White shrimp ( Penaeus vannamei) production is the most relevant aquaculture sector with a production of 4,966 million k g in 2020 and a commercial value of 28,782 million U$. Biofloc Technology (BFT) is a sustainable aquaculture system based on the principle of recycling waste nutrients, particularly nitrogen, into microbial biomass , which can be used in situ by the animals produced or collected and processed. It should be noted that the protein content of feed and feeding regimes can affect not only the growth performance, but also influence water quality through nitrogen excretion, thereby inducing eutrophication. Penaeus vannamei requires 30– 50% crude protein (CP) in its diet, but i t is important to take into account the supplementary nutrients supplied by biofloc , that it has been demonstrated that dietary protein levels of 24% gave comparable results to 32 or 40% protein (Panigrahi et al., 2019) . The feeding regime is a controversial research field that needs to be more studied. Up today, most farmers fed shrimp based on conventional tables, which consider the size and biomass of the organisms to adjust the feeding rate . Tables do not consider the availability of natural productivity of BFT, resulting in a possible overfeeding or underfeeding that eventually might lead to adverse consequences . As consequence, the aim of the present study was to optimize the protein feeding rate using feeds with low protein content at different feeding regimes.
Material and Methods
In the present study, five protein content levels: 30, 34, 38, 42, and 46% were assayed using three feeding rates: 100, 85, and 70% using as reference the feed intake (FI) proposed by Kureshy and Davis (2002) as 100% group. Each group had 4 replicates, therefore a total of 60 experimental units. The feeds were manufactured by adjusting the carbohydrate (Panigrahi et al., 2019) and dietary lipid levels to 10%. All experimental groups were manually fed three times a day. Once shrimp reached an average weight of 0.5 g, they were introduced into the tanks at final densities of 350 shrimp/m2 (super-intensive production conditions). To assess both survival and growth performance, shrimp were sampled weekly (Kuhn et al., 2010). The animals were housed in 90 L tanks at a salinity of 21±0.15 g/L, temperature of 28 ºC, pH 7.5-8.5, oxygen >5 mg/L, and alkalinity >150 mg/L monitored daily by the multiparametric HANNA equipment, HI19829 Model . Ammonium, nitrite, nitrate, alkalinity, and phosphate values were measured weekly using colorimetric kits in a spectrophotometer (Hanna I nstruments). Total suspended solids (TSS) were determined following the protocol of Strickland and Parsons (1972), by weighing the solids retained on glass-fiber Whatman filters (0.45 microns) after filtering the water samples.
Results and discussion
The water quality parameters behaved as expected within the pre-established values, showing tolerable levels for white shrimp production. Regarding the evolution of nitrogenous compounds, as expected in a culture with a mature biofloc, it assimilated the ammonium, nitrite, and nitrate resulting from shrimp production, and the levels remained below 0.09, 0.06, and 170 mg/L, respectively.
In terms of growth performance, the evolution of weight based on protein content or feeding rate showed similar results, finding significant differences mainly at the end of the trial (Figure 1).
A s is expected feeds with a higher protein content (46, 42 and 38%) achieved the highest final weights ( 5.5 ± 0.3, 4. 4 ± 0.6, and 4.7± 0.2 ; Table 1) , in a greement with Correira et.al (2014), with higher growth in diets containing 40% protein. In addition, no distinctions were related to growth according to feeding rate , which may be explained by the consume of natural microbiomase present at Biofloc. In contrast, Kureshy and Davis (2002) , obtained better growth with higher feeding rates; and determined that due to a restriction of feed intake and consequently protein intake, diets with low protein content did not support maximum weight gain. N o significant differences were observed in relation to survival between experimental groups (64%).
In conclusion, up to 5g in super-intensive production a reduction of 70% is possible, what is translate not only into better productivity but also water quality management.
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These results are part of the I+D+i Research Project: “Optimizing shrimp feeding and nutrition in biofloc system (BioFlango )” (PID2020-114574RB-C21) , and the Research Personnel Training Grant (PRE2021-098367) ; supported by MCIN/ AEI/10.13039/501100011033/.