the affects of barometric pressure on fish activity is a mystery that i havent quite fully figured out yet. what the barometor is at and what it will be doing is the #1 factor in fishing to me with moon phases a close #2. i know it affects their swim/air bladder causing it to inflate and deflate. an example would be when pulling crappie from extremely deep water. their bladder swells up so much that if you dont use a needle to deflate it they cannot go any deeper than a foot below the water and end up dying. now i know the barometor doesnt affact them this bad but it does to an extent. my great-grandfather was a carp fanatic and would keep the air bladders from the carp he caught and keep them moist in his kitchen for a few days. i always wondered why he done that when i was little and he would tell me that you could tell what the weather was gonna be by watching how it inflates and deflates. he said he was taught how to watch the bladders when was a young by this old guy who was half indian that lived near him. it wasnt until i was much older, and years after he passed, when i started learning about barometric pressure that it dawned on me why he kept the swim bladders from carp. i grew up right next to grampa and he would come snatch me up at a moments notice to go fishing saying "come on boy! theyre biting!". he was a die-hard carper and didnt fish for anything else, and as far as i can remember we rarely had a day where we didnt catch a bunch of fish. after i started catching on how pressure affects behavior, and before i purchased a weather radio, i sometimes would take an empty 2litr bottle and slightly compress the bottle before putting the cap on and lay it near us at night. when the pressure started rising (=fish biting) that bottle would start popping due to it expanding. same thing happens to a swim bladder in a fish. the sizes of the bladders varies in different species of fish and they no doubt are affected lees/more than others. for example, when there is really high pressure in i see bluegills and carp really shallow in the water column and they wont hit anything, but cats seem to dissapear. trout seem more active during high pressure, and as far as cats go, during high pressure (30.20 and above)channels will still hit but you can forget about the bigger flats but the small ones will sometimes still hit. i visited bass-pro once when the pressure was extremely high (30.50) and all of the cats were supened right under logs or a rock shelf, almost like they were under there so they didnt float to the top. what is still a mystery to me is what does a slightly swollen/deflated bladder do to a fish? do they feel bloated? drive them deeper/shallower? do something to their brain? it does something and thats what i would like to know. i posted this hoping someone might have some scientific data on the subject.