Macroalgae Production in Integrated Multi-Trophic Aquaculture with Shrimp and Fish: Nutrient Uptake and Application of the Produced Biomass
Author: Andrezza Carvalho Chagas (Currículo Lattes)
Advisor: Dr. Luís Henrique da Silva Poersch
Abstract
The use of systems with zero water exchange, such as the Biofloc Technology System (BFT), despite offering greater biosecurity, results in the accumulation of nitrate and phosphate during the production cycle. The possibility of incorporating species from different trophic levels into the system promotes increased productivity by reusing waste for conversion into economically valuable biomass, enabling a circular economy approach. Therefore, this thesis aimed to determine feasible management plans for the macroalga Ulva lactuca in integrated culture systems, focusing on biomass production, macroalgae application, and nutrient absorption. The first experiment evaluated the effect of a symbiotic system on the growth of the macroalga, oyster, and shrimp. Although there was no increase in macroalgal biomass during the experiment, nitrate concentrations were lower in the symbiotic treatment when the macroalga was present in the system. For oysters, the use of the symbiotic system resulted in animal mortality, whereas shrimp exhibited a higher final weight in the integrated system with the symbiotic setup. The second chapter assessed the effects of different concentrations of total suspended solids and nutrients in the biofloc system on the growth, nutrient uptake, and nutritional composition of the macroalga in an integrated system with shrimp. The treatment with 246 mg L⁻¹ of total suspended solids showed better performance in nitrate and phosphate removal, with 55% and 31% removal rates, respectively. It also showed higher protein and chlorophyll-a content compared to the control treatment (without biofloc). The third chapter aimed to evaluate different cultivation structure depths for the macroalga when cultivated in an integrated system with shrimp and fish. Two depths were tested: shallow (up to 10 cm) and deep (up to 25 cm). Macroalgae grown at 10 cm depth showed better growth throughout the cultivation period compared to the deeper treatment. The fourth chapter assessed the effect of macroalga introduction on the microbial community when co-cultured with shrimp. At the end of the experiment, the addition of Ulva to the culture resulted in a decrease in diatoms, cyanobacteria, and ciliates compared to the shrimp monoculture. This may be due to nutrient competition with the macroalga, indicating an advantage of using the integrated system. The fifth chapter of the thesis evaluated the growth, nutrient uptake, and nutritional composition of the macroalga in heterotrophic and chemoautotrophic systems. Results showed higher macroalgal growth rates in the chemoautotrophic treatment by the end of the experiment. However, the heterotrophic treatment presented higher phosphate and nitrate uptake (57% and 56%, respectively), as well as increased protein content in the macroalga. Shrimp performance was not affected by the treatment, and tilapia showed a higher final weight in the heterotrophic treatment. The sixth chapter aimed to evaluate the use of macroalgae as a bioremediator for nutrient absorption from shrimp culture effluent in a biofloc system. Effluents were collected from previous chemoautotrophic and heterotrophic systems, and 1g/L of macroalgae was introduced under constant aeration for 15 days. As a result, a higher nitrate removal rate (28%) was observed in the heterotrophic system, with a relative growth rate of 3.53% per day, indicating better macroalgal performance. The seventh chapter involved applying the produced macroalgal biomass as a feed additive. Different inclusion levels (5%, 10%, and 15%) of macroalgae produced in an integrated biofloc system were tested in tilapia feed, along with a control treatment without macroalgae, to evaluate zootechnical performance, hematological parameters, and antioxidant activity. At the end of the experiment, a salt stress test was performed. No significant differences were found in zootechnical performance and proximate composition among treatments. However, granulocyte counts were higher in the 10% macroalgae inclusion group. After salt stress, the 5% inclusion treatment showed higher antioxidant capacity in muscle tissue, with reduced protein and lipid oxidation. In conclusion, incorporating macroalgae in integrated systems with shrimp and fish demonstrated viability and sustainability through nutrient absorption, biomass production, and inclusion in feed.