Nitrogen transformations and balance in channel catfish ponds

A nitrogen (N) budget was developed for four, 400-m(2):ponds stocked with 550 channel catfish (Ictalurus punctatus) fingerlings that were fed to satiation daily for 133 days with a ration containing 4.85% N. Feed accounted for 87.9% of the N input to ponds. Abundant N from ammonia (NH3), ammonium (NH4+), and nitrate (NO3-) and the high total N: total phosphorus ratio in pond waters prevented appreciable biological N-2 fixation. There were four main N losses: fish harvest (31.5%); denitrification (17.4%); NH3 volatilization (12.5%); accumulation in bottom soils (22.6%). Nitrification averaged 70 mg N m(-2) d(-1), denitrification averaged 38 mg N m(-2) d(-1), and phytoplankton 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 of N recovered in fish and reduce the amount of NH3 excreted by fish. Efficient aeration and water circulation also should enhance nitrification and oxidation of organic N. (C) 2000 Elsevier Science B.V. All rights reserved.