Grouper Culture Using RAS - The Basic Calculations

Last week I visited 2 fish farms that using  recirculating aquaculture system (RAS). Interestingly, both have changed their species to another newly introduced species that having higher price and in demand in the local market. I like their idea of changing the species when the need arise. Having a better priced species is the rule of thumb no 1 in our RAS considerations, but surely is not the only factor that we must consider. More interestingly, they were changing from marine water species to fresh water species. That is the extra benefit of RAS. We may change the “climatic” conditions of our farming system without need to change the location or even the equipments.

What shall we know about RAS before we design it? First, off course the species that we intent to culture, their requirement or acceptable water quality. Please get the information on the acceptable levels of dissolve oxygen (DO), carbon dioxide (CO2), ammonia (NH4⁺ /NH3), nitrite ( NO2 -), nitrate (NO3-), suspended solids (SS) and salinity of our chosen species. Some grouper species have different tolerant towards fluctuation of the quality of some parameters. 

Second, we must quantify the target production over certain farming period (e.g: kg/month, mt/year). By knowing our targeted production, we may calculate the maximum standing stock (MSS), expected oxygen consumption and waste production from our system. We also will be able to calculate the water flow rate, solid and biological  filtrations systems. So, give our system a targeted production, say 12 mt/year.


Third, make a simulation standing stock for our 12 mt/year farm. Are we going to stock our juvenile in batches or all at one go? Say we are going to produce in 12 batches (1 batch/month) at 0.8kg/fish at 90% average survival rate. The survival rates for each month will be different. Normally, the fish survival will be higher when the reach bigger size. The farming period will be 8 month per batch. Therefor, we are going to stock  1280 fish juvenile/batch for each month.   If we are going to use fry of 50g each, in the first month we will have 51 kg of biomass in our system. Next month, with 83% of survival rate and average weight of 70, the same stock will grow becoming 158g each and the total biomass will become 168 kg. At the same time, the 2nd batch will weight 51 kg in biomass making total biomass in the 2nd month will be 219kg. The calculation goes on and on until we reach full 12 batches in the twelve month with biomass of 5034kg. Remember, we are going to sell our yield at the end of 8th month of each batches and restocking new fry in the next month. So, our calculation will results like this:

Standing Stock for 12 mt grouper farm with the Maximum Standing Stock (MSS). Table: Mahmud Ismail.

In order to know the highest standing stock of our system, we simulate the calculation until all batches in full operation. The highest standing stock for our system is 7264 kg which going to take place in the 16th month of operation. This value is called as Maximum Standing Stock (MSS) which indicates the highest load in our production in term of biomass of the fishes, maximum intake of feed and consequently the maximum waste formation. As can be seen, our MSS value is  around 61% of the targeted annual production.


Normally, we feed our grouper fishes around 1.2% to 1.5% of thier body weight per day (for calculation purpose, we use 1.3%). So, during the month 16, our Maximum Feed Consumption (MFC) will be: 7264 x 1.3% = 94.4 kg/day. However, in actual situation, the feeding rates varies according to the size of the fishes, at early stage (smaller size) the feeding rate normally higher then the feeding rate when the fishes are bigger in sizes.  


We shall continue later with calculation for oxygen consumption, carbon dioxide, biological oxygen demand (BOD), suspended solid and ammonia productions from this basic knowledge of MSS and MFC. Later, we shall also calculate the water flow requirement as well as the solid filtration system and the biological filter of our farm.