Introduction
In subtropical and temperate areas, low temperatures limit shrimp culture to summer and fall. The shrimp biofloc culture rearing system (BFT system) in an enclosed greenhouse is a technological alternative aimed at increasing the culture period in these regions. The shrimp farming sector has developed great interest in the BFT, due to the ability of the bioflocs provide a stable culture environment by the maintenance of water quality, low feed-conversion rates (FCRs) through the natural food source and high productivity reaching up to 9 kg.m-3 (Samocha et al., 2017). In order to achieve the highest productivity rates, several authors have been testing the effects of high stocking density rates in order to determine which is the most appropriate (Da Silveira et al., 2020; Krummenauer et al., 2011). However, the production efficiency in culture tanks can be boosted by increasing the culture tank’s carrying capacity, which can be described as the maximum biomass of aquatic cultivated organisms that can be maintained in a rearing system indefinitely, usually described as unit of mass per volume unit (kg.m-3) (Timmons and Ebeling, 2010). Anyway, with the system carrying capacity known, the definition of the most adequate stocking density must consider the shrimp desired final individual weight. In addition, for a better efficiency in the use of the available culture structure, it is also possible to use strategies that maintain the shrimp stocking densities closer to the system carrying capacity throughout the production cycle like multi-phase production system (Van Wyk, 1999). The BFT system at high stocking density requires a vigorous and efficient aeration to keep the oxygen concentration within the desirable concentrations, and maintain a high particle density and organic matter in suspension. The dissolved oxygen is the most critical water quality variation in aquaculture. In this scenario, several technologies and practices has been tested and adopted in order to increase the productivity, for example Samocha et al. (2017) described a system with carrying capacity of 9 kg.m-3 with the possibility of using pure oxygen if it needs. These technological advances have greatly increased the aquaculture system’s carrying capacity in terms of maintaining desirable water quality parameters. Considering that, the multi-phase system permits the maintenance of stocking densities closer to the system’s carrying capacity, and the system’s carrying capacity in terms of water quality maintenance is no longer a limit. The present study intended to show the evolution of the stocking densities from Intensive to Hyperintensive culture of shrimps in BFT systems and determinate the most adequate maximum shrimp biomass in each stage of a multi-phase system.
Material and methods
The present study aims to define the most adequate highest stocking densities for each stage of a multi-phase system, considering that the system’s carrying capacity is able to maintain the water quality parameters at desired levels. This study was divided into four phases according to the size: Phase 1, shrimp were stocked with initial weight of 0.002 g; Phase 2, initial weight of 1.04 g; Phase 3, initial weight of 6.09 g; and, Phase 4, initial weight of 12.51 g. Each phase lasted for 40 days, and the treatments applied were the different stocking densities. Shrimp for all stages were cultured at high stocking densities in biofloc technology system (BFT). The water quality parameters were maintained within the optimum levels for the L. vannamei development.
Results and discussion
The results of the present study confirm the L. vannamei density dependent growth dynamic in all stages of development as reported by several authors (Da Silveira et al., 2020; Krummenauer et al., 2011). The phase 1 or pre-nursery has already been considered intensive culture when the densities reach 2,000 to 5,000 shrimp.m-3. The utilization of the BFT technology in the early stages of L. vannamei can generate shrimp juveniles of excellent quality due to the nutritional benefits taken from the presence of bioflocs. In different experiments survival and growth in all treatments and were significantly different (P<0.05). Better results of each experiment are shown in table 1. However, for phase 4 the quadratic model (y = -1.292e-05x2 + 0.02852x - 1.513) indicate that the support capacity reaches the max biomass (14.22 kg) at 1,125 shrimp.m-3.
Conclusion
In summary, the present study confirms the shrimp density dependent growth pattern even when maintaining optimum levels of water quality parameters. In addition, proposes a biological limit of 14 kg.m-3 for the shrimp culture in BFT system. However, the water quality parameters deterioration is the limiting factor for the system’s carrying capacity, and the shrimp stress behavior generated by the lack of space is the biological limiting factor for the shrimp culture maximum biomass (biological limit). In addition, further studies are needed for better understanding of the shrimp biological limit and for technology development aiming to increase the shrimp culture productivity.
Acknowledgements
This work was developed as part of the ASTRAL (All Atlantic Ocean Sustainable, Profitable and Resilient Aquaculture) project - European Union’s Horizon 2020 research and innovation programme under grant agreement N° 863034 and National Council for Scientific and Technological Development (CNPq) - Brazil.
References
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