Borgmann Aquaponics Hydroponics
Here is a brief explanation about FCR - and why "you" don't gain a kilo of weight with a kilo of "feed"... but some fish do.
Let's start with the utilization of fish feed, which at first glance leads to contradictions regarding the required feed amount.
Important Practical Notes & Limitations
- Growth is not linear: The formula is a strong simplification. Growth often follows a sigmoid curve (slow-fast-slow). In practice, one often starts with a higher number and thins out ("thinned") during growth.
- Oxygen is the most limiting factor: The maxDensity is usually limited by the oxygen solubility in water and the performance of the aeration, not by the volume itself. 20 kg/m³ is already very intensive stocking and requires excellent aeration and filtration.
- Water quality: Ammonia (NH₃/NH₄⁺) and nitrite (NO₂⁻) must be constantly monitored. The growth of the biofilter capacity must keep pace with the fish stock.
- Temperature: The specified optimum must be maintained stably. The lower the temperature, the lower the metabolism and the lower the stocking density must be.
- Feed amount as a control variable: In practice, one often controls via the daily feed amount as a % of fish biomass (Feeding at start (% of end), which decreases over time). The maximum feed amount that the system can process ultimately limits the fish biomass.
Recommended Workflow
- Determine system capacity: How much feed can your facility convert into nutrients daily (depending on plant area, bacteria volume)? A rough guideline: For 1 kg of standard feed, one needs ~50-100 m² of plant growth area.
- Calculate backwards:
- Harvest biomass (kg) = Daily feed amount (kg/day) * FCR * Growth period (days)
- Start biomass (kg) = Harvest biomass / Growth factor
- Start conservatively: Stock initially with only 50% of the calculated maximum stocking density. Increase the feed amount slowly and observe the water parameters (ammonium, nitrite, pH, oxygen).
- Adjust: With optimal water parameters, the stocking density can be increased in subsequent cycles.
Conclusion: The greatest art of aquaponics lies in balancing fish stock, feed amount, and plant growth.
Practical Tip for Implementation
These starting values apply for:
- Well-established biofilters
- Optimal temperature
- Strong aeration (Oxygen > 5 mg/L)
- Regular water quality checks
Always start with the lower limit (e.g., 20 Tilapia/m³ instead of 25) and only increase in later production cycles when you know your system.
The literature tells us
Losses due to the "existence" of fish: Of 100% fed feed, the following are lost:
| Source of loss | Proportion |
|---|---|
| Respiration & Metabolism | 30-45% |
| Feces | 15-25% |
| Nitrogen excretion (NH₄, Urea) | 8-12% |
| Feed residues (undigested) | 5-10% |
| Other losses | 2-5% |
| → Stored as biomass | 40-50% |
1. Definition: What does FCR really measure?
FCR (Feed Conversion Ratio)
FCR = Feed supplied (Fresh weight) / Gain in fish (Fresh weight)
→ FCR is not an efficiency or performance indicator, but a mass ratio on a fresh weight basis.
Example:
FCR = 1.5 means:
1.5 kg feed → 1.0 kg fish gain (Fresh weight)
? Important:
- Feed: ~90 % dry matter
- Fish: ~70–80 % water
2. Why 40–50 % "utilization" still fits with FCR < 2
2.1 Different reference variables
The table describes biochemical/physiological utilization:
- Energy
- Carbon
- Nitrogen
- Dry matter
The FCR refers only to fresh mass.
2.2 Simplified calculation example
Assumption:
- 1.5 kg fish feed
- 90% DM → 1.35 kg dry matter
- Of this, 45 % is stored as biomass
→ 0.61 kg dry matter fish
The fish body contains approx. 25–30 % dry matter
0.61 kg DM ÷ 0.27 ≈ 2.26 kg Fresh fish
Even with only 45 % net utilization of dry matter, >2 kg fresh fish are produced.
In practice, less is produced → FCR 1.2–2.0 is absolutely realistic.
3. The central misconception
❌ "Only 40–50 % is stored → FCR would have to be >2"
✔ Correct is:
- 40–50 % of the dry matter/energy
- become protein & fat
- which, through water binding, lead to much more fresh weight
Water is "free" in the FCR.
4. Comparison: Energy efficiency vs. FCR
| Indicator | Typical value |
|---|---|
| Energy efficiency fish | 20–35 % |
| Protein retention | 30–55 % |
| Dry matter retention | 35–50 % |
| FCR (Fresh weight) | 1.2–2.0 |
→ All values are simultaneously correct, but measure different things.
5. Why fish are still "so efficient"
Compared to land animals:
- ❌ no thermoregulation (poikilothermic)
- ❌ no gravity skeleton
- ✔ ammoniacal N-excretion (energetically cheap)
- ✔ high water binding per g protein
Quote (paraphrased):
“Fish are among the most efficient converters of feed into edible flesh due to low maintenance energy costs and high body water content.”
(Tacon & Metian, 2008)
6. Fachliteratur (empfohlen)
Books
- Halver & Hardy (2002) – Fish Nutrition
Academic Press
→ Standard work on metabolism, energy and protein utilization
https://www.sciencedirect.com/book/9780123196521/fish-nutrition - Jobling (1994) – Fish Bioenergetics
Chapman & Hall
→ very good explanation of energy flows
https://doi.org/10.1007/978-94-011-0798-7
Review
- Tacon & Metian (2008)
Global overview on the use of fish meal and fish oil
https://doi.org/10.1016/j.aquaculture.2008.04.046
7. Summary
- ✔ 40–50 % net utilization of dry matter is realistic
- ✔ FCR < 2 refers to fresh weight
- ✔ Water content of the fish "multiplies" biomass
- ❌ FCR is not an efficiency indicator
This is not a contradiction – only different system boundaries
In-depth literature on these topics, as of 2025-2026
Peer-reviewed studies:
- Masser, M.P., Rakocy, J., & Losordo, T.M. (1999)
- "Recirculating Aquaculture Tank Production Systems: Management of Recirculating Systems"
- SRAC Publication No. 452
- Shows: Nitrification rate decreases by ~50% with a temperature drop from 25°C to 15°C
- Chen, S., Ling, J., & Blancheton, J.P. (2006)
- "Nitrification kinetics of biofilm as affected by water quality factors"
- Aquacultural Engineering, 34(3), 179-197
- Documents Q10 values (temperature coefficient) of 1.8-2.3 for nitrification
- Emparanza, E.J.M. (2009)
- "Problems affecting nitrification in commercial RAS with fixed-bed biofilters for salmonids in Chile"
- Aquacultural Engineering, 41(2), 91-96
- Specifically for trout: At <12°C, nitrification rate drops dramatically
- Zhu, S. & Chen, S. (2002)
- "The impact of temperature on nitrification rate in fixed film biofilters"
- Aquacultural Engineering, 26(4), 221-237
- Empirical formula: Rate = Rate₂₀°C × 1.103^(T-20)
Practical references:
- FAO Technical Paper 529 (2009)
- "Simple methods for aquaculture: Recirculation systems"
- Tables with temperature factors for various fish farming systems
- Timmons, M.B. & Ebeling, J.M. (2013)
- "Recirculating Aquaculture" (3rd Edition)
- Chapter 7: Biofiltration
- Standard reference work with extensive temperature tables
Biofilter efficiency based on retention time
Peer-reviewed studies:
- Guerdat, T.C., Losordo, T.M., DeLong, D.P., & Jones, R.D. (2010)
- "An evaluation of commercial-scale recirculating systems for sustainable aquaculture"
- North Carolina State University
- Shows: HRT (Hydraulic Retention Time) of 15-30 min optimal
- Fdz-Polanco, F., Méndez, E., Urueña, M.A., Villaverde, S., & García, P.A. (2000)
- "Spatial distribution of heterotrophs and nitrifiers in a submerged biofilter for nitrification"
- Water Research, 34(16), 4081-4089
- Documents the relationship between flow rate and nitrification efficiency
- Rusten, B., Eikebrokk, B., Ulgenes, Y., & Lygren, E. (2006)
- "Design and operations of the Kaldnes moving bed biofilm reactors"
- Aquacultural Engineering, 34(3), 322-331
- Specifically for Kaldnes media: Optimal retention time 20-40 minutes
- Eding, E.H., Kamstra, A., Verreth, J.A.J., Huisman, E.A., & Klapwijk, A. (2006)
- "Design and operation of nitrifying trickling filters in recirculating aquaculture: A review"
- Aquacultural Engineering, 34(3), 234-260
- Comprehensive review with hydraulic calculations
Specifically for Moving Bed Biofilm Reactors (MBBR):
- Hem, L.J., Rusten, B., & Ødegaard, H. (1994)
- "Nitrification in a moving bed biofilm reactor"
- Water Research, 28(6), 1425-1433
- Shows: At HRT <10 min, efficiency drops to <50%
- Summerfelt, S.T., & Cleasby, J.L. (1996)
- "A review of hydraulics in fluidized-bed biological filters"
- Aquacultural Engineering, 15(6), 413-430
- Hydraulic modeling of different filter types
Practical manuals & Guidelines:
- Engineering Design of Recirculating Systems (Loyless & Malone, 1998)
- Auburn University Publications
- Practical rules of thumb for hobbyists and commercial operators
- Aquaponics Food Production Systems (Goddek et al., 2019)
- Springer
- Chapter 8: Biofilter Design
- Modern summary of all relevant parameters
- The Conservation Fund's Freshwater Institute
- https://conservationfund.org/our-work/freshwater-institute
- Technical bulletins on RAS systems (freely available)
Online resources:
- University of Florida IFAS Extension
- "Nitrification in Aquaponics Systems" (CIR1229)
- https://edis.ifas.ufl.edu
- FAO Fisheries and Aquaculture
- Technical Papers on RAS
- http://www.fao.org/fishery/topic/13540/en
Most important core statements from the literature:
Temperature:
- Q₁₀ ≈ 2.0 (doubling of rate at +10°C)
- Optimal: 25-30°C
- <15°C: Significant decline
- <10°C: Critically slow
Retention time:
- <5 min: 30-40% efficiency
- 10-15 min: 60-80% efficiency
- 15-30 min: 80-95% efficiency (OPTIMAL)
- 30 min: 95-100% (hardly any further improvement)
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