An orthogonal design to optimize seed production, out-planting, and cultivation of the industrially overexploited red alga Gracilaria edulis (Rhodophyta)

Abstract

Marine algal resources are of significant importance due to their renewable nature and plentiful availability; however, they also are subjected to indiscriminate harvesting. Despite multi-organizational efforts under the Bay of Bengal Program implemented by the Food and Agriculture Organization (FAO) of the United Nations, the Indian agar industry still heavily relies on the natural harvest of Gracilaria edulis from the southeastern coast of the Gulf of Mannar. Experimental cultivation of this species has been successful, but lack of a year-round supply of quality seed material has hampered the prospects of undertaking commercial farming. To maximize the efficiency of artificial seedling production, we used a Taguchi orthogonal array design to determine the optimal temperature, salinity, and pH for seedling production from primary, secondary, and tertiary fragments. The maximum growth was obtained at 25 °C, 20 ppt salinity, and pH 7.5, where the highest daily growth rate (DGR) of 4.88 ± 1.04 % day⁻¹ (K ₃ = 3.194) was recorded in tertiary fragments. The DGR of seedlings grown with shaking as well as static cultures ranged from 8.33 ± 1.50 to 10.26 ± 0.01 % day⁻¹ after 10 days. Analysis of variance (ANOVA) showed that water motion did not significantly affect growth. The out-planting was successful for seedlings derived from laboratory culture. Among the two harvesting methods examined, namely complete and clipping harvest, the former resulted in the highest biomass yield (7.21 ± 0.83 kg fresh wt m⁻²) and DGR (5.93 ± 0.11 % day⁻¹). A statistically significant difference (F = 16.976; p < 0.001 and F = 3.359; p < 0.02) was observed for yield and DGR, respectively, between the methods. It is thus imperative to augment the algal stock for year-round mariculture thereby negating the consequences of overexploitation.