Brian Ott
USDA-ARS Warmwater Aquaculture Research Unit; Thad Cochran National Warmwater Aquaculture Center; Stoneville, Mississippi 38776Earthen ponds used for ictalurid aquaculture are complex and stochastic environments that have broad implications on fish health, physiology, and production. These ponds have traditionally been regarded as phytoplankton-based systems, with much of the nitrogenous waste being assimilated by phytoplankton during photosynthesis. Any aquaculture ponds relying upon phytoplankton as their nitrogenous waste sink can have dramatic daily swings in dissolved oxygen (DO) concentration, pH, and CO2 concentrations. However, there is an upper limit to the phytoplankton population; once that limit is reached an excess of ammonia could accumulate without the addition of other biotic or abiotic processes to handle this. These changes in water quality are accompanied by shifts in physiological parameters that attempt to mitigate how the changes in the pond environment impact the fish. Data accumulated over the previous decade suggest that DO is the limiting factor in catfish production until a certain stocking density, at which point the limiting factor becomes unclear.