Nitrogen Conversion Ratio Efficiency in Semi-Intensive Carp Culture

Last Updated: December 7, 2025
Estimated reading time: 7 minutes

The Nitrogen Conversion Ratio (NCR) is a pivotal metric in sustainable aquaculture, measuring how efficiently a culture system converts nitrogen inputs—from both organic fertilizer and supplementary feed—into harvestable fish biomass. Search intent: explain / apply the concept of nutrient efficiency to optimize semi-intensive farming systems. This post analyzes the nitrogen dynamics of a composite culture system rearing five carp species under varying protein regimes.

Key Takeaways:

• Defining Efficiency: In this study, NCR is expressed as the grams of fish weight produced per 1 gram of Nitrogen added; a higher value indicates better efficiency.
• The Efficiency Paradox: The Control treatment (fertilizer only) achieved the highest nitrogen efficiency (1:5.70) despite having the lowest total yield.
• Optimal Feed Efficiency: Among fed ponds, the 28% digestible protein diet offered the best balance of efficiency (1:5.17) and total production.
• Wasteful Excess: High protein diets (32%) resulted in the lowest NCR (1:3.97), indicating significant nitrogen wastage and environmental loading.
• Seasonal Flux: Nitrogen conversion efficiency peaked in summer (July) and dropped precipitously in winter (January) due to metabolic slowdowns.

Understanding NCR in Composite Culture

In semi-intensive aquaculture, nitrogen enters the pond ecosystem through two primary vectors: organic fertilization (poultry droppings) and supplementary feed. The goal is to maximize the retention of this nitrogen in the fish flesh rather than losing it to the water column as ammonia or to the sediment.

“Supplementary feed at 28% DP (T4) level gave significantly higher increase in fish yield of 51.67Kg year-1 pond-1 with overall pond nitrogen conversion ratio (NCR) of 1:5.17.” (Zeb, 2016, p. Abstract)

The study calculated NCR based on the total nitrogen inputs against the net fish yield. A unique finding was that the Control treatment (T7), which received no feed and relied solely on poultry droppings (0.17g N/100g fish weight), had the statistically highest efficiency ratio of 1:5.70. This means for every gram of nitrogen introduced via manure, the system produced 5.7 grams of fish. This reflects a “starvation efficiency”—where nutrients are scarce, the ecosystem recycles them with maximal tightness, leaving little waste, though the total biomass produced is low.

Student Note: Do not confuse this with FCR (Feed Conversion Ratio). For NCR as defined here (Output/Input), a higher number is better. For FCR (Input/Output), a lower number is better.

Professor’s Insight: The high efficiency of the control pond validates the effectiveness of poultry manure as a standalone fertilizer for low-input systems, even if it cannot support high commercial densities.

Treatment	Protein Level	Total N Added (Kg)	Fish Yield Inc. (Kg)	NCR (1g N : Xg Fish)
T7 (Control)	0%	6.96	24.90	1:5.70
T4	28%	20.38	51.67	1:5.17
T3	26%	19.58	51.48	1:4.86
T1	22%	13.18	36.43	1:4.80
T2	24%	16.74	42.62	1:4.78
T5	30%	17.15	41.62	1:4.28
T6	32%	15.90	36.21	1:3.97
Fig: Nitrogen Conversion Ratios showing efficiency across treatments (Zeb, 2016, p. 69).

The Cost of High Protein

While protein is essential for growth, it is also the primary source of nitrogenous waste. When fish are fed high-protein diets (30-32%), the biological machinery required to process this nitrogen incurs a metabolic cost. The study demonstrated a clear “law of diminishing returns” regarding nitrogen efficiency.

“Significantly lowest nitrogen conversion ratio was observed… due to 32% DP (T6)… This may also be due to catabolization of excess proteins, beyond optimum, a condition to produce and eliminate a lot of ammonia by the fish.” (Zeb, 2016, p. 68, 136)

Treatment 6 (32% protein) had an NCR of 1:3.97. This was the lowest among all treatments. Despite high-quality inputs, the fish could not utilize the extra nitrogen for growth. Instead, they expended energy deaminating the amino acids and excreting the nitrogen as ammonia. This not only represents an economic loss (wasted expensive protein) but also an environmental hazard, as excess nitrogen promotes eutrophication in receiving waters.

Student Note: The process of Deamination removes the amino group from amino acids, converting it to ammonia, which is toxic and must be excreted, consuming ATP (energy).

Professor’s Insight: T4 (28% Protein) represents the “Sweet Spot”—it creates high yield (51.67 Kg) with high efficiency (1:5.17), balancing economic output with environmental stewardship.

Seasonal Dynamics of Nutrient Utilization

The Nitrogen Conversion Ratio is not a static figure; it fluctuates wildly with water temperature. Fish are poikilothermic, meaning their metabolic rate—and thus their ability to synthesize protein—is governed by the environment. The thesis provided a month-by-month breakdown of NCR values.

“The conversion of nitrogen into fish body weight was the highest (1:17.89) during the month of July while it was the lowest (1:0.55) in January.” (Zeb, 2016, p. 67)

In July, when temperatures were optimal (~33°C), the ecosystem was firing on all cylinders. The bacterial decomposition of manure was rapid, plankton blooms were robust, and fish metabolism was high, resulting in massive biomass generation per unit of nitrogen. In contrast, during January (~12°C), the system stalled. Nitrogen inputs (feed/manure) were still applied, but the fish weren’t growing. The ratio dropped below 1:1, meaning more nitrogen was being dumped into the pond than was being retrieved as fish flesh—a definition of pollution.

Student Note: In winter, continued fertilization often leads to Sludge Accumulation rather than plankton production, as bacterial mineralization rates drop significantly.

Month	Avg. Water Temp (°C)	Avg. NCR (All Ponds)	Biological Status
July	~33.25	1:12.87	Peak Growth / High Efficiency
August	~33.45	1:8.97	High Growth
November	~24.44	1:2.51	Slowing Metabolism
January	~12.96	1:0.68	Growth Stasis / N Accumulation
May	~29.02	1:2.28	Recovery / Increasing Input
Fig: Seasonal impact on Nitrogen Conversion Ratios (Zeb, 2016, p. 69).

Synergistic Effects of Manure and Feed

The study utilized a specific fertilization rate: 0.17g Nitrogen per 100g of wet fish weight daily, derived from poultry droppings. This organic input provided the “base load” of nitrogen for the plankton. The supplementary feed provided the “peak load” for somatic growth.

“Poultry manure is a better organic manure for application in fish ponds than does the manure of ruminants because of its higher nitrogen and phosphorous contents and better C:N ratio.” (Zeb, 2016, p. 8)

The data suggests a synergy in T4 (28% Protein). The feed provided adequate amino acids for muscle growth without overwhelming the fish’s excretory system. Simultaneously, the manure maintained the plankton base. In T6 (32%), the feed provided too much nitrogen. The plankton bloomed excessively (highest biomass), but the fish were stressed by the ammonia, breaking the efficiency of the cycle.

Student Note: Poultry manure typically has a C:N Ratio of roughly 10:1, which is ideal for bacterial decomposition and subsequent phytoplankton uptake.

thus section should be in uniqe words for each post, Reviewed and edited by the Professor of Zoology editorial team. Except for direct thesis quotes, all content is original work prepared for educational purposes.

Real-Life Applications

1. Winter Feeding Protocols: Farmers must drastically reduce or stop nitrogen inputs (feed and fertilizer) in December/January. Continuing inputs during these months destroys the NCR, wastes money, and degrades water quality for the upcoming spring.
2. Environmental Compliance: For farms in regulated zones, using a 28% protein diet rather than 32% reduces the nitrogen load in the effluent, helping meet environmental discharge standards.
3. Cost Auditing: Farmers should calculate NCR annually. If the ratio drops (e.g., from 1:5 to 1:3), it indicates feed wastage, over-stocking, or disease—it is a diagnostic tool for farm financial health.
4. Fertilizer Management: The high efficiency of the Control pond suggests that small-scale rural farmers can achieve sustainable (albeit lower) yields using only poultry manure if commercial feed is unaffordable.
5. Polyculture Optimization: Including species like Hypophthalmichthys molitrix (Silver Carp) is essential to capture the nitrogen channeled into plankton, ensuring it ends up as fish biomass rather than algal sludge.

Why this matters: Nitrogen is the currency of aquaculture production. Managing it efficiently determines the line between a profitable, sustainable farm and a polluting, money-losing operation.

Key Takeaways

• Metric Definition: In this context, NCR = Output / Input. Higher is better.
• The Efficiency Champion: Control ponds are the most efficient nitrogen converters (1:5.70), but fed ponds (T4) produce the most food (4303 kg/ha).
• The Limit of Protein: Feeding 32% protein is biologically inefficient (NCR 1:3.97); the extra nitrogen is excreted, not stored.
• Thermal Dependence: Nitrogen efficiency is strictly temperature-dependent; tropical carps cannot convert nitrogen efficiently below 15°C.
• Ecological Balance: The 28% protein diet represents the best compromise, maximizing yield while maintaining a high nitrogen conversion efficiency (1:5.17).
• Feed vs. Fertilizer: Manure provides the ecological foundation; feed provides the growth spike. Both contribute to the total Nitrogen load and must be balanced.

MCQs

1. How is the Nitrogen Conversion Ratio (NCR) defined in this study?
A) Grams of Nitrogen input per gram of fish weight gain.
B) Grams of fish weight gain per gram of Nitrogen input.
C) Percentage of protein in the feed divided by fish weight.
D) Total ammonia nitrogen divided by fish biomass.
Correct: B
Explanation: The thesis presents NCR as a ratio like 1:5.17, meaning 1 gram of Nitrogen input results in 5.17 grams of fish biomass increase.

2. Which month recorded the lowest Nitrogen Conversion efficiency (NCR 1:0.68)?
A) July
B) October
C) January
D) May
Correct: C
Explanation: January had the lowest water temperature (12.96°C), causing fish metabolism to slow down, resulting in very poor conversion of nitrogen inputs into biomass.

3. Why did Treatment 6 (32% Protein) have a lower NCR (1:3.97) than Treatment 4 (28% Protein, 1:5.17)?
A) The fish in T6 died.
B) The plankton levels were too low in T6.
C) The fish in T6 expended energy excreting excess nitrogen.
D) The poultry manure was not applied in T6.
Correct: C
Explanation: Feeding protein beyond the optimal level (28%) forced the fish to deaminate excess amino acids, wasting energy and nitrogen, thus lowering the conversion efficiency.

FAQs

Q: Is a higher or lower NCR better?
A: As defined in this thesis (Fish Weight / N Input), a higher number is better. It means you are getting more fish flesh for every gram of nitrogen you throw into the pond.

Q: Can I rely solely on poultry manure for carp farming?
A: Yes, you can, as shown by the Control treatment. It is highly efficient in terms of nitrogen use, but the total tonnage of fish produced will be significantly lower (approx. 2000 kg/ha) compared to fed ponds (approx. 4300 kg/ha).

Q: What happens to the nitrogen that isn’t converted into fish?
A: It accumulates in the system. It can become toxic ammonia, nitrate, settle into the sediment as sludge, or stimulate excessive algal blooms that can cause oxygen depletion at night.

Q: Did the study use floating or sinking feed?
A: The study used 3mm sinking pellets prepared in a lab extruder. Sinking feeds require good management to ensure they are eaten before they dissolve, which would otherwise skew NCR values.

Lab / Practical Note

Calculating N-Input: To calculate NCR on your farm, you must know the Nitrogen content of your inputs.

• Feed: Crude Protein % / 6.25 = Nitrogen %. (e.g., 28% Protein feed contains ~4.48% Nitrogen).
• Manure: Poultry droppings vary, but this study standardized usage based on N-content. You must perform a Kjeldahl analysis on your manure batch to ensure accurate dosing (0.17g N/100g fish).

External Resources

• Nitrogen cycle in fish ponds (FAO)
• Nutrient budgets in aquaculture (ScienceDirect)

Sources & Citations

Thesis Citation:
Zeb, J. (2016). Optimization of protein level in supplementary feeds for fish rearing under semi-intensive composite pond culture systems (Doctoral dissertation). Department of Zoology, Wildlife and Fisheries, University of Agriculture, Faisalabad. Pages 1-162.

Note on Content: NCR values and seasonal water temperature data were derived from Table 7 (p. 69), Figure 20 (p. 66), and Appendix Tables 21-27.

Invitation: If you are the author of this thesis and wish to provide updates or corrections, please contact us at contact@professorofzoology.com.

Author Box
Jhan Zeb holds a PhD in Zoology from the University of Agriculture, Faisalabad. His research integrates nutritional science with environmental management to optimize semi-intensive aquaculture systems.

Reviewer: Abubakar Siddiq, PhD, Zoology
Note: This summary was assisted by AI and verified by a human editor.