In modern aquaculture, especially within Recirculating Aquaculture Systems (RAS), every cubic centimeter counts. Space optimization not only aims to maximize production but also to ensure the well-being of the fish and the quality of the final product. A recent study delves into a fundamental question: how does “absolute space”—that is, the total volume of the tank—affect the development of fish?
Researchers from Shanghai Ocean University and the Jiangxi Fishery Resources and Ecological Environment Monitoring Center evaluated the impact of three distinct tank volumes on the growth, stress physiology, and nutritional quality of Carassius auratus var. Pengze, a cyprinid of great importance in freshwater aquaculture. The results, far from being obvious, reveal a fascinating plasticity and offer crucial guidelines for designing more efficient and adaptive RAS.
Key findings
- Juvenile fish (~20 g) raised in the smallest tanks (0.075 m³) showed a 26.42% higher weight gain rate compared to those in the largest tanks.
- Contrary to what one might expect, fish in small tanks registered the lowest levels of cortisol, the primary biomarker for stress.
- The muscle of fish in small tanks exhibited a higher crude fat content and significantly superior levels of healthy fatty acids, such as HUFAs and n-3 PUFAs.
- Although the reduced space was beneficial during the initial phase, larger fish (>40 g) demonstrated a higher growth rate in larger-volume tanks during the final stage of the experiment.
How was the effect of space investigated?
To unravel the relationship between tank volume and fish welfare, the scientists designed a 100-day experiment within a single RAS. This ensured that the water quality was identical for all fish, thereby isolating volume as the sole primary variable.
Three experimental groups were established, each with four replicates:
- Group S (Small): 0.075 m³ tanks.
- Group M (Medium): 0.15 m³ tanks.
- Group L (Large): 0.3 m³ tanks.
The researchers stocked juvenile Carassius auratus var. Pengze with an initial weight of approximately 20 grams, maintaining an identical stocking density of 2.68 kg/m³ across all groups. Throughout the study, growth parameters were measured, blood samples were analyzed to evaluate stress and metabolism indicators, and the nutritional composition of the muscle was examined.
Less space, better growth for juveniles?
The growth results were revealing. Group S (small tanks) not only achieved the highest final weight and the highest weight gain rate but also showed the greatest uniformity in fish size, with a coefficient of variation in weight 28.8% lower than that of Group L.
However, the advantage of reduced space was not constant. The analysis by growth phases demonstrated that:
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- Initial phase (0-60 days): Group S clearly dominated, showing the greatest increase in weight.
- Final phase (80-100 days): The trend inverted. Groups L and M surpassed Group S in weight gain, suggesting that as fish grow, they need more space to maintain their rate of development.
This finding indicates a spatial plasticity that is dependent on the developmental stage: confined spaces are optimal for juveniles but limiting for larger individuals.
Stress and physiology: The fish’s response to volume
One of the most surprising discoveries was on the stress front. Serum cortisol, a primary indicator of stress in fish, was significantly lower in Group S. This suggests that for juveniles of this species, a more restricted space may generate a sense of security, reducing the energy expenditure associated with the constant swimming and exploration that occurs in larger tanks.
Observed behavior confirms this: fish in large tanks swam faster and spent more time exploring, whereas those in small tanks displayed more leisurely behaviors, such as waiting in groups in the feeding area.
This reduction in stress and energy expenditure in Group S was reflected in their metabolism: they presented higher concentrations of triglycerides and total cholesterol in their plasma, indicating a greater channeling of energy toward the accumulation of lipid reserves rather than toward physical activity.
Nutritional quality: The impact of space on the fish meat
The optimization of space not only influenced how much the fish grew but also the quality of their meat. Analysis of the muscle composition revealed that Group S presented a significantly higher crude fat content.
More important still was the fatty acid profile. The fish from Group S showed much higher levels of compounds beneficial for human health:
- Highly Unsaturated Fatty Acids (HUFA): Approximately 183% more than in Group L.
- n-3 Polyunsaturated Fatty Acids (PUFA): 126% more than in Group L.
- n-6 Polyunsaturated Fatty Acids (PUFA): 223% more than in Group L.
This improvement in the lipid profile is directly attributed to lower energy expenditure. In the large tanks, the greater locomotor activity demands a high consumption of energy, which leads to the depletion of fat reserves. In contrast, the more sedentary life in the small tanks allows these valuable fatty acids to accumulate in the muscle.
Towards a phased precision aquaculture
This study demonstrates that there is no “one-size-fits-all” tank ideal for the entire cultivation cycle. The most efficient strategy is a phased cultivation approach or an adaptive spatial scaling.
For Carassius auratus var. Pengze, the optimal model would be:
- Juvenile phase (~20 g): Utilize systems with restricted volumes (like 0.075 m³) to maximize the growth rate, minimize stress, and improve the nutritional quality of the product.
- Maturation phase (>40 g): Transfer the fish to larger-volume tanks to prevent space from becoming a limiting factor and thus sustain a high growth rate until harvest.
This precision strategy not only optimizes productive performance but also represents a step forward towards a more sustainable aquaculture that is adjusted to the biological needs of each developmental stage of the fish.
Contact
Luo Guozhi
Shanghai Engineering Research Center of Aquaculture, Shanghai Ocean University
Shanghai 201306, China.
gzhluo@shou.edu.cn
Reference (open access)
Haixin, Z., Yanping, Z., Jiaqin, C., Zijun, W., Hongxin, T., & Guozhi, L. (2025). Spatial optimization in recirculating aquaculture systems: Cortisol-mediated growth modulation and polyunsaturated fatty acid Enrichment in Carassius auratus var. Pengze. Aquaculture Reports, 44, 103027. https://doi.org/10.1016/j.aqrep.2025.103027
Editor at the digital magazine AquaHoy. He holds a degree in Aquaculture Biology from the National University of Santa (UNS) and a Master’s degree in Science and Innovation Management from the Polytechnic University of Valencia, with postgraduate diplomas in Business Innovation and Innovation Management. He possesses extensive experience in the aquaculture and fisheries sector, having led the Fisheries Innovation Unit of the National Program for Innovation in Fisheries and Aquaculture (PNIPA). He has served as a senior consultant in technology watch, an innovation project formulator and advisor, and a lecturer at UNS. He is a member of the Peruvian College of Biologists and was recognized by the World Aquaculture Society (WAS) in 2016 for his contribution to aquaculture.
