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20 Ton Tilapia Farm – Part 8 – Feed Calculations

Making feed estimations for our 20 ton tilapia farm will define the life support systems for the aquaculture system.  We will use the feed conversion ratio to determine the daily feed rate for each stage of growth.  The FCR is often not used correctly so some tips for better application of FCR we will cover.  From this we will develop a feeding strategy later in the series.   We can also use these base feed rates in aquaponics for loading and predicting plant growth. The largest assumption we make in the table below is the feed conversion ratio.  This is the amount of feed it takes to grow 1 kg of fish.  That will provide us with a daily feed rate for the last day in each stage.  Often loading is calculated from this rate, however we will be digging down into the daily feed rate in more detail in the next step of this series then setting out the loading from those calculations. The above table is an overview of the feed calculations for our farm.   Here we use the FCR (Feed Conversion Ratio) to determine the required daily feed.  The FCR is an erroneous or inexact  number.  FCR are a result you work out by the feed input and the weight gain of the fish after or during production which makes it very “plastic” when estimating. The resulting feed rate is very close to farm reported feeding and recommendations.  It is always good to cross check your estimates with known information and adjust as needed. As the feed determines most of the bio-metrics on the farm it is easy to fall into a trap of thinking you will achieve an excellent FCR when most times you will not.  You can attain a 1: 1 ratio with the young fish and as they get larger it declines.  However starting your design with low FCR may see you in a position where your life support systems are overloaded.  Best to keep them a little high for your design purpose and track the FCR during production. The Feed per day is calculated from the Feed as a percentage of body weight of the fish per day (%bw/day).  The growth for each fish per day and biomass increase per day is a result of previous steps where we make assumptions around their condition factor and length growth per month (or per day). The FCR is the most important consideration for calculating the feed rates.  Note in the chart above the Assumed FCR has been lowered for all three stages in production.  Also note the corresponding lower Feed (%bw/day).  While we may target this outcome on the farm through good husbandry and feed management/quality, if the actual FCR is not achieved and is higher the resulting feed per day needed for growth will also be much higher.  The result will be overloading your biofilter, pumping rates will be too low, oxygen levels will be lower which may be the end of the road for your farm. Allow some “wriggle” room in the FCR and it will give you a little more capacity in the life support systems.  Remember what we might do on paper can only be as good as the data we are putting into it and often it will not match actual figures on the farm.  This does not mean we can not get very close, it simple means we need to be cautious. From this information we can break down our feeding to produce a full feed plan for the farm.  In the next part we will cover the feed rates per week through all the production stages which will help us determine the loading on the farm.  From there we go into the fun stuff; the design parameters….

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