Protein Feed Production
6.4 Cost Balance
Table 6.5 shows the cost balance for the described duckweed-based tilapia production.
Tilapia fillet yield is assumed at 33 % of total fish FW.
Table 6.5: Cost balance based on a pond area of 1 ha, producing 10 t (FW) of Nile tilapia per year. Numbers based on estimates for Paraguay, without taxes.
Amount Price per Total Unit Cost Capital Costs
Land (for 1 ha pond area) 1.26 ha 3,000 $/ha 3,780 $
Earthworks 1,500 m3 2 $/m3 3,000 $
Solar Panel (12 V, 130 W, 0.8 m2) 20 (16 m2) 75 $ 1,500 $ Voltage Converter (2,500 W, 12 V/230 V) 200 $ 360 $ Water Pump (230 V, 750 W, 19,000 m3/l) 3 150 $ 450 $
Selling Expenses Tilapia Fillet 3,300 kg 4 $/kg 13,200 $
Total 20,189 $
Income Sales Tilapia Fillet 3,300 kg 8 $/kg 26,400 $
Net Income Per Year 6,211 $
Break Even Point 1.8 years
6.5 Suggestion of System Adoptions for Alternative Nitro-gen Sources
The use of synthetic nitrogen fertilizer can be problematic, due to high volatility seen with high concentrations. Regulations in organic agriculture, environmental safety
con-cerns, energy consumption, availability and costs might also be reasons to look out for alternatives. Several options exist to reduce or circumvent the use of synthetic nitrogen:
Azolla
The needed reactive nitrogen can be fully supplied by Azolla, fixing up to 1,200 kg N/ha/y [Brouwer et al., 2017]. The downside to this approach is that Azolla is clearly inferior to duckweed as fish feed, as discussed in the chapters 4.1.3 and 4.2.3. Therefore, Azolla can only be part of the Nile tilapia´s diet, the percentage here is assumed at 12.3
% at DM basis, the rest is duckweed. The area needed for Azolla will be at 16 % of the original duckweed pond area, as shown in table 6.6.
All the reactive nitrogen will come from Azolla that is fed to the fish, that will then Table 6.6: Parameters of the duckweed and Azolla based aquaculture set-up
Fish Duckweed Azolla
Mat Density FW [kg/m2] - 1.00 1.00
Stocking Density FW [kg/m3] 43.76 -
-DM Content [%] 22.00 5.00 5.00
Standing Biomass DM [t] 0.55 0.42 0.08
Standing Biomass FW [t] 2.50 8.40 1.60
Feeding Rate [FW t/FW t/d] 1.00
SGR [%] 0.83 26.10 19.21
FCR [-] 6.00 -
-Annual DM Output [t] 1.67 40.00 5.60
Annual FW Output [t] 7.60 800.00 112.00
Nitrogen Budget [kg N/y] -155.50 - 192.00
Nitrogen Fixation Rate [kg N/ha/y] - - 1,200.00
excrete the nitrogen, which is then used to fertilize the duckweed. The duckweed pond and the Azolla should be separated from each other, in order to minimize reactive ni-trogen entering the Azolla pond. This would be wasteful, as it would decrease nitrogen fixation activity in the Azolla and unnecessarily compete with duckweed for available reactive nitrogen.
There is no research on the effect of feeding both duckweed andAzolla, but as shown in table 6.6, it is assumed that FCR will be higher and SGR lower.
Also, the duckweed productivity will be lower, due to less reactive nitrogen in the sys-tem. All factors combined, this will lead to a lower fish yield per area, but allows for a nitrogen fertilizer independent system. All other nutrients lost by harvesting the fish, besides nitrogen still need to be added.
Fish Slaughter Waste Recycling
Another option is to process the fish right after the slaughtering and feed the fresh slaughter waste such as fish heads, guts, fins, bones etc. back to the tilapia in the cages.
While intra-species recycling is a serious health hazard for most livestock, fish in general are not susceptible to this practice [Cheng and Lo, 2016].
The fillet (skinless) percentage of the whole fish lies at 33 %, dressing percentage without head at 66 % and with the head at 86 % [Racocy, 2009].
Depending on how the fish are being processed and sold, up to 67 % of the total fish harvest (FW) can be fed back to the fish.
FCR and SGR of the Nile tilapia would likely be improved by incorporating animal protein into their diet. At the same time, the need for fertilizer will be reduced greatly.
The productivity in terms of fish output could be improved by an estimated 4 %, as shown in table 6.7.
The fresh slaughter waste has to be ground to be fed to the fish, ideally in a processing unit directly above the fish cage.
Table 6.7: Parameters of the duckweed based aquaculture set-up with fish slaughter waste recycling
Fish Duckweed Slaughter Waste
Cages Pond Recycling
Organism Nile tilapia Duckweed Nile Tilapia
Area [m2] 114.20 10,000.00
-Depth [m] 0.5 0.50
-Volume [m3] 57.10 5,000.00
-Mat Density FW [kg/m2] - 1.00
-Stocking Density FW [kg/m3] 48.32 -
-DM Content [%] 22.00 5.00 22.00
Standing Biomass DM [t] 0.61 0.50
-Standing Biomass FW [t] 2.76 10.00
-Feeding Rate [t FW/t FW/d] 1.00
SGR [%] 1.03 27.40
-FCR [-] 4.95 -
-Annual DM Output [t] 2.29 50.00 1.53
Annual FW Output [t] 10.41 1,000.00 6.97
Table 6.8: Concentration of elements in mg/kg on a DM basis for the whole body, fillet and carcass of Nile tilapia, after [Gonzales and Brown, 2006].
Element Whole body Fillet Carcass
As can be seen in table 6.8, the concentration of most essential elements is far higher in the carcass than in the fillet. By recycling the carcass back into the system, the loss of major plant nutrients and essential trace elements is greatly reduced.
Alternatively, the fish can be dressed (gutted) and sold, so only the guts would be recy-cled instead of the whole carcass.
Food Waste Recycling
Food waste of restaurants, groceries, cafeterias etc. can be fed to the fish in addition to duckweed. The amount fed should be in accordance with the amount of fish harvested on a DM basis. More than that could overload the system with nutrients. This strategy would provide the big majority of nutrients, sparing most fertilizer needs. Additionally, it would likely improve growth characteristics of the fish and total fish yield, as shown in table 6.9.
Food waste is available at very low costs, but comes with a significant disadvantage: it is potentially contaminated with preservatives, pesticides, packaging material, additives etc. The impact on the nutritional quality of the fish will be low, as more than 95 % of the diet will still be duckweed (DM basis), but it does not come with the same purity as fish fed 100 % out of the system. This might offset a very unique selling point.
Table 6.9: Parameters of the duckweed based aquaculture set-up with fish slaughter
-Stocking Density FW [kg/m3] 48.62 -
-DM Content [%] 22.00 5.00 18.00
Standing Biomass DM [t] 0.61 0.50
-Standing Biomass FW [t] 2.78 10.00
-SGR [%] 1.08 27.40
-FCR [-] 4.80 -
-Annual DM Output [t] 2.40 50.00 2.4
Annual FW Output [t] 10.92 1,000.00 13.33