Effects Of Pond Fertilization And Supplementary Feeding On Growth Perfomance And Economic Return Of Nile Tilapia (Oreochromis Niloticus)

ABSTRACT

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 semiintensive

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 (T1), feeding

alone (T2) and combination of pond fertilization plus supplementary feeding (T3) 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 m2 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/m2 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/m2 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 T2 and T3, respectively. After two months, the

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amount of feed was reduced to 5% and 2.5% for T2 and T3, 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 cm2 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

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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 (T1) 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 (T2) and combination of

fertilization plus supplementary feeding (T3). 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 (T3) 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

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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/m2) 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

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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.