This study aimed to evaluate the in vitro effect of carvacrol on different microorganisms of importance in shrimp farming, as well as its in vivo effect on zootechnical, immunological and microbiological performance, as well as resistance, of Litopenaeus vannamei challenged with Vibrio parahaemolyticus. In particular, the antimicrobial activity of carvacrol was evaluated in vitro by analysis of the minimum inhibitory concentration (MIC) and by agar diffusion disc with Gram-negative and Gram-positive bacteria. The in vivo experiment was conducted using different concentrations of carvacrol (1, 3, 4 and 6 mg mL-1) added to shrimp feed, together with a control diet without carvacrol. After four weeks, zootechnical, immunological and microbiological parameters, as well as resistance, of animals challenged with V. parahaemolyticus were evaluated. The MIC of Vibrio alginolyticus and Vibrio harveyi was 0.078 mg mL-1, while for the other bacteria, it was 0.156 mg mL-1 of carvacrol. The greatest halos of inhibition were observed in V. parahaemolyticus and Vibrio harveyi with significant differences demonstrated for the other microorganisms, except Escherichia coli. The in vivo results showed no significant differences among treatments. In conclusion, the antimicrobial activity of carvacrol was confirmed with Gram-negative and Gram-positive bacteria, and it is suggested that its antimicrobial potential is more effective against Vibrio spp. However, the concentrations of carvacrol used in vivo did not affect the parameters evaluated.

Nile tilapia (Oreochromis niloticus) has an opportunistic omnivorous feeding behavior and studies have demonstrated the consumption of microorganisms by the species from bioflocs, especially in the early phases of life. One may thus assume that when reared in biofloc systems, there may be a reduction in the feeding rates in the Nile tilapia nursery system when compared to the ponds. This study evaluated the effects of different feeding management options in Nile tilapia nurseries in a biofloc system. A total of 700 fingerlings (2.37 ± 0.55 g) were stocked in 20 experimental units (100 L) and inoculated with mature biofloc for the evaluation of five different feeding management options. After 49 days, it was observed that the higher feed rates caused an increase in nitrogen compounds and solids in the water, worsening the feed conversion rate. Furthermore, the excess feed supplied to the tilapia in the biofloc caused a reduction in the utilization of protein nitrogen, an increase in the liver and visceral fat, in addition to an increase in the production of solids and consumption of alkalinizer by fish biomass. On the other hand, low feeding rates cause a decrease in the growth and uniformity of the tilapia juveniles. Taking the results into consideration, it is recommended to use the feed management option proposed in treatment TC (26% below the highest feed rate) in the tilapia nursery in the biofloc system.