International Journal of Aquaculture, 2016, Vol.6, No.7, 1
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2 Discussions
2.1 Growth of African catfish fry
The lack of statistically significant differences between fry in treatment 1 (egg yolk based diet) and fry in
treatment 3 (diet based on soymilk from roasted soybeans) as shown in Table 1 was surprising because egg yolk,
an animal protein source, has a superior amino acid profile to soybeans and fry fed on egg yolk based diets would
thus be expected to show superior growth. These results obtained can be explained to some extent by the
observations by Chow (1980) who explored ramifications of feeding eggs to fish larvae.
Egg yolk is highly nutritious. However, it has a very high energy-protein ratio which could result in inadequate
intake of protein necessary for maximum growth when used as a diet for very young fish. This is because animals,
fish not exempted, eat to satisfy their energy requirements. A fish's food intake increases or decreases
corresponding to a reduction or an increase in the diet's energy density. For diets of fixed protein composition, the
protein requirement may not be sufficiently met if the diet has too high an energy-protein ratio resulting in
increased lipid deposition and reduced growth (Chow, 1980). This concurs with observations by Takeda et al.,
(1975), Wanatabe (1982) and Daniels and Robinson, (1986).
The optimum dietary energy-protein for fish differs among species and at various stages of the life history. Hosain
et al., (1998) recommends an energy-protein ratio of 122 for diets of young African catfish, which was also found
to be optimal for Channel catfish fry (
Ictalurus punctatus
) by Winfree and Stickney (1984). A normal chicken egg
yolk has 32-33% CP and metabolisable energy content of around 5 700 kcal/kg (Chow, 1980; Bologa et al., 2013),
thus an energy-protein ratio of between 173 and 178 which is considerably over the recommended ratio. This, and
the inclusion of fishmeal in both treatments after 1 week, helps to explain why growth of fry fed on egg yolk was
not significantly better than growth of fry whose diet was based on soymilk from roasted beans despite the egg
yolk having a superior nutritional profile. Similar observations on the effect of diets with very high energy-protein
ratios on growth of young fish were made in hybrid striped bass
Morone chrysops
X
M. saxatilis
(Nematipour et
al., 1992). Marammazi and Kahkesh (2011) also documented reduced growth of juvenile Shirbot (
Barbus grypus
)
fed on diets with high energy-protein ratios.
Observations from the present study also show that growth of catfish fry was poor in fry fed on a diet based on
soymilk from raw (unroasted) beans. We attribute this to the presence of anti-nutritional factors that are presented
in untreated soybeans such as trypsin inhibitors and phytic acid. It has been documented that roasting or any other
form of heat treatment helps to deactivate these inhibitors and improve protein utilization by fish. During the
roasting process, active Kunitz soybean trypsin inhibitor decreases from 1.8% of raw soy flour protein to only
0.57% soy flour proteins while the Bowman-Birk inhibitor family comprises 0.38% and only 0.04% of raw and
roasted soy flour respectively (Anderson, 1986). Ogbonna et al., (2014) found that growth of African catfish
fingerling was significantly better in fingerlings fed on a roasted soybean meal diet than fingerlings fed on a diet
of raw soybean meal and similarly attributed the differences to the presence of anti-nutritional factors in raw
soybean.
Results from the study clearly demonstrate that diets based on soymilk from roasted beans can be used to feed
African catfish fry as growth of the fry was highest in fry fed on the diet, alongside fry fed on egg yolk diet, and
was far superior to the growth of fry fed on diets based on soymilk prepared from raw soybeans.
2.2 Survival rates of African catfish fry
Several studies have documented that live food in the form of enriched
Artemia nauplii
, small Daphnia, Moina or
other zooplankton of suitable size are essential for survival and is preferred to dry feed, in this case boiled egg
yolks (Hecht, 2013). This is because the stomach of the young fish is not fully functional at the start of exogenous
feeding. Therefore, high survival rates can only be obtained if feed provided matches the development of the gut
of the fish.
