Optimization of Digestible Protein Level in Semi-Intensive Carp PolyCulture

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

Optimization of the digestible protein level in supplementary feeds is the single most critical factor for balancing economic viability with biological growth in semi-intensive fish farming. Search intent: explain / apply the findings of specific protein optimization trials to practical aquaculture scenarios. This study investigates how varying protein inputs influence the growth, meat quality, and water chemistry of composite culture systems featuring major and Chinese carps.

Key Takeaways:

• Optimal Level: A 28% digestible protein (DP) level yielded the highest net fish production (4303.66 kg/ha/year).
• Species Performance: Grass Carp (Ctenopharyngodon idella) exhibited the highest growth potential among the five species tested.
• ** diminishing Returns:** Protein levels exceeding 28% resulted in decreased growth due to metabolic stress and increased ammonia excretion.
• Ecological Balance: High protein feeds (32%) increased planktonic biomass via nutrient recycling but did not translate to higher fish weight.

Growth Performance Across Carp Species

In composite semi-intensive culture systems, different fish species occupy distinct ecological niches, yet their response to supplementary feed protein varies significantly based on their inherent physiological requirements. The study monitored five commercially important species—Labeo rohita (Rohu), Catla catla (Thaila), Cirrhina mrigala (Mrigal), Ctenopharyngodon idella (Grass Carp), and Hypophthalmichthys molitrix (Silver Carp)—over a one-year period.

“Amongst the five fish species, Ctenopharyngodon idella came out as an exceptional growth performer exhibiting significantly higher final average weight, fork and total length gains… followed by Cirrhina mrigala, Labeo rohita, Hypophthalmichthys molitrix and Catla catla.” (Zeb, 2016, p. Abstract)

The superior performance of Grass Carp suggests a higher aptitude for utilizing pelleted supplementary feeds compared to the filter-feeding Silver Carp or the surface-feeding Catla. While all species survived at a 100% rate, the differential growth rates highlight the importance of species-specific responses in polyculture. The study found that while natural food (plankton) was available due to fertilization with poultry droppings, it was insufficient to support high-density stocking without nutritionally balanced supplementation.

Student Note: The growth pattern for most species followed negative allometric growth (n<3), meaning the fish became more slender as they increased in length, whereas Silver Carp exhibited nearly isometric growth (n=3).

Fish Species	Best Performing Feed (DP %)	Final Avg. Weight (g)
Ctenopharyngodon idella	28% (T4)	1157.54 ± 69.03
Cirrhina mrigala	26% (T3)	1001.22 ± 45.72
Labeo rohita	28% (T4)	943.32 ± 50.12
Hypophthalmichthys molitrix	26% (T3)	731.33 ± 37.53
Catla catla	26% (T3)	618.99 ± 29.27
Fig: Comparative final weights of five carp species under their respective optimal protein treatments (Zeb, 2016, p. 24).

Professor’s Insight: Notice that surface feeders like Catla catla showed the lowest weight gain; this often indicates that sinking pellets in semi-intensive systems are less accessible to surface feeders compared to column or bottom feeders.

Optimizing the Digestible Protein Level

Determining the economic and biological “sweet spot” for protein inclusion is the primary objective of aquaculture nutrition research. Protein is the most expensive component of fish feed. This study tested six iso-caloric feeds ranging from 22% to 32% digestible protein.

“Net fish yield increased as the level of digestible protein in the supplementary diets increased and plateaued at 28% DP level, thereafter fish weight decreased significantly as the level of digestible protein in supplementary diets was further increased.” (Zeb, 2016, p. Abstract)

The results demonstrated a classic bell-curve response. Yield increased steadily from 22% up to 28%. However, at 30% and 32% digestible protein levels, growth performance declined. This reduction is attributed to the energy cost of deaminating excess amino acids. When fish consume more protein than they can synthesize into tissue, the excess must be broken down and excreted as ammonia. This process consumes energy that would otherwise be used for somatic growth, leading to lower yields despite higher quality inputs.

Student Note: The Nitrogen Conversion Ratio (NCR) was best (lowest) at the 28% protein level (1:5.17), indicating the most efficient conversion of supplied nitrogen into fish biomass.

Professor’s Insight: The decline in growth at 32% protein is a textbook example of nutrient toxicity/stress; “more” is not always “better” in animal nutrition.

Proximate Composition and Meat Quality

The nutritional quality of the final fish product is directly influenced by the diet consumed. The study analyzed the flesh of harvested fish to determine moisture, crude protein, total fats, and ash contents. The data revealed a positive correlation between dietary protein intake and muscle protein retention, up to a threshold.

“The 28% DP (T4) level caused maximum accumulation of muscle proteins in Labeo rohita and Ctenopharyngodon idella as 19.15±0.50 and 18.32±0.84%, respectively.” (Zeb, 2016, p. Abstract)

Interestingly, fat accumulation followed a different trend. Fish fed the highest protein levels (32%) accumulated significantly more body fat. This occurs because the carbon skeletons remaining after the deamination of excess amino acids are often converted into lipids for storage. Conversely, control fish (fed no supplement) had the highest moisture content and lowest protein and fat content, indicating a “lean” nutritional status driven by scarcity.

Student Note: There is often an inverse relationship between moisture content and protein/lipid accumulation in fish muscle; as nutrient density increases, water content typically decreases.

Professor’s Insight: For consumers, fish reared on 28% protein offered the best balance of high muscle protein without the excessive fat deposition seen at 32% levels.

Water Quality and Ecological Interactions

In semi-intensive systems, the pond ecology plays a dual role: it processes waste and provides natural food. The study utilized poultry droppings to fertilize ponds, enhancing planktonic biomass. Step-wise regression analysis was used to determine which water quality parameters drove productivity.

“Total hardness, chlorides, pH and total ammonia were the water quality variables that explained the most variability in planktonic productivity of ponds under different treatments.” (Zeb, 2016, p. Abstract)

While higher protein feeds (32%) resulted in the highest planktonic biomass (due to the recycling of nitrogen-rich waste and uneaten feed), this did not translate to higher fish yields. This suggests that at high protein levels, the water quality may deteriorate—specifically regarding ammonia—stressing the fish and inhibiting growth despite the abundance of natural food. The study confirmed that total phosphates and nitrates were limiting nutrients for plankton production.

Student Note: High levels of Total Ammonia in high-protein treatments can become a growth-limiting factor, negating the benefits of the nutrient-dense feed.

Treatment	Digestible Protein %	Net Fish Yield (Kg/ha/year)
T4	28%	4303.66
T3	26%	4288.22
T2	24%	3575.54
T5	30%	3466.94
T1	22%	3034.54
T6	32%	3017.11
T7 (Control)	0%	2073.95
Fig: Net fish yield per hectare across different protein treatments (Zeb, 2016, p. 71).

Professor’s Insight: The drastic drop in yield between T4 (28%) and T5 (30%) highlights how sensitive aquatic systems are to nitrogen inputs; a 2% shift can fundamentally alter the metabolic cost for the fish.

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Real-Life Applications

1. Cost Reduction for Farmers: By capping feed protein at 28% rather than 30% or 32%, farmers can significantly reduce feed costs—the highest operational expense in aquaculture—without sacrificing yield.
2. Waste Management: This system effectively recycles poultry droppings (applied at 0.17g N/100g fish weight), turning a potential agricultural pollutant into high-value fish protein.
3. Water Quality Management: Understanding that high protein inputs increase ammonia levels helps farmers anticipate when to increase aeration or water exchange to prevent stress-induced growth stunting.
4. Species Selection: Farmers aiming for rapid turnover should prioritize Grass Carp and Mrigal in these specific composite systems, as they showed the highest responsiveness to supplementary feeding.
5. Meat Quality Control: Producers can tailor feed protein levels to manipulate the fat-to-protein ratio in fish fillets, catering to specific market demands for leaner or fattier meat.

Why this matters: In developing nations where animal protein is scarce, maximizing fish yield per unit of feed input is essential for food security and economic sustainability.

Key Takeaways

• 28% is the Limit: For semi-intensive composite culture of major carps, 28% digestible protein is the physiological and economic optimum.
• Grass Carp Dominance: Ctenopharyngodon idella outperforms indigenous major carps (Rohu, Thaila, Mrigal) in weight gain under these conditions.
• The Penalty of Excess: Exceeding optimal protein levels causes a “double penalty”: higher feed costs and lower fish growth due to metabolic energy loss.
• Essential Supplementation: Control ponds relying solely on fertilization produced significantly lower yields (2073 kg/ha) compared to optimized fed ponds (4303 kg/ha).
• Nitrogen Efficiency: The most efficient conversion of nitrogen to fish flesh occurs at the 28% protein level; biological efficiency drops as protein input rises further.
• Composite Synergy: The survival rate was 100% across all treatments, proving that these five species are highly compatible and the stocking densities utilized (27% Rohu, 10% Thaila, 10% Mrigal, 7% Grass Carp, 13% Silver Carp) are sustainable.

MCQs

1. According to the study, at what digestible protein (DP) level did the net fish yield reach its maximum plateau?
A) 24%
B) 26%
C) 28%
D) 32%
Correct: C
Explanation: The yield increased with protein levels up to 28% and then significantly decreased at 30% and 32% due to metabolic stress.

2. Which fish species exhibited the highest weight gain under the composite culture system?
A) Labeo rohita
B) Catla catla
C) Hypophthalmichthys molitrix
D) Ctenopharyngodon idella
Correct: D
Explanation: Grass Carp (Ctenopharyngodon idella) showed exceptional growth performance, gaining the most weight (793.32g) and length compared to other species.

3. What was the observed effect of feeding fish a 32% digestible protein diet compared to a 28% diet?
A) Increased fish yield and decreased plankton.
B) Decreased fish yield and increased body fat.
C) Decreased fish yield and decreased body fat.
D) No change in yield or body composition.
Correct: B
Explanation: At 32% protein, the fish yield declined due to energy costs of deamination, but the fish accumulated more body fat from the carbon skeletons of excess amino acids.

FAQs

Q: Why does fish growth decrease after the optimal protein level?
A: When fish consume more protein than they need, they must break down the excess amino acids. This process (deamination) requires significant energy, leaving less energy available for growth. It also increases ammonia production, which stresses the fish.

Q: Did the control ponds receive any feed?
A: No. Control ponds (T7) were fertilized with poultry droppings to stimulate natural plankton growth, but the fish did not receive any pelleted supplementary feed. This resulted in the lowest yields.

Q: Which water quality variable is most critical when using high-protein feeds?
A: Ammonia. The study identified Total Ammonia as a significant variable. High protein inputs lead to higher nitrogen excretion, which can degrade water quality if not managed, negatively affecting fish health.

Q: What is the Nitrogen Conversion Ratio (NCR)?
A: NCR measures how efficiently nitrogen inputs (from feed and fertilizer) are converted into fish biomass. A lower ratio is better. The study found the best NCR (1:5.17) at the 28% protein level.

Lab / Practical Note

Pellet Stability & Feeding: The supplementary feeds in this study were processed into 3mm pellets. When replicating this in a lab or farm, ensure pellets are water-stable to minimize leaching of nutrients before consumption. Furthermore, always monitor water ammonia levels 2-4 hours after feeding high-protein diets (30%+) to detect potential spikes that could harm livestock.

External Resources

• Nutrient requirements of fish and shrimp (NCBI Bookshelf)
• Protein optimization 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: All statistical data regarding growth weights, water chemistry, and feed composition were verified directly from the thesis tables (Appendix Tables 1-50) and Results section.

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 focuses on aquatic sciences, specifically the nutritional optimization of supplementary feeds for major and Chinese carps in semi-intensive systems.

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