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20 Ton Tilapia Farm – Part 5 – Stocking

With more understanding how moralities effect our tilapia fish farm design we move onto stocking calculations which help determine the tank size and loading rates for our life support systems later in the series.  Fish stocking densities can be confusing because while the professional aquaculture community refers to stocking density as an amount of fish per volume of water, eg; kg/m3 or lb/gal there are physical requirements of the fish to consider as well. When staging fish through the farm we need to consider what size and weight they will be at the end of each stage to determine the stocking and tank size requirements.  The more stages and grading in the system design the more calculations are required.  Keep in mind our current calculations are for only one cohort or batch of fish going through the system, which we adjust to suit our overall production for the year a little later on in the series. Knowing what size the fish will be at the end of each stage is critical to defining the size of the fish tanks you will need to maintain a manageable (and sustainable) fish density. Density Sidebar:  A word of caution, avoid stocking at a density just because you can.  The density must be something you can work with practically.  All too often I have seen people stock very high numbers and weight of fish because they read they could somewhere.  We have discussed density in home systems before here. Any density recommendations you hear or read about must be qualified with proven science behind it and the numbers that support it such as feed rate per day, culture tank exchange and oxygen supply.  We will be covering these in detail in later parts of this series.  For the home grower, we covered this in How Many Fish – 4 Questions you should Ask. I have seen farms that push the boundaries of density and not many of them survive even if their life support systems are set up for the load.  In nearly every case the farm running at maximum biomass has failed from human error.  Even lower density farms have failed from simple mistakes on the farm but fewer than those pushing the boundaries. If you can justify running a lower density and still remain commercially relevant and viable, then choose that path.  I understand more than most, clients are looking for the maximum return per m2 or f2 of investment. The general definition of risk is the chance an investment’s actual return will be different than expected.  That is can be no clearer than losing all of your stock, where there is no offset to the risk.  Dead fish don’t sell well. Let’s move onto the density for our farm. Each stage of our production will have a maximum density capacity for the fish in terms of physical size.  You will recall the “Density” in part 2 of this series.  The result of this calculation is the “Check (kg/m3)” above.  Note as the fish get larger, the allowable fish mass also goes up with it.  This is merely a check against setting our density too high for the fish species at various sizes.  While the allowable density does get very high (eg: end of grow out is 121 kg/m3 unless you have a very sophisticated facility and substantial skill, avoid those densities. What is notably important, is the allowable density in the quarantine stage.  We may be easily lead to believe because the fish are so small (6 grams) we can “stuff” in a mountain of fish like cans of sardines.  That is not the case and the density check makes sure we don’t exceed what is physically acceptable for the species and size of the fish.  Very important when crowding very small fish. Density Check Chart From the density checking chart above you will note at 35 weeks we have a fish at around 30 centimeters which is at our harvest weight of 500 grams.  Note at that stage in growth we can run the fish at 121 kg/m3.  This density can be very difficult to manage and with increased density, your risk also increases.  In such a high risk business it is imperative you only take the risks you can afford. We can most certainly run Tilapia at 121 kg/m3 with liquid oxygen but you may compromise on the quality of the fish coming out at harvest in such crowded conditions.  The density you choose is really dependent on the risk you are prepared to accept, the skills you or your staff have and if the system can support the load.  The loading we can do through design however in our scenario we don’t want to push our luck. Consider your business model.  We discussed Quality verses Quantity previously and with a small 20 ton farm, you will be looking to “beat” the very large mass producers on the quality of your fish.  Sure, we can design the overall system to cope with around the 80 kg/m3 but let’s run with a very comfortable 45 kg/m3 and be sure we consistently get our high quality fish to market. In this scenario we may even start running our farm if we are a little less experienced, at 25 kg/m3 until we enter the market and gradually increase the density to the maximum as you gain market exposure and penetration. Remember we have 5 week cycles over 35 weeks so it will only take 6 months to increase your fish production which you can start doing after your first batch out the door after 35 weeks so there is no rush. You will note we have kept the density very low in the quarantine.  This is because my preference is to have plenty of small tanks in the quarantine to make the small fish easy to handle.  You may grade several times while the fish are in quarantine and it is great to have the additional tanks to grade into. For example we may have 6 x 500 liter tanks in the quarantine.  When we first get the fish their stocking maximum is 11 kg/m3 however the total biomass of the fish entering the quarantine is only 2.73kg, so we may put them all into one or two tanks and grade them every week into the other tanks. In the last stage you will notice there is a difference in biomass.  The harvest biomass we are aiming for is 1923 kg per cohort, which we calculated in the first part of the series, however in our density calculations the harvest weight is 1989 kg.  This difference is the same for each of the stages where we have not accounted for any losses as discussed in part 4 (mortality) and assumed all the fish entering each stage will survive.  We have accounted for any culling we may do through between each stage. You will remember the numbers stocked in each stage as per the chart to the left.  We calculate on the stocked number not the number we expect will be leaving the system.  However, between each system we account for the losses there because we will be intentionally culling at the end of each stage.  If we were to have one stage and not cull (unlikely) then the 4089 entering the grow out system (shared life support for the last two stages), will be carried right through to the end regardless of any losses. To clarify that some more.  In the last two stages, they are 15 weeks long each.  We start with 4089 and we select a batch of 10 to 15 out each week for measuring and checking.  Over the 15 weeks we will cull around 200 fish leaving us with 3885.  In the last lot we will check a little less often. It is important to work a density that works for your farm and skill.  When we change the density so too does our tank sizes change.  In the next two parts of this series we will be looking at tank size selection for pre-made tanks, calculating how many of them you need and defining tank dimensions for tanks you make yourself onsite….

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