While total nitrogen concentrations showed a trend of
slight increase towards the later part of the grow-out,
total phosphorus and ammonia levels remained
relatively stable with most values around 0.2-0.3
mg/L and 0.05-0.3 mg/L, respectively (Fig. 3). In
comparison, the water quality in conventional
monospecies culture of walking catfish in Thailand
exhibits extremely low DO (often anaerobic) and
high ammonia (>2-3 mg/L) and sulfide
concentrations (Srisuwantach et al. 1981).
The nutrient inputs in each pond derived from the
catfish feed can be estimated as shown in Table 4.
For pond A, with a feed input of 262 kg and net
catfish yield of 114 kg, the total N and P in the waste
are calculated to be 9.1 and 1.8 kg, respectively. In
comparison with chicken manure, which contains 2%
N and 1% P on average, the P and N input derived
from the fish feed is equivalent to 400 kg of chicken
manure for N and 180 kg for P. While the N:P ratio
in chicken manure was 2:1, in fish waste it was 5:1,
making a more theoretically balanced nutrient source
for pond fertilization.
Although catfish is an air breather that can tolerate
extremely poor water quality, disease has been a
serious problem (Anon. 1981). To
Fig 2 Fluctuation of water temperature, Secchi disc
depth, dissolves oxygen and chlorophyll a
concentrations in ponds A and B.