Field cultivation of Gracilaria edulis under different stocking densities at Mirya Bandar, Ratnagiri, Maharashtra

Introduction

Seaweeds are important marine resources with considerable nutritional, industrial, pharmaceutical, and economic value. They are consumed as food and serve as raw materials for phycocolloids such as agar, carrageenan, and alginates. They also contain vitamins, minerals, essential amino acids, and bioactive compounds with applications in functional foods and pharmaceutical products (Dhargalkar and Pereira, 2005; Rajapakse and Kim, 2011) [9,23] . Their derivatives are widely used in food processing, cosmetics, toothpaste, paints, and other industries, contributing to livelihood generation and the growing blue economy. India possesses a rich diversity of marine macroalgae, including several economically important red, brown, and green seaweeds. Commercial utilization of seaweeds in India expanded during the Second World War, particularly with the development of agar production from Gracilaria species (Rao and Mantri, 2006) [24] . The Maharashtra coast, particularly Ratnagiri, supports diverse seaweed resources, including several species of Rhodophyta and Gracilaria (Redekar et al., 2007; Valanju and Jamdhade, 2014) [26,32] . Among commercially important red seaweeds, Gracilaria edulis (S. G. Gmelin) P. C. Silva, 1952 is an important agarophyte with considerable potential for cultivation. Earlier studies have demonstrated its suitability for open-water, rope, and raft-based cultivation, with favourable growth and biomass production. Cultivation of G. edulis can therefore provide an alternative source of agarophyte biomass while reducing dependence on natural stocks. Stocking density is a critical factor influencing seaweed growth and biomass production through its effects on nutrient availability, light penetration, water movement, and competition among plants. Determining an appropriate stocking density is therefore essential for maximizing productivity under specific environmental conditions. Although the Ratnagiri coast offers considerable potential for seaweed cultivation, information on the optimum stocking density of G. edulis under floating raft culture remains limited.

Hence, the present study was undertaken at Mirya Bandar, Ratnagiri, Maharashtra, to evaluate different stocking densities of G. edulis and their effects on daily growth rate, plant yield, and associated environmental parameters under floating bamboo raft cultivation. In this context, the present study was undertaken at Mirya Bandar, Ratnagiri, Maharashtra, to evaluate the effect of different stocking densities on the field cultivation of Gracilaria edulis using a floating bamboo raft method. The study aimed to determine the optimum stocking density based on daily growth rate (DGR) and plant yield and to assess the prevailing environmental parameters during the culture period. The findings are expected to contribute to the development of suitable cultivation practices for G. edulis and support its potential for sustainable seaweed production along the Maharashtra coast.

Materials and Methods

A suitable site for the cultivation of Gracilaria edulis was selected based on the site-selection criteria suggested by Prema (2013) [22] , and accordingly, Mirya Bandar, Ratnagiri coast, Maharashtra, India (17.039542° N, 73.269939° E) was selected for the present study. A floating raft measuring 1.0 × 1.0 m was constructed using locally available bamboo poles of 3.0-4.0 inch diameter and covered with commercial fishnet of 10 mm mesh size on both the upper and lower surfaces for protection. The raft was anchored at the culture site using 6 mm polypropylene rope attached to natural stones following the method described by Veeragunanathan et al. (2015) [33] . Healthy planting material of Gracilaria edulis (S. G. Gmelin) P. C. Silva, 1952 was procured from Mandapam, Tamil Nadu, packed in wet jute sacks to minimize desiccation during transportation, and acclimatized in natural seawater at the culture site for 2-3 days before stocking. The experiment was conducted for 60 days from December to February, and G. edulis was stocked at five different densities, namely 125 g line⁻¹ (T₁), 187.5 g line⁻¹ (T₂), 250 g line⁻¹ (T₃), 312.5 g line⁻¹ (T₄), and 375 g line⁻¹ (T₅). Each treatment consisted of seven 1 m monolines (n = 7), with each line considered an experimental replicate, and a spacing of 15 cm was maintained between adjacent culture lines (Image 1). The respective quantities of seaweed were uniformly attached to the culture lines according to the assigned stocking density. Growth assessment was carried out at 15 day intervals throughout the culture period by randomly collecting representative samples from each experimental line and recording their biomass using a commercial weighing balance with an accuracy of 0.5 g. The recorded biomass values were used to evaluate the growth performance of G. edulis under different stockingdensity treatments. The daily growth rate (%) was calculated using the formula given by Dawes et al (1994) [7] :

The plant yield (Y) was calculated using the formula of Doty (1986) [10] :

Environmental parameters were monitored at weekly intervals throughout the experimental period. Water temperature was measured using a GH Zeal Ltd. thermometer (London, England) with a precision of±1 °C, while pH was recorded using an Eutech EcoTestr pH pen with a precision of±0.1. Water transparency was measured using a Secchi disc of 20 cm diameter. Water samples were collected weekly for the estimation of nitrite (NO₂⁻), nitrate (NO₃⁻), phosphate (PO₄³⁻), silicate (SiO₄), dissolved oxygen (DO), and salinity following standard analytical methods. The experimental data were statistically analysed following the methods described by Snedecor and Cochran (1967) [29] and Zar (1999) [35] , using SPSS statistical software (Version 16.0).

Results and Discussion

Harvest and Field Observations Harvesting of Gracilaria edulis was carried out after 60 days of culture, following the harvesting period recommended by Kaliaperumal et al. (1990) [17] and Ganesan et al. (2011) [12] . Wenno et al. (2015) [34] reported that plant density influences nutrient uptake and growth, with higher growth generally observed during the early stages of cultivation followed by a decline with prolonged culture duration. In the present study, G. edulis was initially cultured on seven monolines; however, only four lines successfully supported the growth of the seaweed until harvest. Losses in some monolines were associated with several field-related factors, including grazing by pelagic fishes. Species of Gracilaria are consumed by several herbivorous and pelagic fishes, including siganids and rabbitfishes, which can damage the tips, thalli and consequently reduce biomass production (Lundberg and Lipkin, 1979; Kaladharan et al., 1996) [18,16] . Similar losses due to grazing and other environmental factors have been reported in earlier seaweed cultivation studies (Doty, 1973 [11] ; Trono and Azanza-Corrales, 1981 [30] ; Smith et al., 1984 [28] ; Hurtado-Ponce, 1990) [13] . In addition, the growth of epiphytic and competing seaweeds, including Enteromorpha intestinalis, Chaetomorpha antennina, Ceramium diaphanum and Padina tetrastromatica, on the bamboo rafts and protective nets adversely affected the cultured biomass. Periodic cleaning of the rafts and nets was therefore carried out to maintain water circulation and provide adequate space for G. edulis. Attachment of molluscan eggs, occurrence of juvenile crabs, wave action, and water currents also contributed to physical damage and biomass loss during the culture period.

Daily Growth Rate

The mean daily growth rate (DGR) of G. edulis cultured under five stocking densities for 60 days is presented in Figure 1. The highest DGR was recorded in T₂ (187.5 g line⁻¹), with a value of 3.75±0.04% day⁻¹, whereas the lowest was observed in T₁ (125 g line⁻¹), at 3.48±0.03% day⁻¹. One-way ANOVA indicated a significant difference in DGR among the stocking-density treatments (p<0.05). The DGR recorded in T₂ was significantly higher than those of T₁ and T₄, whereas it was statistically comparable with T₃ and T₅. The variation in growth among stocking densities may be related to differences in nutrient availability, light penetration, water movement and competition among thalli. Similar variations in stocking density and growth performance have been reported for Gracilaria species by Kaliaperumal et al. (1990) [17] and Chennubhotla et al. (1992) [6] . The DGR values obtained in the present study were greater than 2% day⁻¹ in all treatments, indicating satisfactory growth performance for cultivation. Similar growth rates above 2% day⁻¹ have been reported for Gracilaria species by Atmadja and Sulistijo (1980) [4] , Hurtado-Ponce (1990) [15] , Gerung (2000) [13] , and Ganesan et al. (2011) [12] . According to the criteria reported by Ask and Azanza (2002) [3] , Anggadiredja et al. (2008) [2] and Hendri et al. (2018) [14] , a DGR exceeding 2% day⁻¹ is considered suitable for commercial seaweed cultivation. The DGR recorded in the present study (3.48-3.75% day⁻¹) was comparable with previously reported values, including 2.61% day⁻¹ in Gracilaria (Veeragurunathan et al., 2015) [33] , 3.5% day⁻¹ in Gracilaria spp. (Dawes, 1995) [8] , 5.0% day⁻¹ in G. gracilis (Anderson et al., 1996) [1] , 3.48% day⁻¹ in G. edulis (Siraimeetan and Selvaraj, 1999) [27] , and 4.67% day⁻¹ in G. dura (Mantri et al., 2009) [19] . Higher values of 4.4%, 7.4%, and 8.96% day⁻¹ have also been reported for G. birdie (Bezzera and Morinho-Soriano, 2010) [5] , G. edulis (Ganesan et al., 2011) [12] and G. verrucosa (Padhi et al., 2011) [21] , respectively. The growth rates observed in the present study therefore fall within the range reported for cultivated Gracilaria species.

Plant Yield

The mean plant yield of G. edulis under different stocking densities, recorded at 15-day intervals over the 60-day culture period, is presented in Figure 2. The highest final plant yield was recorded in T₂ (187.5 g line⁻¹), with 179.70±9.47 g line⁻¹, while the lowest yield was recorded in T₁ (125 g line⁻¹), with 124.20±4.52 g line⁻¹. One-way ANOVA revealed a significant difference in plant yield among the treatments (p<0.05). The yield obtained in T₂ was significantly higher than those recorded in T₁ and T₄, while it was statistically comparable with T₃ and T₅. The Student Newman Keuls (SNK) test further confirmed significant differences in plant yield among the treatments. The superior performance of T₂ suggests that 187.5 g line⁻¹ provided favourable conditions for biomass accumulation under the environmental conditions prevailing at the study site. At lower stocking density, the available culture space may not have been fully utilized, whereas higher densities may have increased competition for nutrients, light, and water movement, thereby limiting biomass accumulation. Based on the four successfully harvested lines, the estimated production was 1.515 kg fresh weight m⁻². This value was higher than the 1.05 kg fresh weight m⁻² reported for Gracilaria cultivated using a raft culture method by Veeragurunathan et al. (2015) [33] , indicating the potential of floating raft cultivation for enhancing G. edulis biomass production under suitable coastal conditions. Overall, the recorded environmental conditions were favourable for the growth and biomass production of Gracilaria edulis. The observed daily growth rate exceeding 2% day⁻¹ further indicates satisfactory growth performance, consistent with the criterion reported by Hendri et al. (2018) [14] for Gracilaria species. These results suggest that the environmental conditions at Mirya Bandar were suitable for the cultivation of G. edulis during the study period. The average optimum plant yield of Gracilaria edulis was achieved at a stocking density of 187.5 g/line, producing 179.7±9.47 g, while the lowest yield was observed at 125 g/line with 124.2±4.52 g. Considering four lines, the total yield reached 1.515 kg fresh weight per square meter. These results confirm that moderate stocking density supports superior growth and productivity compared to lower densities. The findings highlight the importance of density optimization in maximizing biomass yield. Overall, G. edulis demonstrated strong potential for commercial cultivation under raft culture conditions at Ratnagiri, Maharashtra.

Conclusion

The present study demonstrated the feasibility of cultivating Gracilaria edulis using a floating bamboo raft system at Mirya Bandar, Ratnagiri, Maharashtra, during the premonsoon season. Among the five stocking densities evaluated, 187.5 g line⁻¹ (T₂) recorded the highest daily growth rate (3.75±0.04% day⁻¹) and final plant yield (179.70±9.47 g line⁻¹), indicating favourable biomass production under the prevailing environmental conditions. The recorded water quality parameters remained within suitable ranges throughout the 60-day culture period and supported satisfactory growth of G. edulis. Although some biomass losses occurred due to grazing, epiphytes, wave action, currents, and other field-related factors, the overall growth performance was satisfactory, with DGR values exceeding 2% day⁻¹ in all treatments. The estimated production of 1.515 kg fresh weight m⁻² from the successfully harvested culture lines further indicates the potential of floating raft culture for G. edulis production along the Ratnagiri coast. Therefore, a stocking density of 187.5 g line⁻¹ may be considered suitable for field cultivation of G. edulis under conditions similar to those of the present study. The findings provide useful baseline information for optimizing raft-based seaweed cultivation in the Maharashtra coastal region and support its potential for sustainable seaweed production and livelihood development.

Further studies covering different seasons, culture durations, raft configurations, and larger-scale field trials are recommended to validate and optimize production at a commercial scale.

Acknowledgement

AcknowledgementsThe author also extends their deep sense of gratitude to the Professor, Department of Aquaculture, College of Fisheries, Ratnagiri, for their valuable guidance. The first author wishes to express his heartfelt thanks to the Associate Dean and Major Advisors, College of Fisheries, Ratnagiri, for their constant support, encouragement throughout the course of the study.

References

1. Anderson RJ, Levitt GJ, Share A. Experimental investigations for the mariculture of Gracilaria in Saldanha Bay, South Africa. Journal of Applied Phycology. 1996;8(4):421-430.
2. Anggadiredja TJ, Zatnika A, Purwoto H, Istini S. Rumput laut. Jakarta: Penebar Swadaya; 2008. 147 p.
3. Ask EI, Azanza RV. Advances in cultivation technology of commercial eucheumatoid species: a review with suggestions for future research. Aquaculture. 2002;206(3-4):257-277.
4. Atmadja WS, Sulistijo. Experimental cultivation of red algal genera Eucheuma and Gracilaria in the lagoon of Pari Island, Indonesia. In: Furtado JI, editor. Tropical ecology and development, Part II. Kuala Lumpur: International Society of Tropical Ecology; 1980. p.1121-1126.
5. Bezerra AF, Marinho-Soriano E. Cultivation of the red seaweed Gracilaria birdiae (Gracilariales, Rhodophyta) in tropical waters of northeast Brazil. Biomass and Bioenergy. 2010;34(12):1813-1817.
6. Dawes CJ, Lluisma AO, Trono GC. Laboratory and field growth studies of commercial strains of Eucheuma denticulatum and Kappaphycus alvarezii in the Philippines. Journal of Applied Phycology. 1994;6:2124.
7. Dawes CP. Suspended cultivation of Gracilaria in the sea. Applied Phycology. 1995;8:310-313.
8. Dhargalkar VK, Pereira N. Seaweed: promising plant of the millennium. Science and Culture. 2005 MarApr:60-61.
9. Doty MS. Estimating returns from producing Gracilaria and Eucheuma on line farms. Monographs in Biology. 1986;4:45-62.
10. Doty MS. Farming the red seaweed Eucheuma for carrageenans. Micronesica. 1973;9:59-73.
11. Ganesan M, Sahu N, Eswaran K. Raft culture of Gracilaria edulis in open sea along the south-eastern coast of India. Aquaculture. 2011;321(1-2):145-151.
12. Gerung SG, Ohno M, Yamamoto H. Growth rates and agar properties on some species of Gracilaria Grev. from Manado, Indonesia. Bulletin of Marine Sciences and Fisheries, Kochi University. 2000;19:9-14.
13. Hendri M, Rozirwan R, Apri R, Handayani Y. Gracilaria sp. seaweed cultivation with net floating method in traditional shrimp pond in the Dungun River of Marga Sungsang Village of Banyuasin District, South Sumatera. International Journal of Marine Science. 2018;8.
14. Hurtado-Ponce AQ. Vertical rope cultivation of Gracilaria (Rhodophyta) using vegetative fragments. Botanica Marina. 1990;33:477-481.
15. Kaladharan P, Vijayakumaran K, Chennubhotla VSK. Optimization of certain physical parameters for the mariculture of Gracilaria edulis (Gmelin) Silva in Minicoy lagoon (Laccadive Archipelago). Aquaculture. 1996;139:265-270.
16. Kaliaperumal N, Chennubhotla VS, Kalimuthu S, Ramalingam JR, Muniyandi K. Environmental factors influencing the growth of Gracilaria edulis in culture. Marine Fisheries Information Service. 1990;105:10-11.
17. Lundberg B, Lipkin Y. Natural food of the herbivorous rabbitfish (Siganus spp.) in Northern Red Sea. Botanica Marina. 1979;22:173-181.
18. Mantri VA, Thakur MC, Kumar M, Reddy CRK, Jha B. The carpospore culture of industrially important red algae Gracilaria dura. Aquaculture. 2009;297(1-4):8590.
19. National Fisheries Development Board. Guidelines for seaweed cultivation. Hyderabad: National Fisheries Development Board; 2015. Available from: NFDB Guidelines for Seaweed Cultivation. Accessed 2022 Jan 1.
20. Padhi S, Swain PK, Behura SK, Baidya S, Behera SK, Panigrahy MR. Cultivation of Gracilaria verrucosa in Chilika Lake for livelihood generation in coastal areas of Orissa State. Journal of Applied Phycology. 2011;23(2):151-155.
21. Prema D. Site selection and water quality in mariculture. Cochin: Central Marine Fisheries Research Institute; 2013. p.35-43.
22. Rajapakse N, Kim SK. Nutritional and digestive health benefits of seaweed. Advances in Food and Nutrition Research. 2011;64:17-28.
23. Rao PS, Mantri VA. Indian seaweed resources and sustainable utilization: scenario at the dawn of a new century. Current Science. 2006;91:164-174.
24. Rao PSN, Gupta RK. Algae of India: a checklist of Indian marine algae (excluding diatoms and dinoflagellates). Kolkata: Botanical Survey of India; 2015. p.93.
25. Redekar PD, Sadawarte RK, Kovale SR. Composition and seasonal occurrence of marine algae (seaweeds) along Ratnagiri coast. Shirgaon, Ratnagiri, Maharashtra: College of Fisheries; 2007. p.1-3.
26. Siraimeetan P, Selvaraj M. Experimental cultivation of Gracilaria edulis in Gulf of Mannar at Tuticorin. Seaweed Research and Utilisation. 1999;21(1-2):121124.
27. Smith AH, Nichols McLachlan J. Cultivation of seamoss (Gracilaria) in St. Lucia, West Indies: a preliminary report. Hydrobiologia. 1984;116/117:249251.
28. Snedecor GW, Cochran WG. Statistical methods. 6th ed. New Delhi: Oxford and IBH Publishing Co.; 1967. 593 p.
29. Trono GC, Azanza-Corrales. The seasonal variation in the biomass and reproductive states of Gracilaria in Manila Bay. International Seaweed Symposium. 1981;10:743-748.
30. Trono CG Jr. Eucheuma and Kappaphycus: taxonomy and cultivation. Bulletin of Marine Sciences and Fisheries, Kochi University. 1992;12:51-65.
31. Valanju NM, Jamdhade VM. Study of marine macroalgal biodiversity at Kurli-Kasop beach, Ratnagiri district of Maharashtra. Bionano Frontier. 2014;7(1):172-175.
32. Veeragurunathan V, Eswaran K, Malarvizhi J, Gobalakrishnan M. Cultivation of Gracilaria dura in the open sea along the southeast coast of India. Journal of Applied Phycology. 2015;27(6):2353-2365.
33. Wenno PA, Syamsuddin R, Zainuddin EN, AmboRappe R. Cultivation of red seaweed Kappaphycus alvarezii (Doty) at different depths in South Sulawesi, Indonesia. Aquaculture, Aquarium, Conservation and Legislation: International Journal of the Bioflux Society. 2015;8(3):468-473.
34. Zar JH. Biostatistical analysis. New Delhi: Pearson Education India; 1999. p. 228-232.