The influences of supplementing Diatomite in the brackish water biofloc system for white-leg shrimp farming:
White-leg shrimp (Penaeus vannamei) plays an important role over the world due to big demand and because this is a species that can adapts to a wide salinity range with good growth. Biofloc (BFT) is one of the typical farming models with many advantages for growing white-leg shrimp.
Biofloc system focuses on maintaining the balance between the ratios of carbon and nitrogen (C: N) in the water as an essential condition for the growth of heterotrophic bacterias. However, microalgae is the second largest group that grows in this farming system, it plays an important role in maintaining the water quality and also is a nutrient source. The system of idiotrophic biofloc (dominated by microalgae) contains a higher level of lipid than in the heterotrophic system (dominated by bacteria), it may affects the growth and feed using efficiency of cultivated species. There was a report in the past that showed the supplementation of sodium silicate into biofloc environment helps to enrich types of diatom.
The role of silica in shrimp farming
The silica compounds were used to increase the solubility of silica in the water. However, its effects on the water quality and shrimp’s growth due to the difference of physical and chemical properties between the zeolite sources. Using zeolite (silicate mineral from natural or synthetic sediment) can improve the doubling time of diatom cell and stability of culture medium; however, zeolite has poor solubility and release low silica. Therefore, it is recommended to use other silica that has higher availability like diatomite and its derivatives.
Diatomaceous earth (D.E.) also known as diatomite is a sedimentary rock that formed from diatoms, after diatom dies, its dead body will be settle and formed diatomite after thousands of year. The main composition of diatomite is silicon dioxide (SiO2).
Supplementing diatomite into tiger shrimp pond that applies biofloc technology
This result is from the research of Emmanuel Martínez-Montaño and co-workers in 2020 to evaluate the influence of diatomite on the water quality, growth rate of white-leg shrimp P. Vannamei that grown in seawater (E1) and brackish water (E2) of the biofloc technology.
Two independent experiments (E1 & E2), each has 3 treatments with biofloc was evaluated: the treatment D with weekly diatomite supplementation (10gr/m3); the treatments D combine with a continuous inoculum of diatom Chaetoceros muelleri (DD); compare to a group of shrimp that grown by biofloc.
Experiment 1 (E1): seawater with a salinity of 32% for 47 days. Creatures will be cultivated 7 days before the experiment to promote the growth of biofloc, the tanks are fertilized with urea mixture (1g/m3), phosphoric acid (1.5 g / m3), and wheat ( 20 g / m3) for three days, together with 25 g / m3 of industrial ground feed and molasses until shrimp stocking: then molasses and wheat are daily supplemented as a carbon source to maintain the ratio 20: 1 of C: N.
During the farming process, a mixture of molasses and cornstarch which were hydrated for 24 hours and be added daily to each experimental shrimp tank. Then shrimp (lot 1) were stocked with the density of 200 shrimps/m3 (shrimp PL19) in each experimental unit. The feeding rate was 10% of the estimated biomass per tank until shrimp reached an average weight of 1 g; after that, it was adjusted to 5% for the remaining of the test. The bodyweight of random of 20 shrimp per experimental unit was measured and recorded.
Experiment 2 (E2): brackish water with a salinity of 14% within 49 days. Before the experiment, water containing biofloc from three control experimental units of E1 was pumped into a 9 m3 tank to adjust the salinity to 14 %. This biofloc tank was used to grow new shrimp (lot 2) until shrimp reached an average weight of 1.42 ± 0.45 g. Weekly feeding rates and biological indicators were measured according to the methods described for E1. Biofloc was maintained for a C: N 20: 1 adjustment in molasses and cornmeal tanks as described for E1.
Diatomite improves cultivated shrimp in the system of brackish-water biofloc
After seven weeks at E1, there were no significant differences were noted in survival, weight gain (WG), specific growth rate (SGR) and feed conversion rate (FCR) between DD treatment and the control one. However, in the brackish water biofloc system (E2), diatomite supplementation promoted significant growth of baby shrimp, measured by final weight, weight gain, and final biomass compared to the un-supplemented group of diatomite.
Systems with autotrophic biofloc (dominated by microalgae) provide higher lipid and lower protein content, compared to heterotrophic systems (dominated by bacteria). Particularly, for farming of white-leg shrimp, it is recommended to use mixed biofloc (both microalgae and idiotrophic bacteria), since it helps to increase productivity and decrease production costs by using less commercial feed and oxygen. Thus, improvement of shrimp’s growth is associated with supplementation of diatomite, which promotes flocs in the tank.
The intensive production model of white-leg shrimp in marine biofloc systems showed a significant increase in PO4 which is associated with fecal accumulation and decomposition of excess feed. In this experiment with seawater, diatomite supplementation was related to a decrease in PO4 levels. Silica compounds such as diatomite and zeolite have been extensively used to remove dissolved PO4 through adsorption and exchange of ions. Besides, the reduction of PO4 levels observed in biological tests with biofloc in seawater may be related to the effect of diatomite on microalgae growth, which is effective in nitrogen removal and recycling. and PO4 compounds in white-leg shrimp farming. Explaining this, microalgae is the second major group growing in the BFT system and SiO4 is one of the essential nutrients required for proper growth of algae, especially for diatoms, as well as in freshwater biofloc systems.
In the system of brackish biofloc E2, the settable solids concentrations were much higher in the diatomite-supplemented treatments compared with the control group. Our results recommend using diatomite to improve the growth and feed efficiency of shrimp cultured in brackish water biofloc systems.