Study area. An experimental seaweed culture site was designed in the sheltered intertidal zones of Chowfoldondi (21.503248, 91.994117) of Cox’s Bazar district (Fig. 3), along the Bay of Bengal’s north-eastern coast. The study area is on the Bakkhali River (the second largest river in the district), which has moderate wave action and is directly connected to the Bay of Bengal. This area is considered a biologically diverse ecosystem, having intertidal mudflats with various salt marshes, seagrasses, cord grasses, seaweeds, and also some mangrove vegetation. Additionally, this place is a safe habitat for numerous types of fish, reptiles, oysters, mussels, crabs, snails, shrimp, and so on. The yearly average rainfall and temperature follow at 3770 mm and 25.6°C, respectively, in the Cox’s Bazar district. This tropical climatic area’s wind speed average is about 8.3 miles per hour. The study was performed for a period of 90 days from January 2022 to March 2022.
Seed collection. Young, wild G. verrucosa seed was initially collected from the intertidal zones of the Nuniachara coast (21.474700, 91.964450) of Cox’s Bazar Sadar. This sand flat site is a natural bed of seaweed with some other seagrass, salt marsh, and mangrove vegetation. Permission to collect seaweed samples and their culture practices was obtained from the local government in accordance with local and national legislation. The botanical identification of seaweed species was checked and confirmed through the published literature5,31. Dr. Md. Enamul Hoq, Former Director of BFRI, validated the botanical identification of seaweed species as the voucher specimen has already been placed at BFRI herbarium [BFRI (MFTS-RS-18/19–038). A fresh sample was collected in an open box with adequate seawater and an aeration facility and then immediately transferred to the cultivation site to keep the fresh quality.
Experimental culture raft setup. Several bamboo poles (7.0–10.0 cm diameter) made into a square (5 m x 5 m) frames were prepared to provide the seaweed culture layout. The four corners of the raft were steadied tightly between and among themselves to keep the raft shape intact. Two more bamboos were fastened tightly at the opposite end to make the structure stronger. Four recycled plastic drams were attached to the structure's four corners, ensuring that it was always floating on the water. A 1.50 cm mesh size plastic net was placed in the lower part of the frame to minimize the wave action, and crop loss caused by plant rupture from the base, especially during adverse weather. All of the rafts were rope-tied, placed in the culture site, and anchored to help stabilize the structure. The anchor of the structure was placed in such a way that it could raise and fall vertically during the tidal action. Each experiment had three replications.
Seaweed seeding. The younger pieces of G. verrucosa were used for seeding with an average of 5 ± 0.4 gram of fresh weight in each knot and 5 cm size in the rope twists. No fertilizer, growth hormone, or any other chemicals were used during the culture period. Partial harvesting was done when the seaweed reached an average standard length. The partial harvesting took place by cutting off the algae hanging on the surface, allowing the base on the surface to expand further. Seaweed biomass production was measured as the fresh weight of seaweed per unit culture area (Kg m-2) and was calculated using the given Eq. 32.
Y = (Wn − W0)/A (1)
Here, Y = seaweed biomass production; Wn = raw weight on day n; W0 = beginning raw weight; A = culture unit’s area.
Daily growth rate % was calculated using the formula given Eq. 33.
DGR % = ln (Wf / Wo) / t x 100 (2)
Here, Wf = final raw weight (g) at t day; Wo = initial raw weight (g); t = cultivation period (days).
Water quality variables. The culture site was studied throughout the 90 days culture period from January 2022 to March 2022. Sampling was carried out twice a month. Multiple water quality parameters such as temperature, pH, dissolved oxygen (DO), salinity, TDS (total dissolved solids), transparency, alkalinity, ammonia, nitrite, nitrate, phosphate, and silica were measured. Temperature, pH, dissolved oxygen, salinity, and TDS were all measured on the spot using HANNA HI-98194 multiparameter. Measurement of water transparency was performed with a Secchi disc. Water samples from 0-100cm depth were collected (Van Dorn water sampler) and immediately transported to the laboratory. Alkalinity, ammonia, nitrite, nitrate, phosphate, and silica in water were analyzed following the methods of HANNA (Hanna COD and Multiparameter Bench Photometer, 230V - HI83099 procedure manual).
Effect of rope material. This experiment was carried out to find out the rope material’s effect on the yield performance of G. verrucosa. In this study, four types of rope materials (T1 = nylon rope, T2 = plastic rope, T3 = coir rope: acquired from coconut husk, and T4 = jute rope) were used. Among them, coir and jute ropes are biodegradable, while the other two are non-biodegradable. Best yield performances along with the rope materials’ sustainability in saline water were observed.
Effect of culture type. In this study, different culture types, such as long line and square net methods, were used to observe the yield performance of G. verrucosa. Here, T1 stands for the long line method and T2 stands for the square net method.
Effect of raft shape. In this experiment, different shaped rafts were prepared and observed for their effect on the yield performance of G. verrucosa. Here, T1 (square shape) and T2 (triangular shape) rafts were prepared and observed for their yield performance.
Effect of seeding intensity. This experiment was conducted to observe the effect of seeding intensity on the biomass production of G. verrucosa. Here, T1, T2, and T3 indicate 50, 100, and 150 seeds per square meter, respectively. This results in 250gm, 500gm, and 750gm initial seaweed seeds per square meter for cultivation. During harvesting, fresh seaweed samples were collected separately and washed carefully with running fresh water to remove dirt or any other impurities. The fresh sample was then weighted with the help of a digital balance.
Effect of harvesting phase. In this study, after initial seeding, harvesting was performed at four different intervals (T1 = 15 days, T2 = 30 days, T3 = 45 days, and T4 = 90 days). Here, in the case of T1 a total of six partial harvests, in the case of T2 a total of three partial harvests, in the case of T3 a total of two partial harvests, and in the case of T4 zero partial harvests were performed throughout the 90 days of the culture period. The best harvesting intervals were evaluated through the maximum yield performance of G. verrucosa.
Effect of water depth. In this study, several vertical culture units were placed at different water levels in the open sea. T1 represents the first unit that was placed at the water's surface, T2 represents the second unit that was placed below the first one at a 25cm depth from the water's surface, and T3 represents the third unit that was placed below the second one at a 50cm depth from the water's surface. The possibility of vertical expansion of seaweed culture was evaluated through the maximum yield performance of G. verrucosa at each depth.
Seasonal appearance of epiphytic algae. Epiphytic algal occurrences on G. verrucosa and the associated culture materials were documented during the three-month (January–March 2022) cultivation period. Seaweed samples were collected, washed with clean water, and preserved in silica gel in a sample vial. Finally, seaweed samples were confirmed through microscopic examination and cross-checked against the published literature5,31.
Cost-benefit analysis of seaweed culture. The cost-benefit analyses of G. verrucosa culture for 20 rafts (5m x 5m) in a six-month culture period were analyzed. The economic turnover of G. verrucosa culture through biomass production can be estimated. All operation and maintenance costs were considered here as input materials. Based on the local market, all prices of input materials were included and expressed in USD.
Statistical analysis. The experimental data was analyzed using standard statistical techniques. Statistical package SPSS version 20.0 (IBM Co., Chicago, IL) was used to examine the data. One way ANOVA and T-test were used to determine the significance of each parameter among different treatments. Level of significance was set at 95% probability level.