Effects of pond fertilization and supplementary feeding on growth perfomance and economic return of Nile tilapia (oreochromis niloticus)

M.Sc. Dissertation

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.