**5. Economics and types of carbon sources**

equilibrium), an inoculum of a previous BFT culture can be used once sanitary conditions are

It is important to note that as long as the production cycles advance, nitrifying (chemoautotrophic) bacteria play a major role in N-compound control. In addition, suspended particles or solids (bioflocs) also will be increasing over time. With this information in mind, carbon addition could be reduced (or even stopped), preventing the excess of solids (bioflocs) in the cultured system that will lead an excessive DO consumption [65] and shrimp/fish gill occlusions [66]. For the maintenance phase, the monitoring of TAN values is an important tool for water quality maintenance. When values of TAN are higher than 1.0 mg L−1, external carbon source application is recommended with a C:N ratio of 6:1 [36]. In such phase, the use of monosaccharide and oligosaccharide carbohydrate-rich types (e.g., molasses and other sugars) is recommended due to the faster bacteria assimilation and consequently TAN reduction. Same examples of C:N calculations for the phase I and phase II are presented as followed. For both examples, the carbon content of the feed will be considered 50% (based on dry matter). For the carbon source, molasses was chosen and its content in such case is also 50%. It is important to note that the carbon content will change according to the dry matter composition and type of carbon source. In a practical way, dry matter of the feed will be 90%. Fish and shrimp assimi-

**Example 1** (**initial and formation phase using a C**:**N ratio of 20**:**1**) **in a tilapia culture tank** 

C: 4 kg of feed × 0.9 (90% dry matter) × 0.7 (30% of fish assimilation or 70% of waste that remains in water)/2 (carbon content of the feed is ~50% based on dry matter) = 1260 g of C

N: 4 kg of feed × 0.9 (90% dry matter) × 0.7 (30% of fish assimilation or 70% of waste that remains in water) × 0.3 (30% crude protein content of feed)/6.25 (constant) = 121 g of N. **The** 

If I want a C:N ratio of 20:1, 121 g of N in feed × 20 = I need 2420 g of C. But I already have

If the molasses has 50% of carbon content (based on dry matter), 1 kg of molasses represents 500 g of carbon. So, 1160 g of carbon requirement will represent **2320 g** (**or 2**.**3 kg**) **of molasses**

**Example 2** (**maintenance phase and C**:**N ratio of 6**:**1**) **in a** *L*. *vannamei* **culture tank** (**30 m**<sup>3</sup>

tank = 0.002 g × 30,000 L = 60 g of TAN

)

1260 g of C (calculated in feed). So 2420 g–1260 g of C = I really need 1160 g of C.

satisfactory.

100 Water Quality

lation will be considered 35 and 20%, respectively.

*Calculation 1* (*C*:*N content in the feed*)

**results indicated a** ~**10**:**1 C**:**N ratio of feed**.

(**applied daily until biofloc maturation**).

**that indicates 2**.**0 mg L**<sup>−</sup>**<sup>1</sup> TAN values**.

*Calculation 1* (*TAN in water*)

For 2.0 mg L−1 of TAN in a 30 m3

*Calculation 2* (*adjusting the C*:*N ratio*)

*Calculation 2* (*adjusting the C*:*N ratio*)

**that receives 4 kg of feed** (**30**% **of crude protein**) **per day.**

The carbon sources applied in BFT are often by-products derived from human and/or animal food industry, preferentially cheap and local available. Cheap sources of carbohydrates such as molasses, glycerol, and plant meals (i.e., wheat, corn, rice, tapioca, etc.) will be applied before the fry/postlarvae stocking (fertilization protocols) and during grow-out phase, aiming to (i) provide food for the first stages of growth and (ii) to maintain a high C:N ratio and to control N-compound peaks in the culture tanks, respectively [67].

Depending of the carbon source chosen, organic fertilization could be considered as an important item of The production costs. Local available sources should be tested, but bacteria assimilation's characteristics will certainly need to take into account. Monosaccharide and oligosaccharide simple carbohydrate-rich types (e.g., glucose, sucrose-rich sugars, etc.) versus polysaccharide complex-rich types (e.g., starch and cellulose) will lead different bacteria assimilations, nutritional value, and growth. Crab et al. [16] evaluated the effect of different carbon sources for *Macrobrachium rosenbergii* postlarvae. Besides the price, different sources will lead diverse nutritional value of the flocs. The authors observed that when using glycerol as compared to glucose and acetate, higher values of n-6 PUFA were observed.

For each phase (initial and formation phase or maintenance phase), different sources should be chosen according to the price and purpose. For example, dextrose (high purified sugar) versus molasses; refined sugar versus grains by-products, etc. Grains and tubercles contain high levels of carbon (carbohydrates), as polysaccharides. Some grains used as carbon sources additionally contain protein and lipids. García-Ríos [68] compared three carbon sources into the BFT tilapia fry culture and found that corn meal contains 11.79% of protein and 2.8% lipid; meanwhile, wheat has 15.5% of protein and 3.73% lipid. The unrefined sugar (monosaccharide) without protein and lipid promoted the best growth and the highest protein content into the tilapia tissue. It is possible that the chemical structure of sugar presented a high bioavailability to heterotrophic bacteria, hence, fast increase of bacterial biomass.
