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Nile Tilapia (Oreochromis niloticus) is the most cultured fish species in tropical and
subtropical countries. Desirable characteristics of farmed Nile tilapia include tolerance to a
variety of aquaculture environments and consumption of a wide range of natural food
organisms. In developing countries including Tanzania, Nile tilapia is cultured under semi-
intensive system. Under this system, supplementary feeding is imperative for optimum
growth. However, commercial feeds are very expensive and feed cost accounts for 40 to
70% of the culture operational costs. The best way to reduce production costs is
fertilization of ponds to stimulate natural food production that can be eaten by fish. This
minimizes the amount of supplementary feeds provided without significantly affecting the
growth of the fish, and hence, increases yield and profitability.
A study was carried out to evaluate the effects of pond fertilization alone (T 1 ), feeding
alone (T 2 ) and combination of pond fertilization plus supplementary feeding (T 3 ) on water
physico-chemical parameters, growth performance and profitability of pond cultured Nile
tilapia. The study also assessed the quantity and quality of periphyton found in the ponds
subjected to the three treatments. The experiment was conducted in nine earthen ponds,
each with an average size of 177 m 2 for 180 days. Sex reversed Nile Tilapia (O. niloticus)
fingerlings with an average size of 0.9 g were collected from Ruvu fish farm and stocked
at a density of 3 fish/m 2 seven days after initial fertilization of ponds. Urea and
Diammonium phosphate (DAP) were applied into the ponds under fertilization treatments
at a rate of 3 g/m 2 and 2 g/m2 one week before stocking and then weekly during the
experimental period. Mash feed containing 25.1% crude protein (CP) was fed twice daily
at 1000 and 1600 hours. During the first two months, the fish were fed at a feeding rate of
10% and 5% of fish body weight (FBW) for T 2 and T 3, respectively. After two months, the amount of feed was reduced to 5% and 2.5% for T 2 and T 3, respectively. The fish were fed
at these feeding rates up to the end of the experiment.
Pond water physico-chemical parameters i.e. dissolved oxygen (DO), pH, temperature,
total dissolved solids (TDS), conductivity and salinity were measured weekly at dawn
while Secchi disk readings were measured weekly after dawn hours. Diurnal
measurements were done at three hours intervals for 24 hours at the beginning of the
experiment and then at three months intervals up to the end of the experimental period. A
total of 500 ml of water samples were collected weekly for alkalinity, total nitrogen (TN),
nitrate and phosphorus determination. For periphyton collection, eight nets, each with 20
μm-mesh size and an area of 1250 cm 2 were placed full submerged in water in each pond
for periphyton to attach. The nets were taken out from pond water after every two months
and put in a bucket containing water and then scrubbed to collect periphyton and
zooplankton. The periphyton samples were stored in vials for determination of dry matter,
organic matter, crude protein, phosphorus and ether extract. Four ml of the periphyton
solution were taken and preserved at 4% concentration of formalin for species
identification. A random sample of 30 fish from each pond was taken biweekly and each
fish was measured individually for body weight and length. After being measured the fish
were returned back to their respective ponds. At the end of the experiment feed conversion
ratio (FCR), feed conversion efficiency (FCE), fish body weight gain, growth rate (GR),
specific growth rate (SGR), condition factor (K) and proximate chemical composition of
the fish body were determined.
The data were analysed using one-way ANOVA to assess the effect of treatment on water
physico-chemical parameters, fish body weight gain, growth rate, specific growth rate,
proximate chemical composition, survival rate and gross margin. R studio software version 3.5.0 (2018) was used to analyse the data. Duncan’s new multiple range test and
Tukey’s were used to assess the significance of the differences between pairs of the
treatment means at p = 0.05.
Results indicate that ponds subjected to fertilization alone (T 1 ) had significantly higher
(p ≤ 0.05) dawn dissolved oxygen (DO) (4.35 ± 0.04 mg/l), pH (8.24 ± 0.01) and Secchi
disk reading (25.3 ± 0.1 cm) than the ponds under feeding alone (T 2 ) and combination of
fertilization plus supplementary feeding (T 3 ). Ponds under fertilization alone had the
lowest values for water conductivity (1322 ± 3.28 mg/L), salinity (0.660 ± 0.0 mg/L) and
TDS (670 ± 1.70). Furthermore, the results show that phosphorous (0.33 ± 0.01 mg/L), TN
(20.82 ± 0.24 mg/L) and nitrate (11.85 ± 0.12 mg/L) concentrations were higher (p ≤ 0.05)
in the ponds under fertilization alone than in the ponds under other treatments. Water
alkalinity was lower (181.97 ± 3.25 mg/L) in the ponds under the combination of
fertilization plus feeding treatment than in other treatments. Higher values of water
alkalinity were observed under the ponds subjected to feeding alone (194.39 ± 2.43 mg/L)
and fertilization alone (191.82 ± 2.45 mg/L) treatments, but the difference of water
alkalinity between the two treatments was insignificant (p > 0.05). Mean water
temperature during the experimental period did not differ significantly among the
treatments. The values of DO, pH and temperature within 24 hours showed the peak
values at 1500 hours while the lowest values were observed at 0600 hours in all
treatments.
Results on growth performance indicate that fish cultured under the treatment of
combination of pond fertilization plus supplementary feeding (T 3 ) had significantly higher
daily weight gain (1.5 ± 0.1 g/day), feed conversion efficiency (FCE) (0.5 ± 0.0) and gross
margin (28 499 967 ± 3 173 413 TZS/ha/year) than the fish reared under the other treatments. The survival rate of the fish reared in ponds subjected to different treatments
did not differ significantly (p > 0.05). In addition, fish reared in ponds under feeding alone
showed higher FCR (4.1 ± 0.3) than those grown under combination of fertilization plus
supplementary feeding (2.0 ± 0.1). The cost of producing one kg for fish reared in ponds
under feeding alone (TZS 8 446 ± 380.6) was significantly higher (p < 0.05) than cost of
producing one kg for fish reared under fertilization alone (TZS 5 284 ± 327.4) or
fertilization plus supplementary feeding treatment (TZS 5 824 ± 166.7). However, the
costs of producing one kg of fish did not differ significantly (p > 0.05) between
fertilization alone treatment and combination of fertilization plus supplementary feeding.
Fish condition factor (K) differed significantly among the treatments. The fish cultured
under fertilization alone had the highest condition factor (2.54 ± 0.0) while those reared
under feeding alone showed the least value (2.05 ± 0.0). The highest periphyton biomass
(47.35 ± 7.64 g DM/m 2 ) was obtained in ponds under combination of fertilization plus
supplementary feeding treatment. Ether extract (EE) was significantly higher in fish body
muscles (18.33 ± 0.19%) and periphyton (1.84 ± 0.07%) in samples from ponds subjected
to fertilization alone than in the samples from ponds under feeding alone and combination
of fertilization plus supplementary feeding. Positive correlation was observed between CP
and EE of fish body muscle and those of periphyton. Higher values of CP for both fish
(69.14 ± 0.33%) and periphyton (11.40 ± 0.16%) were observed in ponds under the
combination of fertilization plus supplementary feeding than in other treatments.
The analysis of correlation between fish growth rate and periphyton quantity and quality
revealed that as periphyton quantity (biomass, OM) and quality (CP) increased, fish
growth rate also increased. Periphyton community composition differed (p ≤ 0.05) among
the treatments. Higher species abundance was observed in the ponds subjected to
combination of fertilization and supplementary feeding. The phytoplankton classes observed were Bacillariophyceae, Chlorophyceae, Cyanophyceae, Euglenophyceae and
Zygnematophyceae while zooplankton classes were Eurotatoria, Heterotrichea and
Oligohymenophorea. Therefore, from this study it is concluded that the combination of
weekly fertilization plus supplementary feeding at 2.5% of the fish body weight is the best
feeding strategy. This is because it results into higher periphyton quantity (biomass and
organic matter), quality (crude protein) and species composition which ultimately lead to
higher fish growth, carcass quality (crude protein) and gross margin compared to the other
treatments. The chemical composition of fish body muscles is closely related to
periphyton chemical composition. |
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