In present study, the water treatment filters were efficient and at the densities tested are within the appropriate range for the cultivation of C. macropomum. The temperature and pH remained within the range recorded for farmed C. macropomum in other cultivation systems (Silva et al., 2013; Santos et al., 2014; Silva and Fujimoto, 2015; Sousa et al., 2016; Saint-Paul, 2017; Costa et al., 2019). However, the lettuces have optimal growth with temperatures around 24 ºC (Rakocy et al., 2006; Rakocy, 2007), suggesting that the temperatures here observed (25–30 ºC) did not remain within values recommended for cultivation of this vegetable. The increase in the water flow rate is a factor that might favor lower temperatures and higher DO concentrations in nutritive solution, due to less exposure of the nutrient solution during the time of increased incidence of solar radiation (Genuncio et al., 2012). It is essential that the water flow rate between fish tank and wetland increased, reducing the timer control the interval of every water pump activation. Another suggestion is the choice of lettuce cultivars adapted to higher temperatures, as observed by Rodrigues et al. (2008) who studied the performance of lettuce cultivars in Manaus, in Brazilian Amazon region. Moreover, mean temperature, TDS, DO, pH, nitrogen compounds, and alkalinity values found in this study were similar to reported for other aquaponics systems (Sikawa and Yakupitiyage, 2010; Roosta, 2014; Zou et al., 2016; Cerozi and Fitzsimmons, 2016; Costa et al., 2019).
It has been reported that constraints to the use of aquaculture water for lettuce production are due to a low macronutrients’ concentrations, low dissolved oxygen levels and high suspended solids concentrations (Sikawa and Yakupitiyage, 2010; Sace and Fitzsimmons, 2013). In present study, the organic waste volume generated trial seems that has not provided adequate nutrition for lettuce. However, in hydroponic systems, the growth of lettuces suffers significant influence of with ionic concentration and water flow rate of the nutrient solution, which are relevant variables at the nutrient availability and water retention (Genuncio et al., 2012; Guimarães et al., 2016).
Leaves number, total fresh mass, and yield of the lettuce varieties studied were relatively superior that the values reported by Sikawa andYakupitiyage (2010); however, were lower than the values reported for aquaponics systems using effluent enriched with a nutrient solution (Seawright et al., 1998; Rakocy et al., 2006). In addition, interactions between fish stocking density and leaf number, total fresh mass, and yield of lettuces were found here. Both varieties of lettuces had an increase in biomass, possibly due to the increased availability of nutrients in aquaponic system. Although no additional nutrient solution was added in our aquaponic system, except hydrated lime, the lettuce biomass obtained was like values reported for grew lettuces hydroponically using saline wastewater from fish farming (Guimarães et al., 2016). On the other hand, number of leaves, total fresh mass, and yield of lettuces, in present study, were relatively lower than the values reported for grew lettuces hydroponically with uses of nutrient solutions (Genuncio et al., 2012) and in an aquaponics system of lettuces with the addition of biofertilizers in different substrates (Jordan et al., 2018). Therefore, our results showed the potential use of C. macropomum cultivation tank water for supplying a family hydroponic production of lettuces, because for the commercial production of lettuces is necessary add nutrients to aquaponics systems.
Stocking density is critical for successful of C. macropomum production because it influences numerous growth performance parameters (Silva et al., 2013; Santos et al., 2014; Silva and Fujimoto, 2015; Sousa et al., 2016; Costa et al., 2019), as well as the sanity. With the increasing of C. macropomum cultivation there is also an increase in the load of ectoparasites such as Ichthyophthirius multifiliis, Anacanthorus spathulatus, Notozothecium janauachensis and Mymarothecium boegeri, due to a poor environmental quality (Dias et al., 2015; Baia et al., 2019), which may lead to economic losses due to epizooties in fish farming. However, we found not difference in the infection levels by I. multifiliis and monogeneans A. spathulatus, N. janauachensis and M. boegeri between the different densities of C. macropomum used in aquaponic system. In addition, mean abundance of these parasites was lower that for C. macropomum farmed in ponds (Dias et al., 2015) and in net-cages (Baia et al., 2019).
An inverse relationship between stocking density of fish and growth has been reported for C. macropomum in different farming systems, suggesting that high stocking densities can negatively influence their final yield (Santos et al., 2014; Silva and Fujimoto, 2015; Sousa et al., 2016; Costa et al., 2019), and this fact was also observed in present study. The results showed that C. macropomum did not present apparent stress in the three stocking densities used. Still, a worsened in the growth parameters values was noticed in the final of the study with increasing stocking density. The effects observed include the final weight decreased, FCR, and consumed food increased, suggesting that C. macropomum farmed in small tanks can be sensible to the intraspecific competition. Nevertheless, survival was not affected, and productivity has increased with increasing stocking density, which are therefore positive parameters.
Survival of C. macropomum in aquaponic system were similar to reported for this fish farmed in other systems of production (Santos et al., 2014; Silva and Fujimoto, 2015; Sousa et al., 2016), suggesting therefore that the cultivation conditions were suitable for development of this Amazonian fish. Thus, hypothesis of a higher survival and productivity in handling with low stocking density of C. macropomum was not confirmed in this study, corroborating that this fish species tolerates intensification during fingerlings phase and can be raised in both semi-intensive and intensive farming systems with high survival and productivity; in addition, in aquaponic systems may be used densities beyond 80 and 120 fish m3.
High fish stocking densities in a reduced space can be led to the occurrence of problems related to stress, and it could interfere negatively in the growth and development of individuals. In general, in small volume tanks, the feeding area is reduced, causing the food dispute to be great, hence, there is an energetic expense that leads to an increase in feed consumption to maintain vital functions and continue to grow (Gomes et al., 2004). Although the growth performance parameters of C. macropomum fingerlings in aquaponic system has been negatively affected by the increasing stocking density, the parameters were compatible, and even superior to those reported for this same fish in others farming intensive systems (Silva et al., 2013; Santos et al., 2014; Silva and Fujimoto, 2015; Sousa et al., 2016). Therefore, results indicated that C. macropomum has a good performance in aquaponic system using constructed semi-dry wetlands, and the fish yield observed suggests that this Amazonian fish is a species suitable for the aquaponics systems. As 80 and 120 fish densities were the most productive, it is recommended for the rearing of C. macropomum fingerlings at a cycle of up to 110 days.
In conclusion, survival and productivity of C. macropomum in aquaponic system were high, in contrast to other growth parameters observed in traditional amazon fish farming system. The parasitic infection levels in gills were low, despite the high density of fish, demonstrating that aquaponics is isolated system that can be efficiency on reduce the occurrence of numerous parasite species being good option for intensive farming C. macropomum and other fish species. The water treatment using semi-dry wetland were efficient and at the densities tested are within the appropriate range for the cultivation of C. macropomum. Aquaponics system using semi-dry wetland is a potential system for intensive C. macropomum production with small environmental impacts.