Valorizing Hempseed Meal as a Circular Bio-Ingredient for Sustainable Fisheries Development

Abstract

The increasing demand for sustainable aquafeeds necessitates the development of alternative protein sources that support both economic efficiency and ecological responsibility. This study evaluates the potential of using hempseed meal (a nutrient-dense agro-industrial by-product) as a functional ingredient in carp aquaculture diets. The paper presents a proof-of-concept evaluation demonstrating the potential of hempseed meal as a circular bio-ingredient that aligns with the principles of sustainable aquaculture, rather than providing a comprehensive assessment of its long-term physiological effects on fish. A 90-day feeding trial was conducted under controlled pond conditions to assess the effects of graded hempseeds meal inclusion levels on growth performance, feed utilization, and environmental sustainability indicators for three Cyprinus carpio varieties. Four isonitrogenous and isoenergetic diets were formulated: a control diet (R1) based on conventional plant protein sources such as soybean and pea meal, and three experimental diets containing 5%, 10%, and 20% hempseed meal (R2–R4). Growth indices including absolute weight gain (WG), average daily gain (ADG), specific growth rate (SGR), and feed conversion ratio (FCR) were determined, and data was analyzed via two-way ANOVA with Tukey HSD post hoc testing. Results indicated that 10% hempseed meal inclusion produced optimal growth responses, improving specific growth rate by 12.6% and reduced feed conversion ratio by 10.8% compared to the control. The most pronounced effects were observed for Frăsinet carp variety (SGR 1.23%·day⁻¹; FCR 1.39). Environmental assessments demonstrated that substituting conventional protein sources (soybean and pea meal) with hempseed meal at 20% inclusion valorized 200 kg of hemp press cake per ton of feed, reduced conventional protein use by 33.3%, diverted up to 80% of waste from disposal. These findings validate hempseed meal as a sustainable, cost-effective, and nutritionally viable alternative to conventional protein sources in freshwater aquaculture, advancing circular bioeconomy strategies and supporting low-carbon fish production systems.

1. Introduction

The rapid expansion of aquaculture as an important source of animal protein has heightened the need for sustainable, cost-effective feed ingredients that prioritize ecological responsibility [1,2]. Conventional fish feed is heavily reliant on fishmeal and soybean meal and faces important challenges such as high costs, competition with human food systems, and environmental issues such as water pollution [1,2]. Consequently, alternative protein sources from agricultural by-products and food processing residues are gaining interest, promoting a circular economy, and minimizing environmental harm [3,4]. Hemp (Cannabis sativa L.) by-products, specifically hempseed meal or cake obtained from oil extraction, stand out as promising additions to aquaculture diets [5,6]. By integrating advanced agrotechnical technologies with diverse production streams, the industry can achieve environmental neutrality or even positive ecological impacts [7,8].

Hemp is a versatile crop [9] increasingly valued for its rich composition of polyunsaturated fatty acids, essential amino acids, and bioactive compounds [10,11]. Cold-press oil extraction produces a defatted meal containing 30–35% protein, high fiber, and residual lipids, representing a valuable yet underexploited resource [12]. Its balanced amino acid profile makes it suitable for omnivorous fish such as carp, which efficiently utilize plant-derived proteins [13], while its natural aroma and flavor compounds may enhance feed intake and growth performance [14].

Previous studies have shown that partial replacement of soybean or fishmeal with hempseed meal in salmonid and cyprinid diets does not impair growth, feed conversion, or health parameters [15,16]. In some cases, hemp supplementation improved lipid metabolism and immune responses due to omega-3 and omega-6 fatty acids [17,18]. Moreover, hemp derivatives can stimulate feed acceptance, an important factor in alternative diet formulation [11,19]. Collectively, these findings indicate that hempseed by-products—often considered waste—can be effectively reused in aquafeeds without compromising fish performance.

The ecological implications of incorporating hempseed meal into fish nutrition are significant. Valorizing agricultural by-products supports sustainable development by reducing waste, dependence on conventional feeds, and aquaculture’s environmental footprint [20,21,22]. The hemp oil industry yields substantial residual meal, often posing disposal or low-value utilization issues if unused [7,23,24,25,26]. Its integration into aquafeeds adds economic and ecological value: transforming a low-market by-product into a resource that aids food production, eases pressure on marine stocks, and promotes the circular bioeconomy [3,27,28]. Hemp cultivation further provides agronomic benefits, including low pesticide needs, carbon sequestration, and soil improvement [29,30].

This present paper addresses the lack of experimental evidence on the use of cold-pressed hempseed meal as a sustainable feed ingredient for freshwater carp varieties. By systematically testing graded inclusion levels while maintaining balanced nutrient profiles, the research evaluates the potential of hempseed wastes to improve the quality and efficiency of the fodder and support circular, and low-waste aquaculture practices.

Within this context, the research explores the opportunity to integrate hempseed oil extraction residues into carp feeding strategies under controlled pond conditions. Three fish varieties of economic importance for Romanian market, namely common carp (Cyprinus carpio), Salonta carp (Cyprinus carpio var. specularis), and Frasinet carp (Cyprinus carpio—an improved breed with an elongated body but does not have a distinct subvariety)—were selected to evaluate the growth potential and feeding behavior associated with diets supplemented by hempseed meal. Young specimens, initially weighing between 100 and 102 g, were raised over a three-month period on formulated diets incorporating different percentages of hempseed meal, offering valuable insights into nutritional efficacy and behavioral responses.

This study addresses a gap in sustainable aquaculture by demonstrating the practical valorization of hempseed meal—a by-product of oil extraction—as a functional and sustainable ingredient in carp diets. Unlike earlier laboratory-based studies, it provides field-scale, quantitative evidence under realistic pond-culture conditions, bridging the transition from experimental potential to applied feasibility. By integrating agro-industrial wastes into feed formulations, the research advances circular bioeconomy principles, reducing waste and environmental impacts while enhancing feed sustainability. The findings offer a scientific framework for incorporating plant-based by-products into aquafeeds, supporting both environmental responsibility and the economic efficiency of freshwater aquaculture. The primary aim of this study is a proof-of-concept investigation into the substitution of conventional feed protein with hempseed meal in the diet of Cyprinus carpio to validate its efficacy as a circular bio-ingredient.

2. Materials and Methods

2.1. Experimental Site and Research Approach

The experiment was carried out under controlled pond-culture conditions at the National Institute of Research–Development for Machines and Installations Designed for Agriculture and Food Industry (INMA Bucharest), Romania, during the period May–July 2024. The study employed a completely randomized block design, comprising four dietary treatments with graded levels of hempseed meal inclusion in the formulated feeds. Each treatment was replicated in three independent ponds to ensure statistical reliability and minimize environmental variability. The feeding trial extended over a three-month growth period, during which water quality and environmental parameters were maintained within optimal ranges for carp culture. The experimental design is presented in Figure 1.

2.2. Oils Extraction from Hemp Seeds Using Cold Pressing

The cold pressing of hemp seeds was carried out using a mechanical extraction process that maintained the temperature below 50 °C, thereby preserving thermolabile bioactive compounds such as omega-3 and omega-6 fatty acids, tocopherols, and natural antioxidants. This technique yields a pure, unrefined oil characterized by its natural flavor, nutritional integrity, and high oxidative stability, making it suitable for both food and cosmetic applications. In contrast to solvent-based or hot extraction methods, cold pressing eliminates the risk of thermal degradation and chemical contamination, ensuring a cleaner and more environmentally sustainable production process. For the present study, oil extraction was performed using a prototype mechanical oil press (INMA-PRO-OIL 1) developed at the INMA Bucharest Institute, as illustrated in Figure 2.

The Jubileu Secuieni variety of hemp seeds (developed by SCDA Secuieni, Secuieni, Romania), with an estimated yield of 1200–1600 kg seeds/ha, was processed for oil extraction. Prior to processing, the seeds were pre-dried to optimize the calculation of the oil extraction rate. The average cold-press extraction yield obtained from Jubileu hemp seeds was approximately 29% oil, representing about 84% of the total potential oil content from the seeds. The remaining 71% corresponded to hempseed cake (hempseed meal), retained around 10–12% of the unextracted residual oil.

This by-product is particularly valuable as a feed ingredient due to its balanced nutritional composition. After oil extraction, hempseed meal contains between 30–35% crude protein, 8–12% crude lipid, 20–25% crude fiber, 5–7% ash, and 25–30% nitrogen-free extract (NFE). The protein fraction provides a favorable amino acid profile, notably rich in arginine, methionine, and glutamic acid, which are essential for the growth and metabolic health of fish. The residual oil fraction contributes a high proportion of polyunsaturated fatty acids, particularly linoleic acid (C18:2 n-6, 50–60%) and α-linolenic acid (C18:3 n-3, 15–20%)—that play vital roles in maintaining membrane integrity, immunity, and overall physiological performance in aquatic organisms. Moreover, the presence of natural antioxidants such as tocopherols and phenolic compounds enhances oxidative stability and may improve the health status of fish when incorporated into feed formulations.

Due to its moderate protein content, balanced fatty acid composition, and high digestibility, hempseed meal represents a sustainable and nutritionally valuable alternative to conventional plant-derived protein sources such as soybean meal. Its inclusion in aquafeed formulations can reduce dependence on imported protein ingredients, enhance feed sustainability, and contribute to circular bioeconomy principles by valorizing agricultural by-products.

2.3. Experimental Design for Fish Rearing Conditions

The study was conducted over a three-month period to assess the effects of hempseed meal supplementation in fish feed on somatic growth parameters in three cyprinid varieties (Cyprinus carpio): Common carp, Salonta carp, and Frăsinet carp. For each carp variety, four experimental diet formulations were developed, incorporating hempseed meal at varying levels (0% to 20%) to assess its impact on feed composition and performance.

Each experimental diet was evaluated in triplicate, employing three independent replicates per variety, to ensure statistical robustness. Fish exhibiting comparable initial body length and mass were allocated to each rearing tank (100 fish per tank), thereby maintaining batch homogeneity. Feeding was performed twice daily, with rations calculated as a percentage of the total biomass, while key environmental parameters—including water temperature, dissolved oxygen concentration, and pH—were rigorously maintained at constant levels throughout the experimental duration.

Individual fish weights were measured monthly, starting from the initial moment (month 0) until the end of the experiment (month 3). For each variety and each feed diet, the average values per batch and per replicate were recorded. Consequently, the resultant dataset comprised longitudinal measurements at four discrete time points (months 0, 1, 2, and 3), across four treatment regimens (R1–R4) and three replicates per variety.

Each pond was considered an experimental unit (n = 3 per treatment per variety). Mean values for growth parameters were calculated from the pooled measurements of all fish within each pond. Statistical analyses were performed using pond-level means to avoid non-independence of replicates.

Data processing involved recording the raw body mass measurements into a matrix with two independent variables: Factor A (Diet)—percentage of hempseed meal in the feed (0, 5, 10 and 20%); and Factor B (Measurement Interval), denoting the temporal assessment points (0, 1, 2, and 3 months). The dependent variable was the average body weight (g) of the fish. For each of the fish variety, datasets were evaluated for homogeneity of variance and normality of distribution. Arithmetic means and standard deviations were subsequently computed for every factorial combination.

The statistical analysis involved a two-way analysis of variance (two-way ANOVA) with the following model structure:

$$Y_{ijk}=\mu +A_i+B_j+{(A\times B)}_{ij}+{\epsilon }_{ijk}$$

(1)

where $Y_{ijk}$ is the observed mean body mass, $\mu$ is the overall mean, $A_i$ is the effect of the feeding regimen, $B_j$ is the effect of time (month of measurement), ${(A\times B)}_{ij}$ is the interaction between regimen and time, and ${\epsilon }_{ijk}$ is the experimental error associated with observation k.

The statistical significance of the differences was assessed at 95% and 99% confidence levels (p < 0.05, p < 0.01, respectively). In cases where significant differences were obtained between diets, the post hoc Tukey HSD test was applied, to identify homogeneous groups and significant differences between treatments. Furthermore, temporal dynamics of body mass were graphically depicted for each variety. The relationship between hempseed meal inclusion levels and cumulative weight gain was assessed through the coefficient of determination (R²) derived from a linear regression model. Data processing and statistical calculations as well as the graphical representations were generated using Microsoft Excel (Microsoft Office Home and Business 2021).

The juvenile fish specimens with an initial body weight of 100–103 g were stocked in three cubic meters water ponds. Fish were acclimated for two weeks prior to the beginning of the trial and fed a basal diet without hempseed meal during this period. Throughout the experimental period, water quality parameters (temperature, dissolved oxygen, pH, and ammonia concentration) were monitored and values were maintained within optimal ranges for carp culture. The filtration unit Hailea G12000 (Guangdong Hailea Group Co., Ltd., Foshan, China), an automated feeding system Eheim Twin Duo (EHEIM GmbH & Co. KG., Deizisau, Germany), and a water-quality-monitoring system Atlas Scientific Wi-Fi Hydroponics Kit (Atlas Scientific Environmental Robotics, New York, NY, USA) were all installed in the fish tanks, which were part of a recirculating system.

2.4. Experimental Diet Formulations

The experimental diets were designed to evaluate the effect of hempseed meal addition on the growth of juvenile fish of the Cyprinidae family. Four feed variants (R1–R4) were formulated, differentiated by the percentage of hempseed meal: 0%, 5%, 10% and 20%, gradually included in the mixture. The 0% level was used as a control, representing the standard feed used in the feeding of carp juveniles, based on established vegetable protein sources.

The inclusion of hempseed meal at varying levels (0%, 5%, 10%, 20%) as a partial substitute for conventional protein sources in cyprinid aquafeeds maintained isonitrogenous and isoenergetic diet compositions. To maintain comparability between diets, the crude protein content was standardized at 30%, the optimal value for carp juvenile fish, which have a high metabolic rate and require a consistent protein intake for the formation of muscle tissues. Adjustments were aimed at gradually reducing pea flour and soybean meal, as the proportion of hempseed meal increased. In this way, a nutritional balance was maintained, avoiding major variations in the content of essential amino acids and metabolizable energy. Hempseed meal, sourced from defatted seeds, provided 25–50% protein, balanced amino acids, and polyunsaturated fatty acids. Maintaining constant proportions for wheat bran (15%), corn meal (10%) and alfalfa meal (15%) ensured a uniform pellet texture and comparable density between diets, essential factors for feed acceptability in juvenile fish. Minor variations in lipid (2.23% to 3.52%), fiber (9% to 10.3%), and ash content were considered, to provide additional energy for growth and enhance digestion and intestinal transit regulation.

Dry ingredients were mixed homogenously in a high-shear mixer, then were cold-extruded through a 3-mm die into pellets, air-dried to 8–10% moisture, sieved for uniformity, and refrigerated for storage.

To ensure comparability among dietary treatments, all feed formulations were designed to be iso-nitrogenous (30% crude protein) and iso-energetic, maintaining balanced nutritional profiles suitable for juvenile carp. Lipid levels were allowed to vary slightly (<1.3%) within the normal range reported for cyprinid feeds, since carp, as omnivorous variety, rely primarily on carbohydrates and proteins rather than lipids as their main energy source. This approach ensured a realistic nutritional framework consistent with commercial feeding practices. The choice of iso-nitrogenous and iso-energetic rather than iso-lipidic formulations was therefore intentional, aiming to evaluate the contribution of hempseed meal to growth performance under practical pond-culture conditions. Nevertheless, it is acknowledged that future experiments employing iso-lipidic formulations would allow finer assessment of metabolic energy utilization and enhance inter-diet comparability.

The composition and characteristics of the experimental feeds are found in

Table 1. The composition and characteristics of the experimental feeds for growing cyprinids in controlled environments.

Ingredient/Proximate (%)	Diet 1 Hempseed Meal 0%	Diet 2 Hempseed Meal 5%	Diet 3 Hempseed Meal 10%	Diet 4 Hempseed Meal 20%
Ingredient composition
Hempseed meal	0	5	10	20
Pea flour	10	7.5	5	0
Soybean meal	50	47.5	45	40
Wheat bran	15	15	15	15
Corn flour	10	10	10	10
Alfalfa meal	15	15	15	15
Estimated proximate composition
Crude protein	30	30	30	30
Crude lipid	2.23	2.56	2.88	3.52
Crude fiber	9	9.33	9.65	10.3
Ash	5.3	5.36	5.42	5.55
Nitrogen-free extract (NFE)	53.47	52.75	52.05	50.63

, where proximate composition values were determined analytically. Ingredient-level proximate data from supplier certificates were used initially to balance formulations.

No vitamin–mineral premix was added to the experimental diets, as the purpose of the trial was to evaluate the specific effects of hempseed meal inclusion as a protein source, rather than to formulate a fully balanced commercial feed. The experimental design intentionally maintained a simplified composition to isolate the macronutrient-level influence of hemp-derived proteins and lipids on fish growth. Although mineral and vitamin supplementation can enhance long-term physiological development, the three-month experimental period focused primarily on short-term growth performance, during which macronutrient composition plays the dominant role. Future feed optimization trials will include standardized vitamin–mineral premixes to ensure complete micronutrient coverage and to evaluate potential biochemical and physiological responses.

Fish were fed twice daily (09:00 and 17:00 h) at a feeding rate equivalent to 3–4% of body weight, adjusted weekly according to biomass estimates. Feed intake and feeding behavior were observed to evaluate potential effects of hemp residues on feed attractiveness.

Although the proposed diets were isonitrogenous and isoenergetic, small differences in lipid (±1.3%), fiber (±0.8%), and ash (±0.6%) contents may have influenced digestibility and energy utilization. These variations are inherent to the substitution of soybean and pea proteins with hempseed meal, which contains more fiber and residual oil. Nevertheless, the overall growth trends suggest that carp efficiently utilized these mixed plant-based proteins without adverse effects on feed efficiency. Both soybean and pea meals were proportionally reduced to accommodate hempseed meal inclusion, reflecting a practical approach to feed diversification. This method was chosen to simulate realistic feed manufacturing conditions, where protein inputs often come from multiple plant sources.

2.5. Growth Performance Calculations

Growth indices were determined for each experimental unit (replicate tank/batch) and averaged across treatments (diet formulations). For calculations requiring individual fish data, the individual weights recorded at the specified sampling points were used.

Absolute weight gain (WG) is the total increase in body weight over a specific period, and was calculated with Equation (2):

$$WG=W_f-W_i$$

(2)

where $W_f$ is the final body weight (g), and $W_i$ is the initial body weight (g).

Average Daily Gain (ADG) is the average weight gained per day over a specific period—Equation (3).

$$ADG={\displaystyle \frac{W_f-W_i}{t}}$$

(3)

where $W_f$ is the final body weight (g), $W_i$ is he initial body weight (g), and $t$, is the time period (days).

Specific Growth Rate (SGR) is the rate of weight gain expressed as a percentage of body weight per day, accounting for exponential growth, as depicted in Equation (4).

$$SGR={\displaystyle \frac{ln\left(W_f\right)-ln\left(W_i\right)}{t}}\times 100$$

(4)

where $W_f$ is the final body weight (g), $W_i$ is the initial body weight (g); $t$ is the time period (days) and $\mathrm{l}\mathrm{n}$ is the natural logarithm

Feed Conversion Ratio (FCR) is the amount of feed required to produce one unit of weight gain, and is provided in Equation (5):

$$FCR={\displaystyle \frac{Total Feed Consumed \left(\mathrm{g}\right)}{Weight Gain \left(\mathrm{g}\right)}}$$

(5)

3. Results

3.1. Growth Performance Evaluation

The effect of incorporating hempseed meal into four feed formulations (R1–R4) on the growth of juvenile Common Carp, Frăsinet Carp, and Salonta Carp was assessed over a three-month period. Figure 3 depicts the average absolute weight gain, highlighting variety-specific responses to varying dietary hempseed meal levels.

Although standard deviations (SD) were not displayed on the graph to preserve visual clarity, statistical analysis confirmed that variability among replicates remained low throughout the experiment. The SD values for all treatments were within 3–8% of the mean (3–5% for first two months and 5–8% for the third month), indicating consistent growth responses among fish fed different hempseed meal diets. The slight increase in variability observed during the final month reflects normal biological differences in individual growth performance, rather than diet-related effects.

Figure 4 depicts the mean relative growth of the three carp varieties, with standard deviation error bars reflecting the variability in growth performance across treatments.

Among the tested variety, the Frăsinet carp demonstrated consistently superior growth performance across all diets, attaining a peak relative growth of 201.16 ± 2.68% with the R4 (20% hempseed meal) diet. This indicates both superior growth potential and a high level of consistency, as shown by the low standard deviation. The Common Carp showed a steady increase in growth with each successive diet, reaching 179.92 ± 8.95% under R4, suggesting that the formulations positively influenced growth performance, though variability remained moderate. In contrast, the Salonta Carp demonstrated a more irregular pattern: growth improved from R1 to R3 (up to 174.76 ± 5.88%) but declined slightly with R4 (167.03 ± 5.16%). This suggests a potential nutritional or metabolic threshold beyond which the diet may not further enhance performance.

Figure 5 illustrates the specific growth rate (SGR) of the three juvenile carp varieties, comparing the effects of the four dietary diets (R1–R4) with varying hempseed meal inclusion.

The analysis of specific growth rate (SGR) values across the four dietary diets (R1–R4) reveals distinct patterns in the growth performance of juvenile Common Carp, Salonta Carp, and Frasinet Carp over the three-month trial, likely influenced by varying levels of hempseed meal inclusion. For Common Carp, SGR progressively increased from 0.98%/day on R1 to 1.16%/day on R4, indicating a positive dose-dependent response to higher hempseed meal content, which may enhance nutrient utilization. Salonta Carp exhibited a narrower SGR range of 0.98–1.12%/day, peaking at R3 before plateauing at R4 (1.09%/day), suggesting a saturation threshold where additional hempseed meal yields diminishing returns, possibly due to variety-specific metabolic limits or anti-nutritional factors in higher inclusions. In contrast, Frasinet Carp demonstrated superior responsiveness, with SGR values ranging from 1.07%/day on R1 to a maximum of 1.23%/day on R4, underscoring its potential for hemp-based diets and highlighting inherent genetic advantages in nutrient assimilation compared to the other varieties.

No mortality was recorded throughout the 90-day experimental period, indicating that all diets were well accepted and did not induce any adverse physiological effects.

Figure 6 illustrates the Feed Conversion Ratio (FCR) of juvenile fish fed with the four dietary diets with varying hempseed meal inclusion, highlighting the comparative feed efficiency across varieties and diets.

The feed conversion ratio serves as an indicator of the efficiency of feed utilization for fish mass deposition, with lower values denoting superior conversion efficiency. In all three carp varieties, dietary inclusion of hempseed meal elicited a significant reduction in FCR, indicative of improved nutrient utilization rate and feed metabolic efficiency.

3.2. Environmental Impact Assessment

Table 2. Assessment of the environmental benefits of reintegrating waste generated during oil extraction into technological processes.

Objective	Subpoint	Calculation/ Estimation	Assessment (per 1 ton Feed)
Circular Economy— Valorization of by-product	Quantity of hemp press cake valorized	Hemp_kg = (%Hemp × 1000)	R1: 0 kg; R2: 50 kg; R3: 100 kg; R4: 200 kg
	Share of feed from by-product	% Hemp in diet	R1: 0%; R2: 5%; R3: 10%; R4: 20%
	Annual valorization potential (100 t feed/year)	(Hemp_% × Production)/100	R4: 20 t hemp waste reused annually
	Economic value recovered (0.05 €/kg saving)	Hemp_kg × 0.05 €/kg	R4: 10 €/ton feed saved
Circular Economy— Resource substitution	Reduction in pea + soybean use	Baseline − Current	R2: −50 kg; R3: −100 kg; R4: −200 kg
	% reduction in conventional protein	(Reduction/600) × 100	R2: 8.3%; R3: 16.7%; R4: 33.3%
	Soybean reduction (kg, %)	500 − Soybean	R4: −100 kg (20% reduction)
Waste reduction	Waste diverted from disposal	Hemp_kg = (%Hemp × 1000)	R2: 50 kg; R3: 100 kg; R4: 200 kg
	Waste reduction % (if 250 kg hemp waste/ton seed)	(Hemp_kg/250) × 100	R4: 80% waste diverted
	Avoided disposal cost (0.02 €/kg)	Hemp_kg × 0.02	R4: 4 €/ton avoided waste disposal cost
Carbon footprint	Carbon footprint (kg CO2-eq/ton feed)	Σ(Ingredient_kg × EF)	R1: 1375; R2: 1347.5; R3: 1320; R4: 1265 kg CO2-eq/ton of waste
	Carbon footprint per kg protein (300 kg protein/ton feed)	CF/300	R1: 4.58; R2: 4.49; R3: 4.40; R4: 4.22 kg CO2-eq/kg protein

illustrates the evaluation of the positive environmental impact resulting from the reintegration into technological processes of the waste generated during oil extraction operations.

4. Discussion

4.1. Comparative Analysis of Growth and Feed Conversion Efficiency

The inclusion of hempseed meal in carp diets significantly influenced growth performance and feed conversion efficiency across all tested varieties. Moderate inclusion levels (5–10%) consistently improved weight gain and reduced feed conversion ratios (FCR) compared with the control diet, confirming hempseed meal’s nutritional potential as a sustainable plant-based ingredient. Similar improvements in growth indices were previously reported for cyprinids and other omnivorous fish varieties supplemented with oilseed by-products [31,32]. The optimal 10% inclusion identified in this study aligns with reports indicating that moderate substitution levels typically balance nutrient availability and digestibility while avoiding excess fiber or anti-nutritional effects [33].

Similar research studies have reported that 10% hempseeds meal inclusion in fodders improved growth and reduced FCR in Common carp without adverse health effects [34].

4.2. Mechanistic Interpretation

The superior performance of hemp-based diets can be attributed to their balanced amino acid profile, high digestibility, and favorable lipid composition. Hemp proteins are rich in arginine, methionine, and glutamic acid, essential for tissue growth and metabolic regulation [35]. The residual oil fraction, dominated by polyunsaturated fatty acids (linoleic and α-linolenic acids), likely enhanced nutrient absorption and membrane integrity, contributing to improved growth rates. Additionally, bioactive compounds such as tocopherols and phenolics in hempseed meal may exert antioxidant and immunostimulant effects, supporting metabolic health and feed efficiency.

However, excessive inclusion levels (20%) did not yield further performance gains, likely due to elevated fiber content and anti-nutritional compounds that can limit digestibility. This plateau effect is consistent with earlier findings on plant-derived protein sources in aquafeeds [34,35].

4.3. Inter-Varieties Variability

Growth responses differed among the three carp varieties. Frăsinet carp exhibited the highest growth rate and feed conversion efficiency, followed by Common carp and Salonta carp. This suggests a genetic predisposition of the Frăsinet variety toward more efficient nutrient assimilation, possibly related to selective breeding for improved muscle yield and metabolic adaptability. The strong diet–time interaction observed for this varieties (p < 0.001) highlights its capacity to utilize hemp-derived nutrients effectively.

Common carp displayed consistent but moderate improvements, while Salonta carp showed slight performance variation at higher hemp inclusions, indicating a possible metabolic threshold. These differences underscore the importance of tailoring hempseed meal inclusion levels to varieties specific digestive physiology to maximize feed efficiency and economic return.

4.4. Environmental and Circular-Economy Implications

Beyond nutritional benefits, incorporating hempseed meal into aquafeeds supports circular-economy objectives by valorizing an agro-industrial by-product of oil extraction. At a 20% inclusion level, approximately 200 kg of hemp press cake per ton of feed is reused, corresponding to 20 tons annually for a 100-ton production scenario. This substitution reduces the demand for conventional protein sources such as soybean and pea meal by up to 33%, alleviating pressure on land-intensive crops.

From an environmental standpoint, waste redirection from oilseed processing prevents disposal impacts and contributes to lower greenhouse gas emissions. The estimated reduction in the feed’s carbon footprint—approximately 8% between the control and the highest hemp inclusion—demonstrates the potential of hempseed meal to decrease aquaculture’s environmental intensity. Thus, integrating hempseed meal into fish feeds promotes both economic efficiency and ecological sustainability, aligning with global goals for low-carbon and circular food systems.

4.5. Limitations and Future Perspectives

While the results confirm hempseed meal as a promising alternative protein source, several limitations should be acknowledged. The study focused on short-term growth performance under controlled conditions and did not include physiological or biochemical evaluations (e.g., plasma metabolites, enzyme activities). Future research will integrate these analyses to elucidate the metabolic pathways influenced by hemp-derived nutrients.

This research focuses specifically on growth performance and basic environmental parameters (e.g., feed conversion efficiency). In-depth physiological studies are outside the scope of this initial valorization study and are reserved for future work.

The experimental design also focused exclusively on three carp varieties, limiting the generalization of results to other freshwater or marine varieties. Future research should extend to carnivorous fish and omnivorous varieties with different digestive capacities to assess the broader applicability of hempseed meal, as a sustainable protein ingredient.

From a sustainability perspective, further investigation into the economic and environmental implications of large-scale hempseed meal utilization in aquafeeds is needed, including life cycle assessment (LCA) and cost–benefit analyses. Research into optimized cold-pressing parameters or partial defatting processes could also enhance the nutritional value of hempseed meal by balancing its residual oil and fiber content.

In summary, while the current findings strongly support the use of hempseed meal as a viable partial replacement for conventional protein sources, future work should focus on long-term growth trials, digestibility optimization, and biochemical validation to strengthen the evidence base for its integration into sustainable aquaculture feed systems.

Future studies will employ iso-lipidic formulations to better isolate the metabolic effects of dietary fat and improve cross-diet comparability. Additionally, follow-up experiments will include physiological and biochemical analyses, including blood plasma biochemistry, antioxidant enzyme activity, and muscle composition, in order to deepen the metabolic responses to hempseed meal inclusion and provide a more comprehensive understanding of the nutritional and metabolic impact.

Future studies will include physiological and biochemical assessments such as blood plasma biochemistry, antioxidant enzyme activity, and muscle composition to validate the metabolic responses associated with hempseed meal inclusion.

5. Conclusions

This study demonstrates that moderate inclusion of hempseed meal in carp diets enhances growth performance and feed conversion efficiency without adverse effects on fish health. The results confirm hempseed meal as a viable, plant-based alternative to conventional protein sources, contributing to more sustainable and resource-efficient aquaculture practices. By valorizing hempseed meal, an agro-industrial by-product, as an aquafeed ingredient, this approach reduces waste generation and dependence on conventional protein sources, thereby strengthening circular bioeconomy practices in aquaculture. Future studies will expand on these findings by integrating physiological and biochemical analyses to further elucidate the nutritional and metabolic effects of hempseed meal in freshwater aquaculture systems.

The most significant contribution of this study is the quantified demonstration of the hempseed meal potential as a circular bio-ingredient, fully supporting the principles of a sustainable fisheries model. The findings confirm the feasibility and significant ecological advantage of this valorization strategy, establishing a proof-of-concept for enhancing sustainability within the aquaculture sector.

Future research will focus on optimizing feed formulations to enhance nutritional and production outcomes, as the present study was primarily designed to validate the practical feasibility of incorporating hempseed meal into carp diets.