Factors affecting growth performance of oreochromis mossambicus in a low technology aquaponic system

Abstract

The aim of this study was to enhance fish production in low technology aquaponic systems. Growth parameters and somatic indices were assessed in order to determine the effect of feeding frequency, stocking density and recirculation rate on the growth performance of Oreochromis mossambicus in low technology aquaponic systems. Weight gain (%), specific growth rate (SGR), food conversion ratio (FCR), thermal growth coefficient (TGC), condition factor (CF), hepatosomatic index (HSI), viscerosomatic index (VSI) and survival rate (SR) were assessed for each factor. Haematological parameters were also checked to assess the stress levels of fish under different treatments of feeding frequency, stocking density and recirculation rate. Red blood cell (RBC) count, white blood cell (WBC) count, haematocrit, haemoglobin and serum glucose levels were also assessed for each factor. Water quality parameters (pH, dissolved oxygen (DO) concentration, total dissolved solids (TDS), ammonia (NH3), nitrate (NO3-), nitrite (NO2-) and phosphorus (P)) were monitored over the duration of the experiment. Spinacia oleracea growth parameters (shoot length and the number of leaves), were analysed in order to determine its growth performance when integrated with O mossambicus. The effect of feeding frequency on the growth performance of O. mossambicus in low technology aquaponic systems was evaluated over a duration of 28 days. The fish were stocked in 500 L fibreglass tanks. The fish feeding frequency treatments were T1 (once daily), T2 (twice daily) and T3 (thrice daily). The highest %WG, SGR, TGC and SR were recorded in T3, which was significantly different from other treatments (p < 0.05, ANOVA). The lowest FCR was recorded in T3. The best growth performance that was observed in the T3 treatment may be due to better feed intake and feed utilization that comes with frequent feeding. The fish feed was also available for prolonged periods. No significant differences (p > 0.05, ANOVA) in SR were observed between treatments. Mortalities that occurred in each treatment were not related to feeding frequency, they may have been due to handling. No significant differences (p > 0.05, ANOVA) in organosomatic indices (CF, HSI and VSI) were observed with respect to feeding frequency. The health status of the fish was not affected by the different feeding frequencies. Additionally, feeding frequency did not significantly affect (p > 0.05, ANOVA) all the haematological parameters. Different feeding frequencies did not cause any significant changes in RBC count, WBC count or serum glucose. The highest blood performance was recorded at the highest feeding frequency (3 times daily). This suggests better health status of O. mossambicus at the highest feeding frequency, where the fish may have been able to feed efficiently. The pH and DO concentration range were 6.99-7.20 and 5.70-8.60 mg/L respectively. No significant (p > 0.05, ANOVA) differences in pH and DO were observed between different treatments. Water quality parameters remained within acceptable limits for growth of tilapia. NH3, NO3- and NO2- concentrations increased with feeding frequency and the duration of the experiment. This could be due to increasing accumulation of fish waste as feeding frequency increased. Feeding frequency resulted in an increase in P concentrations. This may be a result of increasing accumulation of fish waste in the system. The highest plant growth was also observed in T3. The maximum plant growth in T3 was due to the availability of nutrients in the system for a prolonged period during the day. These results show that feeding frequency had an effect on the growth performance of O. mossambicus in low technology aquaponic systems. The feeding frequency of 3 times daily may be the optimum feeding frequency for the growth of O. mossambicus and S. oleracea in aquaponic systems. The effect of stocking density on the growth performance of O. mossambicus in low technology aquaponic systems was evaluated over a duration of 28 days. The fish were stocked in 500 L fibreglass tanks. The fish stocking density treatments were low stocking density (1.87 kg/m3), intermediate stocking density (2.50 kg/m3) and high stocking density (3.13 kg/m3). The highest %WG, SGR, TGC and SR were recorded in fish that were stocked in ISD. The lowest FCR was also recorded in fish that were stocked in ISD. A significant difference (p < 0.05, ANOVA) in SGR and FCR was observed between ISD and other treatments. The best growth performance that was observed in the 2.50 kg/m3 treatment was due to efficient feeding. At intermediate stocking densities, the was no competition for food and space and as a result, fish stress did not occur. The fish may have been able to channel their energy to feeding instead of stress. SR was not significantly affected (p > 0.05, ANOVA) by stocking density. Mortalities that occurred in each treatment were not related to stocking density; they may have been due to handling. The highest organosomatic indices (CF, HSI and VSI) were observed in the ISD treatment. This could suggest that the fish had the best condition and best health status. Additionally, the highest RBC count, WBC count, haematocrit, haemoglobin, and serum glucose were observed in fish that were stocked at the HSD treatment. This could mean that the fish stocked at the highest density experienced stress which was reflected in high RBC count, WBC count and serum glucose. The highest blood performance was recorded at the lowest stocking density. This could suggest better health status of O. mossambicus at the lowest stocking density, where the fish were able to focus their energy on feeding instead of stress resulting from crowding and competition for food. No significant differences in pH were observed between different treatments. The lowest DO concentration was observed in HSD. Water quality parameters remained within acceptable limits for growth of O. mossambicus. NH3, NO3-, and NO2- concentrations increased with stocking density and the duration of the experiment. This could have resulted from the increasing accumulation of fish waste as stocking density increased. The concentration of P also increased with the stocking density and the experimental duration. This may be owing to the increasing accumulation of fish waste. The highest plant growth was also observed at HSD. This may be due to the increasing accumulation of fish waste in the system that is directly related to the increasing fish biomass with increasing stocking density. These results show that stocking density had an effect on the growth performance of O. mossambicus in low technology aquaponic systems. The intermediate stocking density of 2.50 kg/m3 may be optimal for the growth of both O. mossambicus and S. oleracea in aquaponic systems. The best fish growth (SGR and FCR) was observed in the intermediate stocking density compared to high stocking density and low stocking density. The effect of recirculation rate on the growth performance of O. mossambicus in low technology aquaponic systems was evaluated over a duration of 28 days. The fish were stocked in 500 L fibreglass tanks. The recirculation rate treatments were LRR (0.5 L/min), IRR (1.5 L/min) and HRR (2.5 L/min). The highest %WG, SGR, TGC and SR were recorded in fish that were stocked in HRR treatments. The lowest FCR was also recorded in HRR. A significant difference (p < 0.05, ANOVA) in SGR and FCR was observed between HRR and LRR. The best growth performance of O. mossambicus that was observed in the HRR treatment may be due to good water quality (high DO conc. and low TDS). When the water quality is good, fish are able to feed efficiently in the stress-free environment. SR was not significantly affected (p > 0.05, ANOVA) by recirculation rate. Mortalities that occurred in each recirculation rate treatment were not related to recirculation rate; they may have been due to handling. The highest (p < 0.05, ANOVA) organosomatic indices (CF, HSI and VSI) were observed at HRR. Poor fish health in terms of CF, HIS and VSI was observed in LRR. The highest (p < 0.05, ANOVA) haematological parameters were observed in LRR. This could be due to fish stress resulting from the low DO concentrations in the water. The highest blood performance of 3.22 was observed at the high recirculation rate treatment. This could suggest that the high recirculation rate resulted in the best health of O. mossambicus. No significant differences(p > 0.05, ANOVA) by recirculation rate. Mortalities that occurred in each recirculation rate treatment were not related to recirculation rate; they may have been due to handling. The highest (p < 0.05, ANOVA) organosomatic indices (CF, HSI and VSI) were observed at HRR. Poor fish health in terms of CF, HIS and VSI was observed in LRR. The highest (p < 0.05, ANOVA) haematological parameters were observed in LRR. This could be due to fish stress resulting from the low DO concentrations in the water. The highest blood performance of 3.22 was observed at the high recirculation rate treatment. This could suggest that the high recirculation rate resulted in the best health of O. mossambicus. No significant differences (p > 0.05, ANOVA) in pH were observed between different treatments. The pH remained within acceptable limits for growth of O. mossambicus. The lowest DO concentration was recorded in the LRR treatments. In LRR, the lowest DO concentration was observed. The dissolved oxygen levels in LRR treatment were below acceptable limits for growth of O. mossambicus. Recirculation rate led to a decrease in NH3, NO3- and NO2- concentrations, while experimental duration led to their increase. This could be attributed to the decreasing accumulation of fish waste as recirculation rate increased. The P concentration also decreased with increasing recirculation rate and increased with the duration of the experiment. This could be as a result of the decreasing accumulation of fish waste as recirculation rates increased. The highest plant growth was observed in IRR. The maximum plant growth in IRR may be due to stable DO concentrations and nutrients in the system from the accumulating fish waste. These results show that recirculation rate had an effect on the growth performance of O. mossambicus low technology in aquaponic systems. The intermediate recirculation rate (1.5 L/min) may be optimum for the growth of both S. oleracea and O. mossambicus in aquaponic systems. A low technology coupled aquaponic systems was used in this study. The incorporation of a sedimenter into the system presents a solution to the build up of solid waste in the tanks and the subsequent water quality deterioration. This can also minimise the constant water exchanges. The optimisation of the coupled aquaponic system can allow higher fish and plant densities without compromising the water quality.The balance of fish and plant ratios can allow for optimal growth of both entities without water quality deteriorations.The choice of the fish and and plant species can also affect the nutrient build up in the water. Fish polyculture and plant intercropping are also functional in the optimisation of they system’s overall production. Optimal feeding frequencies can also be used without worrying about the nutrient build up.