possibly eating really small fry or zoo plankton ? many folks dont know black crappie eat super tiny things ....1/4 inch long in some cases ....
get you some super tiny jigs ...like 1/64 with a really small body .....maybe try to indent the food item ?
do you have those tiny ghost shrimp in those waters like some places ? they really like them .....
possibly even an aquatic insect hatch of some variety .....?
this is all of course if they are not gulping due to oxygen depletion ....