A nitrogen (N) budget was developed for four, 400-m2 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 N2 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 m2 d1, denitrification
averaged 38 mg N m2 d1, and phytoplankton removed NO3N at 24 mg N m2
d1. Mineralization of feed N to NH3 averaged 59 mg N m2 d1. As feed is the largest
N input in catfish ponds, improved feeds and feeding practices can 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