Water quality parameters remained within the range suitable for the growth of both species (Van Wyk and Scarpa 1999). Temperature is one of the most important parameters, as it can affect food consumption, growth and survival of the species in culture environments (Hostins et al. 2015; Wang et al. 2006). According to Van Wyk and Scarpa (1999), the Pacific white shrimp L. vannamei can survive in a wide temperature range (15–35°C), however, for the best zootechnical performance it should be between 28–32°C (Walker et al. 2011). Zuñiga et al. (1988) found that growth of L. vannamei was negatively affected at temperatures below 23°C and Ponce-Palafox et al. (1997) observed that temperatures of 20°C negatively affect the species’ food consumption and growth.
There is little information regarding the ideal temperature range for growth and survival of F. brasiliensis, especially in the grow out phase. However, it is known that native species, both F. brasiliensis and F. paulensis show greater tolerance to low temperatures when compared to L. vannamei (Poersch et al. 2006; Wasielesky Jr. et al. 2016). According to Hostins et al. (2015), evaluating the effect of temperature in the nursery stage, found that the best temperature for their growth is 27°C. The average temperature of the present study was 25.5°C, being within the acceptable range for the growth of both cultivated species. During the first weeks of culture, the average temperature in both treatments was near 29°C, in the optimal range for growth. However, there was a gradual decrease in temperature starting in the fifth week, reaching 21°C at end of the experiment. The progressive decline in temperature throughout the second half of the experiment period was one of the main factors responsible for the decrease in weekly weight gain and growth of both species, especially L. vannamei. The reported temperature drop was caused by successive cold fronts (coming from the South) that passed through the region where the experiment was carried out.
Dissolved oxygen (DO) is a key parameter for successful shrimp culture (Zhang et al. 2006) and DO concentrations above 5.0 mg L− 1 are recommended (Durai et al. 2021; Van Wyk and Scarpa 1999). In BFT systems, the DO requirement is higher, as besides the consumption by the shrimp, there is an additional consumption by the microorganisms present in the bioflocs (Cohen et al. 2005; De Schryver et al. 2008; Hargreaves 2013). In the present study, the average DO concentration was 6.5 mg L− 1, above the recommended level for shrimp culture throughout the experiment. The pH, alkalinity, and salinity were within the values recommended by Van Wyk and Scarpa (1999).
The continuous monitoring of TSS concentration in BFT systems is essential for the maintenance of water quality as well as for the best zootechnical performance of the shrimps (Avnimelech 2009; Gaona et al. 2011; Samocha et al. 2007). In this study, there were no TSS significant differences (p > 0.05) between treatments and were within the recommended range for penaeid species (Gaona et al. 2017, 2011; Schveitzer et al. 2013). Although it does not have a recommended value for F. brasiliensis, the TSS values were similar to that found by De Souza et al. (2011); Emerenciano et al. (2012); Hostins et al. (2015) for culture in biofloc system. From the sixth and seventh weeks for the LV and the FB treatments, respectively, the clarification process was started aiming at decreasing the TSS concentration as suggested by Gaona et al. (2011) and by Zemor et al. (2019). According to Vinatea et al. (2010), turbidity has a direct relationship with TSS concentration as an escalation in suspended particles increases turbidity in the culture water, decreasing light penetration. In the present study, turbidity did not show significant differences between treatments and followed the variation of TSS throughout the experiment.
Ammonia is the main nitrogenous form of crustacean excreta (Regnault 1987) and in high concentration can be toxic and harmful to shrimp, as can be its metabolites (nitrite and nitrate) (Furtado et al. 2015; Lin and Chen 2001, 2003). The values of these nitrogenous compounds found in this study, remained within tolerable limits for both species (Campos et al. 2015; Van Wyk and Scarpa 1999), causing no negative effects on the zootechnical performance of farmed shrimp. Although from the 50th experimental day, the nitrite concentration in the LV treatment gradually increased, remaining higher compared to the FB treatment until the end of the experiment (Fig. 1b), these levels did not exceed the recommended limit for L. vannamei (Lin and Chen 2003). The continuous clarification process may have caused this increase in nitrite concentration in the LV treatment. It is possible to observe that from the 6th week of culture, when the solids removal process started in this treatment, the nitrite concentration also started to increase. The intense clarification to remove TSS may have also been responsible for removing heterotrophic and nitrifying bacteria present in the biofloc, leading to an increase of nitrite in the system. Ray et al. (2010) observed that the removal of suspended solids decreases the bacterial abundance from the microbial community in the BFT system. It is therefore likely that the intense clarification process from the 6th week onwards caused a reduction of bacteria and a greater increase in the total ammonia nitrogen in the water as the bacteria are responsible for its removal (Ebeling et al. 2006).
The concentration and behavior of orthophosphate in a culture are related to the constant nutrient input, with the decomposition of uneaten feed and the excretion of cultured organisms being the main source of phosphorus in the system (Barak et al. 2003; Samocha et al. 2017). Samocha et al. (2017) reported that they found orthophosphate concentrations of 32 mg L− 1 in culture farms with no harm to shrimp performance. In this present study, the average orthophosphate concentration found in both treatments was much lower than reported by Samocha et al. (2017).
The results of zootechnical performance showed significant differences among treatments, with higher values of final weight, weekly weight gain, biomass, survival and productivity for L. vannamei when compared to F. brasiliensis treatment. Some studies have shown good zootechnical performance results for F. brasiliensis cultured in biofloc system. However, these encouraging results are reported only in the nursery phase (De Souza et al. 2011; Emerenciano et al. 2012; Hostins et al. 2015). Information on the grow out in the BFT system is lacking for both F. brasiliensis and L. vannamei. Several studies corroborate the results regarding the adaptability of later species in this culture system (Brol et al. 2021; Krummenauer et al. 2014; Reis et al. 2019).
Stocking density is a factor that plays an important role in survival rate and other zootechnical parameters. In this study, significant differences were found in survival rates between LV and FB treatments. In other comparative studies survival rates were not statistically different (e.g. Peixoto et al. 2003; Sandifer et al. 1993), though stocking densities were lower compared to the present study. Da Silveira et al. (2020) and Krummenauer et al. (2011) tested higher stocking densities than that used in this study (for L. vannamei) and found no significant differences in survival rates between treatments. Lopes et al. (2012) observed that F. brasiliensis could be reared at densities of up to 600 m− 2 in a BFT system in the nursery phase without affecting survival. Although there are no studies showing the effects of different stocking densities of this species in the grow out phase, Krummenauer et al. (2006) analyzed different stocking densities for the native pink shrimp, F. paulensis and recommended that this species can be reared at stocking densities between 40 to 120 shrimp m− 2, obtaining suitable zootechnical performance indices. In the present study, the density used was 100 shrimp m− 2, suggesting that the survival rates of both species reared in the present study were not affected by the stocking density used.
Determining the protein digestibility of feedstuffs that make up shrimp feed is important for developing balanced, high quality diets (Ayisi et al. 2017; Cruz-Suárez et al. 2009; Lee and Lawrence 1997). In addition, effective diets based on the response of different shrimp species can offer many advantages, such as a better feed conversion rate and faster growth (D’Abramo et al. 1997). In this study, both treatments were fed with commercial feed specifically developed for L. vannamei based on the nutritional requirements of this species. This possibly favored the occurrence of lower AFCR rates for L. vannamei compared to F. brasiliensis.
Regarding the utilization of biofloc as food supplement for these species, it seems that the contribution was greater for the L. vannamei. Bufford et al. (2004) also showed a significant contribution of biofloc for the nutrition of this species. The structure of its third maxilliped probably has contributed to the better nutrition and overall zootechnical performance of L. vannamei. This appendix is the largest and outermost buccal apparatus in decapod crustaceans (Eap et al. 2020; Garm 2004) and performs several functions, including the manipulation of food particles (Alexander et al. 1980; Parra-Flores et al. 2019). In L. vannamei the endopods of the third maxilliped are covered by longer, more abundant and feathered bristles (Fig. 4a and 4b) that facilitate the capture of biofloc particles, whereas F. brasiliensis (Fig. 4c and 4d) has simpler and shorter bristles, that makes particles’ capture more difficult. Kim et al. (2015) also observed the same difference between the third maxilliped of L. vannamei and other penaeid species and showed that the former has a higher efficiency in capturing biofloc. Krummenauer et al. (2020) using the stable isotope analyses showed that bioflocs can contribute up to 86% of the feed of L. vannamei in the grow out phase. These results corroborate those found in this study and show that L. vannamei can have better performances in this type of culture system.