Influence of low salinity on microbial composition and performance of Litopenaeus vannamei juveniles grown in a super-intensive system without water renewal
Considered environmentally friendly, due to the minimal use of water and the emission of effluents, production systems carried out without water renewal present the additional advantages of the possibility of cultivating marine shrimp far from the valued coastal region and close to consumption centers, as well as complementation of the animals' diet through its characteristic microbial community. The shrimp Litopenaeus vannamei, due to its accelerated growth and tolerance to a wide range of salinities, represents a species of interest for cultivation in such production systems. In this context, the objective of the present study was to evaluate the influence of salinity on water quality, microbial composition and performance of L. vannamei juveniles grown in a system without water renewal.The experimental system was set up inside a greenhouse, consisting of 16 circular tanks (0.36 m2 of bottom area) with a useful volume of 163L. Groups of 110 prawns, with an average initial weight of 0.24 ± 0.08g, were randomly distributed in the experimental units (300 prawns / m2) and cultivated for 40 days in different salinities (0, 2, 4 and 25 ‰) with four repetitions per treatment. Prior to storage, over 10 days the prawns were properly acclimated to the different salinities. Feeding (commercial diet - 42.5% CP) was carried out with the aid of trays, twice a day, at an initial rate of 8% of the biomass in each tank and, subsequently, adjusted according to the observed consumption. The physical, chemical and biological parameters of the water (temperature, dissolved oxygen, salinity, pH, alkalinity,ammonia, nitrite, nitrate, phosphate, total suspended solids and chlorophyll a) were monitored periodically. The microbial community present in the different salinities was characterized and analyzed for its proximate composition. The performance of the animals (final weight, weight gain, specific growth rate, final biomass, individual total food consumption, feed conversion and survival) was evaluated. Most water quality parameters were not significantly influenced by salinity, except for the concentration of total suspended solids and pH. Despite the non-significant differences (P> 0.05), a trend towards intensification of the nitrification process was observed with the increase in salinity, with lower values of ammonia and higher values of nitrite and nitrate at the highest salinity (25 ‰) .There was a tendency towards a reduction in the concentration of ciliates and an increase in the concentration of flagellates with the increase in salinity. Microalgae were predominantly represented by diatoms at the highest salinity and by chlorophytes at low salinity. The percentage of crude protein in the microbial flakes decreased as the salinity increased, while the ash content showed the opposite behavior. Survival, final weight, final biomass and total individual food consumption were positively influenced by the increase in salinity (P <0.05). The other performance parameters, despite also showing a tendency to improve with the increase in salinity, did not differ between treatments (P> 0.05). In the present study, the best results were obtained with juveniles of L.vannamei grown in a system with no water renewal at 25 ‰ salinity, however, satisfactory productivity rates were also verified at 4 ‰ salinity, suggesting the feasibility of carrying out cultivation at low salinity.