of the turbulence of the storm surge.

Mossoi, it's an interesting question. Do you have any specific sources that confirm this, or is it 'common knowledge' amongst fishermen? Also, is does this relate to freshwater fish, seawater fish, or both?