Comparing Partial and Full Harvest Strategies on the Cultivation of Ulva Ohnoi Using Water from Marine Shrimp Biofloc Systems

This study investigated the effects of different partial harvest frequencies on the cultivation of the green macroalga Ulva ohnoi using water from whiteleg shrimp Penaeus vannamei biofloc systems over 6 weeks. Specimens of U. ohnoi reproduced in the laboratory were stocked at 1 g of fresh weight (FW) l(-1) in 50 l tanks. Four experimental groups were evaluated: three partial harvest frequencies (every 7, 14 and 21 days), where biomass exceeding the initial stocking of 1 g FW l(-1) was removed from the tanks, and full harvest at 42 days, at which point the partial harvest treatments were also fully harvested. Each group had six replicates for a total of 24 experimental units. Weekly, water from a shrimp biofloc tank was filtered with a 20 & micro;m bag filter and used to fill the algae tanks, whose previous water was discarded. Weighing the algae after removing water with a manual centrifuge was performed weekly. Standing biomass was highest in the 42-day group from week 4 onwards (p < 0.05), while growth rates showed significant differences primarily in weeks 2 and 3, with the 7 day partial harvest frequency showing higher rates (p < 0.05). Final yield and nitrogen and carbon removals did not differ between treatments (p > 0.05), whereas phosphorus removal was significantly higher in the 42-day harvest group (p < 0.05). This indicates that cumulative biomass production was similar among harvest regimes, supporting the feasibility of a single 42-day harvest. In conclusion, a 42-day harvest interval can simplify operational management by reducing frequent harvests while maintaining yield and improving nutrient removal in U. ohnoi cultivation within a decoupled biofloc-based integrated system with shrimp compared with different partial harvest regimes.