Introduction
Implementing biofloc technology (BFT) has allowed an increase in shrimp culture densities, but at the cost of raising the requirements for dissolved oxygen (DO) in the water of production units, becoming one of the biggest risk factors in aquaculture production. Because BFT systems rely on technologies such as blowers and hydraulic pumps to generate and replenish oxygen in the culture water, the risk of energy outbreaks, generator, or pump failures could lead to a potential drop in oxygen levels, with resulting mortalities in the cultured organisms. The high oxygen consumption in BFT systems depends on several factors such as the density of cultured organisms like shrimp , salinity, and water temperature. Temperature, for example, acts directly on the solubility of gases in water and regulates the metabolism of the animal by increasing or decreasing its consumption over time. Salinity also directly affects the oxygen solubility in water, when salinity increases the dispersion decreases, and when it decreases the portion of DO in water increases. Due to several factors, the oxygen concentration in the water can be depleted, which requires emergency strategies to overpass a lack of aeration, affecting shrimp survival. For this reason, we ran several experiments to test the factors affecting oxygen consumption in biofloc systems and the effect of different re-oxygenation strategies o n P . vannamei survival.
Material and Methods
The experiments were performed in 20L plastic tanks with individual aeration supplies, placed in thermostatic baths for temperature maintenance throughout the experiment, using total suspended solids (TSS) at 500mg/L, all treatments with three replicates. Dissolved oxygen levels were initially measured in all tanks, and then the air supply was cut off and again measured at 20-min intervals until levels reached hypoxia. Then the survival was evaluated by counting the number of dead animals after exposure to stresses. First, we tested high (30º and 36° C) and low (16º and 22°C) temperatures at different densities (150 and 600 cam/m³), using individuals with an average weight of 15g, to determine the consumption time and survival. Then, we determined the DO consumption time in a P. vannamei culture and survival using different salinities (5 and 30‰) and different stocking densities (150 and 600 cam/m³) . Finally, we determined the effect of different re-oxygenation methods (hydrogen peroxide and manual aeration) on the survival of P. vannamei shrimp in BFT systems. The re- oxygenation was performed in 4 different ways: Abrupt (RA - total reinsertion of DO up to 5 mg/L in less than 20 min), Gradual (RG - DO rising 1 mg/L every 20 min), with hydrogen peroxide (RPH - 7ml/m³) and with hydrogen peroxide powder (RPP) in the amounts indicated by the manufacturer (SwBio). Survival was observed for 3 days after reoxygenation
Results
T he experimental units at a density of 600 reached critical levels faster than those at 150. There was also an increase in the time required for the system to reach the hypoxia scenario considering the temperature of the experimental units. The temperature directly affected survival, which reduced the animal’s metabolism, directly influencing energy output and productivity.
In the salinity trials, the stocking density was the most significant factor in DO consumption. The densities of 600 cam/m³ took about 1h40min at 30‰ and 2h at 5‰ to reach hypoxia, consuming about three times faster the DO than the treatments with a density of 150 cam/m³ that took 4h20min at 30‰ and 5‰. At low salinity, outside its isosmotic point, osmoregulatory stress combined with hypoxia leads P. vannamei to a high mortality rate, at a salinity of 5‰ and density of 600 cam/m³, survival was only 29.1%, thus demonstrating that this combination of stress factors is highly detrimental to production.
In the re-oxygenation trials, survival was largely affected by critical oxygen depletion, showing significant differences between treatments . The animals can maintain themselves in the basal state to prevent mortality as long as there is no other problem associated with DO deprivation. The lack of oxygen combined with re-oxygenation is a stressful factor for the animals and may affect their ability to maintain homeostasis and metabolis m, directly affecting survival and may have caused mortality when these organisms were exposed to combined hypoxia/re-oxygenation stress.
Conclusion
Based on our results, we recommend determining a strict protocol for dissolved oxygen management in shrimp biofloc culture systems. Monitoring and maintaining adequate TSS concentrations, animal culture density, and water temperature is also critical. We also emphasize the importance of not letting dissolved oxygen levels go under the value of 2 mg/L. Our data and observations from both experiments showed that the density of the cultured animals and the biofloc in the BFT system can directly influence the consumption of dissolved oxygen in the water, and consequently affect the production performance of the shrimp.
All references are available from the corresponding author.