Comparative performances of different farming systems and associated influence of ecological factors on Gracilaria sp. seaweed at the south-east coast of the Bay of Bengal, Bangladesh

The production performance and biochemical composition of agar-enriched Gracilaria sp. were evaluated under three farming systems (off-bottom net, floating long-line and off-bottom long-line) from October 2021 to April 2022. For this study, 25 off-bottom long-line (14 m × 5 m), 20 off-bottom net (1.8 m × 3.5 m), and 10 floating long-line (10 m × 3 m) were established and managed in a participatory farming approach with coastal communities at the south-east coast of the Bay of Bengal, Bangladesh. While growing to a length of 30–40 cm, Gracilaria sp. was partially harvested every 15–30 days, depending on the culture months. The average biomass production in the floating long-line system was 135% and 67.2% higher than the off-bottom long line and off-bottom net culture systems, respectively. Harvested Gracilaria sp. from floating culture was very clean, mostly free from encrusting organisms and impurities, and had an attractive reddish coloration. Multivariate analysis demonstrated that the growth performances of Gracilaria sp. were positively influenced by high salinity, dissolved oxygen, and nutrient concentration and negatively influenced by high temperature, turbidity, and total suspended solids. Gracilaria sp. harvested from the floating long-line system had higher amount of protein and lipids compared to the off-bottom systems. The total essential amino acid content of Gracilaria sp. in the floating long-line system was 18.4% and 13.3% higher (p < 0.05) than in off-bottom long-line and off-bottom net culture systems, respectively. Among the 10 minerals analyzed, Ca, K, and Zn contents were significantly higher (p < 0.05) in the floating long-line system. Among fatty acids, the n6-PUFA content of Gracilaria sp. in the floating long-line system was 11.2% higher (p < 0.05) than in the off-bottom long-line system. The Gracilaria sp. from the floating long-line culture system contained 18.8–21.3% higher agar than the off-bottom systems. The study also revealed that Gracilaria sp. obtained from the floating long-line system contained significantly lower heavy metals (Cr, Cd, Pb, and As) than the off-bottom systems. The findings of the present study demonstrated that raft-based floating long-line systems could be a promising culture technology for the production of agar-enriched Gracilaria sp. along the coastal areas of Bangladesh and elsewhere.