International Journal of Aquaculture, 2016, Vol.6, No.7, 1
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Results from our experiment demonstrate that providing feed in a ‘milk’ or filtrate form as was the case with the
soymilk has the same effect on the digestive tract. That is to say, soymilk matches with the ontogeny of the
digestive system of the fish. This is supported by observations on survival rate of fry in our study which showed
that survival rate of fish in this experiment was significantly higher in fry fed on diets based on soymilk prepared
from raw soybeans and soymilk from roasted soybeans (treatments 2 and 3). Survival of fry in both treatments
was significantly higher than survival of fry fed on a diet based on egg yolk diet (treatment 1). de Graaf and
Jensen (1996) obtained survival rates of 40% over a 50-day experimental period following what was described as
‘good feeding and management practices’. This is consistent with the results of the present study and supports our
conclusion that soymilk is ideal for African catfish fry nursing to improve survival.
The differences in survival of fry fed on diets based on soymilk and those fed on diets based on egg yolk can also
be attributed to differences in cannibalistic activities in the treatments. During the course of the experiment, a total
of 18 ‘jumper’ fry were isolated and graded from tanks in which fry were fed on egg yolk based diet. In contrast,
only 4 were graded from fry fed on diets based on soymilk prepared roasted soybeans and none from fry fed on
diets based on soymilk prepared from raw soybeans. This implies that cannibalism was greatest in fry fed on diets
based on egg yolk.
The differences in cannibalistic activity were more likely a result of the differences in the form in which the feed
was presented than the type of the feed itself. Almazán et al., (2004) showed that a reduction in swimming activity
results in a lower incidence of aggressive behavior. Hecht (2013) suggests that feeding frequency, food availability
influences swimming activity of fry and Broom (1998) demonstrated that aggression and other behavior patterns
can be altered by feed availability. The soymilk was presented in form of a filtrate that was dispersed evenly into
the rearing units. The egg yolk, on the other hand, was presented in form of crumbs that were likely not evenly
distributed in the tanks. This caused intra-tank differences in feed availability causing an increase in swimming
activity in search of feed and the resulting aggression. This also likely resulted in some individual fry having
better access to the feed than others (the jumpers). This, in turn, explains the high cannibalism observed.
From this study, it is evident that diet of African catfish fry based on soymilk prepared from roasted soybeans is
best for obtaining high survival rate of the fry as in both treatments where soymilk was used to feed fry, survival
rate was statistically higher than in the treatment where egg yolk was instead used to feed fry among fry in
treatment 1. While the study has demonstrated positive effects of diets based on soymilk from roasted beans on
growth and shows higher survival rates of African catfish fry than previous studies in the country (Iita, 2013), the
study did not vary the application rate (or feeding rate) of the soymilk. Therefore, it is difficult to assert that the
application rate used in this study (1 ml of soymilk / litre of culture media) is suitable for optimal growth and
survival of African catfish fry. That is to say, it is possible that the application rate used in this study is below or
above some hypothetical optimal application rate that can be established through further research by investigating
growth and survival response of fry when soymilk application rate is varied.
2.3 Water Quality
Temperature values were within recommended limits for African catfish larvae and fry growth and survival.
Barton et al., (2002) recommends that temperatures should be within 23
o
C - 30
o
C agreeing with FAO (1996). The
study was limited by a lack of automated water heaters at the experiment to regulate water temperature resulting
in mean temperatures that were below the optimum 28
o
C for growth of African catfish as reported by Hecht (2013)
as presented in Table 2. Fish growth decreases at temperatures above or below the optimum temperature (Stickney,
1994), and this suggests possible higher growth rates of fry than presently reported in this study, if temperature
was regulated.
pH values recorded in all treatments were within recommended limits during the duration of the experiment as
they were within the recommended range of 6.0-9.0 while DO levels were similarly within optimal levels as they
were above 6 mg/l in all treatments (FAO, 1996).
