Raft cultivation of Gracilaria edulÿ (Gmel.) Silva

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1 Proc. Indian Acad. Sci. (Plant Sci.), Vol. 100, No. 2, Ap 1990, pp Printed in India. Raft cultivation of Gracilaria edulÿ (Gmel.) Silva K SUBBARAMAIAH and P C THOMAS Central Salt and Marine Chemicals Research lnstitute, Ma Mandapam , India MS received 31 October 1989; revised 17 March 1990 Algal Research Station, Abstract. The single-rope floating raft cultivation of Gracilaria edulis was t at Krusadai Island for one year. In 3 harvests mean biomass annual yield of 4 kg (wet) m- was obtained which is the highest recorded for the alga. The raft cultivation of the alga at different levels has shown that maintaining the cultivation topes at the top level will gire better yield. Keywords. Gracilaria edulis;, single-rope floating raft; alga cultivation. 1. lntroduction Cultivation of seaweeds in the sea was termed 'marine agronomy' by Doty (1977). Among the economic marine algae only 11 genera are commercially cultivated to a certain extent in Japan, China, Philippines in the far east and these include Porphyra tenera, P. yezoensis, P. haitanensis, Gelidiella acerosa, Gloiopeltis furcata, Eucheuma denticulatum, E. striatum, E. gelatinae, Gracilaria verrucosa, Laminaria japonica, Undaria pinnatifida, Macrocystis pyrifera, Monostroma grevillei, Enteromorpha intestinalis and Caulerpa racemosa. Till now, only 4 major ma crop plants, viz., Prophyra spp. Eucheuma spp. LaminarŸ japonica and Undaria pinnatifida have actually been domesticated, for which the crop harvested exceeds that taken from wild populations (Tseng 1981a). In India experimental field cultivation of agarophytes, Gracilaria edulis (Raju and Thomas 1971) by the long line rope method, and of Gelidiella acerosa (Patel et al 1986; Subbaramaiah and Thomas 1989) by the bottom culture on coral stones have been developed, and methods for their field cultivation are available with the Central Salt and Ma Chemicals Research Institute. Newer cultivation techniques, such as the single rope floating raft technique (SRFT) which have been practised for Laminaria on a commercial scale (Tseng 1981a) and for Gracilaria (Li Ren-Zhi et al 1984) on a small scale, is tried with G. edulis at Krusadai Island in the gulf of Mannar. The efficacy of the SRFT and the possible crop yields obtainable by adopting this technique for G. edulis field cultivation forros the subject matter of this paper. 2. Materials and methods 2.1 Floating raft set-up In the SRFT, the main structure is a long coir rope (2"5 cm dia., 30 m long), attached to two wooden stakes with two anchor cables made of synthetic fibres, and kept afloat with bamboo/coconut (kernelless) floats. The length of the cable was 4 m, 123

2 124 K Subbaramaiah and P C Thomas which was twice the depth of the sea. Each raft is kept afloat by means of floats. The cultivation ropes also of the same diameter coir rope and 1 m in length are attached to the floating rope by hanging. A stone is attached to the lower end of the cultivation rope so as to keep it vertical without floating to the surface. Generally 10 fragments of G. edulis are inserted in each rope. The distance between two cultivation ropes is 25 cm and the distance between two rafts is about 2 m (figure I). 2.2 Experimental field observations An experimental site was selected on the southern side of Krusadai Island in the lagoon at a depth of more than 2 m. G. edulis collected from the same locality was used as seed material. Planting was done on 20 m of the cultivation ropeg on 25 August Growth in length of plants was recorded at fortnightly intervals, separately for the plants situated at the upper, middle and lower parts of the cultivation ropes. Harvesting was done once in 3-4 months, up to July 1989 by hand picking leaving behind sufficient biomass of plants for further growth. The monthly extension growth rate was calculated by using the formula, EGR = final length-initial length x 31/number of days of growth and is expressed as cm mon -I. The relative growth rate (RGR, dry wt. g-1d-1) for all harvests was calculated as RGR = In (wt/wo)/('t-'o) where Wo and wt are initial and final dry weights and 'o and 't are initial and final times, respectively. 3. Results and discussion 3.1 Growth and crop yield characteristics Fragments [4 cm long and 12 g (wet) per one cultivation rope] were planted over 20 m of the cultivation rope. Growth curves for the plants from different levels show differential pattern attaining peak length of 13.5, 15 and 24 cm in the lower, middle and upper parts before harvest in 95 days (figure 2). Mean extension growth rate showed decrease from the top to the middle and to the lower level, that is 8.3, 5.0 and 3.8(cm mon -1) respectively (table I). Crop yield also exhibited gradual [ ]Om ] Figure 1. G. edulis cultivation rail. Note that each cultivation rope has its uppr end tied to the floating ropr and its lower end tied to a weighing stone.

3 Rail cultivation of G. edulis ? Upper ; {'~./ 2,~,00,. wef /. 50 ~00 Days Figure 2. Growth curves (length cm) of G. edulis plants grown at the upper, middle and lower parts of the cultivation rope before the first harvest (95 days growth). Table I. Growth and harvest data of the experimental cultivation of G, edulis. Month November March July Total Days of growth " 312 No. of harvest Top Extension growth* Crop yield** Middle Extension growth Crop yield Lower Extension growth Crop yield Mean crop yield** Relative growth rate (dry wt. g- ' d- ') *cmmon-i; **gm -1 wet. decrease from top to the lower level (table 1); the plants from the middle and top levels giving 99 and 185% increase respectively over those of the lower level. The plants grew to maturity in 3-4 months (figure 3) after which a harvest was made, as the plants harvested 3 months after planting were known to yield good quality agar (Thomas and Krishnamurthy 1976). In the 3 harvests made in November 1988, March and July 1989, the biomass crop yield showed gradual decrease, although the relative growth rate remained almost the same g dry wt. g-~d -1 (table 1). During growth 98-t34-fold increase in biomass was obtained before harvest.

4 126 K Subbaramaiah and P C Thomas Figure 3. Growth of G. edulis plants on the cultivation rope before harvest after 95 days. In experimental cultivation of G. edulis by long line rope method, maximum length of plants up to 30-2 cm anda mean annual crop yield of 3.2 kg (wet) m- was reported (Raju and Thomas 1971). Later Krishnamurthy et al (1977) obtained a mean annual crop yield of 3.43 kg (wet) m- 1 in scale-up field cultivation of the alga. In the present study, the mean annual crop yield achieved (without reference of the depth) of 4.04 kg (wet) m- 1 is higher than the yields reported for the alga earlier, and is attributed to the raft technique employed. By the same raft cultivation technique, Li Ren-Zhi et al (1984) obtained a much higher yield of 3.3 kg m -~ (for G. verrucosa and G. sjoestedtii) in 5-6 months in the intertidal zone in Qingdao, China. The observed difference in crop yield as compared to that in the present study is probably due to species difference and variations in environmental factors such as temperature, light, nutrients, grazing, etc. The superiority of the raft method lies in growing the plants in the selected water level (Tseng 1981a). In the present study, the variation in the annual crop yield of G. edulis from kg (wet)mpoints to a possibility of obtaining greater crop yield at the top level. As maximum yield has been seen in the upper parts, itis suggested that for greater production, the growth on the cultivation topes can be equalized and maintained at maximum for the alga by tying together the two adjacent hanging topes of two adjacent rafts, so that the ropes become horizontally disposed instead of hanging vertically.

5 Raft cuhivation of G. edulis 127 Acknowledgement The authors are grateful to Prof. M facilities. M Taqui Khan for encouragement and References Doty M S 1977 Eucheuma--Current marine agronomy; in Marine plant biomass of the pacific northwest coast. A potential economic resource (ed.) R W Krauss (Oregon: Oregon State University) pp Krishnamurthy V, Raju P V and Thomas P C 1977 On augrnenting the seaweed resources of India; Seaweed Res. Util Li Ren-Zhi, Chong Ren-yi and Meng Zhao-Cai 1984 A prelimina D' study of raft cultivation of Gracilaria verrucosa and Gracilaria sjoestedtii; in XI lnternational Seaweed Symp., Netherlands, Hydrobiologia 116/ Patel J B, Gopai B V, Nagulan V R, Subbaramaiah K and Thomas P C 1986 Experimental cultivation of GelidieUa acetosa at Ervadi in India; Proc. Symp. Coastal Aquacuhure Raju P V and Thomas P C 1971 Experimental field cultivation of Gracilaria edulis (Gmel.) Silva; Bot. Mar Subbaramaiah K and Thomas P C 1989 Effect of seedling density and growth regulators on field grown red alga Gelidiella acetosa; Indian J. Mar. Sci Thomas P C and Krishnamurthy V 1976 Agar from cultured Gracilaria edulis (Gmel.) Silva; Bot. Mar Tseng C K 198ia Marine Phycoculture in China; International Seaweed Symposium (ed.) Tore Levring. pp Tseng C K 1981b Commercial cultivation; BioL Seaweeds