Effects of bottom soil management practices on water quality improvement in channel catfish Ictalurus punctatus ponds

Abstract

The bottoms of four ponds on the Auburn University Fisheries Research Unit, Auburn, AL, were dried for 1 month and tilled with a roto-tiller (dry-till treatment). Four other ponds were dried and tilled, sodium nitrate (200 kg ha −1) was applied to oxidize bottom soil, and soils were tilled again to mix bottom soil with sodium nitrate (dry-till with sodium nitrate treatment). Four ponds were not subjected to bottom drying, tilling or sodium nitrate treatment (controls). Channel catfish ( Ictalurus punctatus) fingerings were stocked at 15,000 ha −1. Selected water quality variables were measured at 1–2-week intervals during the growing season. Concentrations of most variables increased over time because feeding rates were progressively increased as fish grew. Compared to controls, both treatments had lower concentrations of soluble reactive phosphorus, nitrate nitrogen, nitrite nitrogen, total ammonia nitrogen, total suspended solids and turbidity, and higher values of pH, Secchi disk visibility, total alkalinity, total hardness, and calcium hardness ( P<0.01). Ponds of the dry-till treatment had in addition lower concentrations ( P<0.01) of total phosphorus and total nitrogen than control ponds. Concentrations of dissolved oxygen and chemical oxygen demand did not differ significantly among the treatments ( P>0.05). Compared to the control, both treatments had lower concentration of chlorophyll a from June to July ( P=0.004), but controls had lower concentrations of chlorophyll a from August to September than both treatments ( P<0.001). Organic carbon, total phosphorus, and dry pH in soil samples did not differ among the treatments ( P>0.05). The findings suggest that water quality improvement can be achieved by drying and tilling pond bottoms between crops. Applying sodium nitrate to dry, tilled pond bottoms did not increase the degree of water quality improvement.