Nitrogen transformations and balance in channel catfish ponds

A nitrogen (N) budget was developed for four, 400-m(2):ponds stocked with 5 50 channel catfish (Ictalurus punctatus) fingerlings that were fed to satia tion daily for 133 days with a ration containing 4.85% N. Feed accounted fo r 87.9% of the N input to ponds. Abundant N from ammonia (NH3), ammonium (N H4+), and nitrate (NO3-) and the high total N: total phosphorus ratio in po nd waters prevented appreciable biological N-2 fixation. There were four ma in N losses: fish harvest (31.5%); denitrification (17.4%); NH3 volatilizat ion (12.5%); accumulation in bottom soils (22.6%). Nitrification averaged 7 0 mg N m(-2) d(-1), denitrification averaged 38 mg N m(-2) d(-1), and phyto plankton removed NO3-N at 24 mg N m(-2) d(-1). Mineralization of feed N to NH3 averaged 59 mg N m(-2) d(-1). As feed is the largest N input in catfish ponds, improved feeds and feeding practices carl increase the proportion o f N recovered in fish and reduce the amount of NH3 excreted by fish. Effici ent aeration and water circulation also should enhance nitrification and ox idation of organic N. (C) 2000 Elsevier Science B.V. All rights reserved.