A STUDY ON MINERAL SUPPLEMENTATION IMPROVING WATER QUALITY IN AQUACULTURE PONDS

Catfish(Ictalurus punctatus) has been the main freshwater aquaculture economical species in both America and China mainland.To study the effect of NaNO3 as a soil oxidant and possible decline against blue-green algae and potential extension prospect of mineral supplementation on water quality improvement in freshwater aquaculture ponds,the experiment including three treatments,NaNO3,KCl+MgCl2 and NaNO3+KCl+MgCl2,as mineral supplementation and NaNO3 as a soil oxidant,and the control with not any mineral supplementation or fertilizers,was designed and applied to 12 freshwater earthen ponds stocked with catfish at the density of one individual per cubic meter water in Auburn University,Alabama,U.S.A.During the culture period of 182 days,the application dosage of NaNO3,KCl,or MgCl2 was 2g,100g,and 20g per cubic meter water per time,and the application times of NaNO3,or KCl+MgCl2 was 21 and 2 times in total,respectively,moreover,the application times of NaNO3+KCl+MgCl2 included 21 times for NaNO3 application and 2 times for KCl+MgCl2 application in total.17 water quality parameters,including total ammonia nitrogen,nitrite nitrogen,nitrate nitrogen,reactive phosphorous,turbidity,DO,pH,alkalinity,chloride,chlorophyll a,BOD5,potassium,sodium,calcium,magnesium,total algae density and blue-green algae relative density,and 5 bottom soil parameters,including redox potential,total nitrogen,total phosphorous,soil respiration rate and organic carbon,in the treatments and control ponds were monitored at interval.The results showed that all the treatments caused the blue-green algae relative density of ponds to decline significantly(P0.05),which would decline the possibility of catfish being off-flavor;The treatment of KCl+MgCl2 promoted nitrification positively in ponds water-body,which would promote total ammonia nitrogen,whose high concentration would be harmful to aquaculture species growth,transferring to nitrite nitrogen and then continuing to be nitrate nitrogen,which would be not harmful to aquaculture species growth.The treatment of NaNO3+KCl+MgCl2 increased bottom soil redox potential significantly(P0.05),which would reduce some harmful deoxidized composites diffusing from soil surface to water;Only the treatment of NaNO3+KCl+MgCl2 increased catfish production at 6.6% and declined feed conversion ratio at 6.2% significantly(P0.05);NaNO3 cooperated with KCl+MgCl2 took effect of improving water and bottom soil quality in aquaculture ponds,where the blue-green algae relative density of ponds being declined and ponds bottom soil redox potential being increased,in result,which blocked and reduced some harmful deoxidized composites diffusing from soil surface to water,and reduced fish reaction against the bad water quality,which saved energy consumption for fish,decreased feed conversion ratio and increased growth rate.Therefore,NaNO3+KCl+MgCl2 treatment would develop a new path to improve water and bottom soil quality in aquaculture freshwater ponds,and have a great extension prospect.Moreover,if NaNO3+KCl+MgCl2 treatment apply to other higher value species aquaculture ponds,for instance,shrimp or eel,those species aquaculture effect should be better than that of catfish,because NaNO3+KCl+MgCl2 treatment would have similar effect on the improvement of water and bottom soil quality in the ponds as in catfish ponds,furthermore,the value of production increased and the cost of feed saved would be more than the cost of all mineral supplementation to the ponds.