International Journal of Aquaculture 2015, Vol.5, No. 41, 1-20
17
consumption). In Castro et al. (2011), fish 100% fish
meal diet consumed the most oxygen than other
phytase supplemented diets, indicating the lowest
dissolved oxygen for the diet, which also observed in
this study. Dissolved oxygen measured in this study
are in the range of requirement for
Clarias gariepinus
between 0.50-3.0mg/l as reported by Manuel et al.
(2014). However, it has also been shown that an
oxygen level of 3.2 mg/l (60 mmHg) or higher is
desired for African catfish held at 25 "C (Manuel et al.,
2014). Measurement of ammonia concentration
showed significant reduction in all diet, except for fish
meal diet and diet without phytase where levels were
signifi
c
antly elevated due to increased excretion as
observed by Castro et al. (2011), compared to phytase
soyabean diet. According to Castro et al. (2011),
nitrogen excretion and oxygen consumption may be
used to estimate the oxidation of nutrients from the
diet and, usually, low values indicate a better use of
the protein source (less excretion) and less energy
spent in its oxidation (less consumption). In this study,
fish fed 100% soya bean with phytase showed the
lowest ammonia level, while control fish fed 100%
fish meal (S0P0) showed the highest ammonia level as
observed by Castro et al. (2011). The high ammonia
level in tank of fish fed diet S0P0 may possibly
explain the reduction in survival of the fish (Manual et
al., 2004) compared to phytase-supplemented diets
(Table 7). Farmer et al. (2011) reported that elevated
ammonia levels is stressful to fish, which could lower
fish growth and survival (Lemarie et al., 2004). The
lack of significant change in pH may suggest an
improved water quality, which was not impaired by
phytase supplementation to diet. Temperature was
slightly raised, but are within the range for the fish
(Manual et al., 2013). The improvement in dissolved
oxygen with phytase supplementation in the diet
without significant change in pH may possibly obviate
the negative effect of temperature (Wagner et al.,
1997). Temperatures ranging between 18°C and 28°C
occur in the natural habitat of this species (Manual et
al., 2013). The significant reduction in nitrate,
resulting from a reduction of ammonia by oxidation
(WHO, 2011) as observed in Table 12, may indicate
improved oxygen carrying capacity of blood and
reduced gill permeability, making it less toxic to fish
compared to nitrite (Schram et al., 2012). Levels of
nitrate observed in this experiment are still tolerable
by the fish and are far below the maximum of 140
mg/l nitrate-nitrogen, which may explain improved daily
feed intake (Figure 3) with phytase supplementation
(Schram et al., 2012). The correlation between
treatment, phytase and water quality shows phytase is a
non-polluting feed supplement. Improvement in growth
by phytase without impairing water quality (Table 14)
shows it is an environmentally sustainable method of
sustaining aquaculture production. Economic analysis
(Table 15) showed that soyabean effectively replaced
greater amount of fish meal with low incidence of cost,
but higher profit index (Table 16) compared to
fishmeal at low phytase of 250FTU/g. Cost savings is
equally derived with phytase supplementation in soya
bean based diet of the fish. The highly reduced cost of
phytase (N143.12/kg) mean it is not only a cheaper
and environmental alternative to inorganic phosphorus
(Lei et al., 2013), but a better economy for enhancing
aquaculture production with minimal cost implications.
In conclusion, the study has demonstrated that
although phosphorus level in water was not measured,
which may limit its scope, phytase supplementation at
low level in soyabean diet of
Clarias gariepinus
could
effectively utilize phosphorus from phytate, cut and
manage pollution in aquaculture environments, and
improve overall growth and economy of fish when
compared to the use of fish meal-based diets and diet
with no phytase supplementation.
Table 16 Economic analysis of phytase supplementation in soyabean based diet of
Clarias gariepinus
Phytase (FTU/g)
Cost of feed consumed (N)
Value of fish (N) Incidence of cost
Profit index Cost savings (N)
0
500.62±162.91
407.33±218.47 0.49±0.15
0.75±0.34 -21.66
250
455.93±131.82
417.47±172.68 0.41±0.04
0.88±0.23 34.29
500
428.66±145.56
342.26±158.40 0.44±0.05
0.76±0.23 14.44
750
500.94±159.04
387.68±200.23 0.47±0.07
0.72±0.25 -9.02
1000
463.37±139.73
354.53±190.80 0.47±0.08
0.71±0.30 -12.63
Sig
P>0.05
P>0.05
P>0.05
P>0.05
