Seaweeds are a large and diverse group of marine macrophytic algae, a primitive type of plants lacking true root, stem and leaf. It can be found in intertidal and subtidal coastal region to considerable depths of ocean, floating freely or attached to substrate.These are simple in their structural compositions because they take up nutrients into their blades or fronds directly from the seawater. India possesses 434 species of red seaweeds, 194 species of brown seaweeds and 216 species of green seaweeds.They are classified into these three groups according to their unique photosynthetic pigments, which give them their characteristic colors and unique properties. Kappaphycus alvarezii is a fast-growing species which has wide acceptance for seaweed farming in India.
Seaweeds are raw material for extraction of phycocolloids (agar, alginate and carrageenan), which have applications in the food, beverages, pharmaceutical, chemical, cosmetic and textile industries. In Kerala coast,the agar yielding seaweeds such as Gelidium, Gelidiella, Gracilaria and Plerocladia; the agaroid yielding seaweeds Hypnea and Acanthophora, the algin yielding seaweed Sargassum and edible seaweeds Ulva, Caulerpa, Enteromorpha and Porphyra occur in appreciable quantities.
Seaweeds are considered as medicinal food of the 21st century. They are high in trace elements, vitamins, and unique bioactive compounds, making them highly nutritious for people, animals and plantsand have yielded molecules for anti-HIV drugs. They also provide a strong base for growth promoters for several plants and are expected to be the major source of bio fertilizers to start organic agricultural revolution.
Since they are grown by vegetative propagation method, fragments from grown plant are used as seeding material. The seedling to be collected should be brittle, shiny and must have young branches with sharp pointed tips and there should not be any grazing or whitened thallus (which can be the indication of diseases). It is collected by leaving the basal portion for regeneration. Small wiry plant like Gelidiella acerosa which is attached firmly to substratum, is collected using a scalpel. The lengthy plants like Sargassum and Turbineria can be collected easily by hands alone. Caulerpa racemosa, Ulva lactuca and Enteromorpha compressa which grow on the intertidal and shallow water areas can be collected by hand picking. Acanthophora spicifera and Hypnea valentiae is also collected by hand picking. Gracilaria edulis grows attached to small stones, pebbles and as epiphytes on seagrass and available upto 1-2 m depth. It is slender and easy to remove from the substratum. The seed materialis collected in polythene bag or plastic bucket containing seawater. The water is changed frequently on the way for long distance transportation.After the material is brought to the shore, itis transferred to plastic bin or FRP tank containing clean seawater. In the case of seaweed farming unit, if the conditions are favourable in the field for further cultivation, the remnants of first harvest are allowed to grow further or a part of the harvested materialis used as seed material for subsequent crop.
The farming area must be well sheltered from very strong wave, current and wind. The bays, creeks, lagoons and coral reefs are suitable areas for the farming of seaweeds. The site should have a rapid water turnover, but not heavy enough to damage the farm. The ground should be stable enough to permit easy installation of stake or bamboo. The soil of the site should be clayey and not of humus sand or mud.
Water quality parameters
Single Rope Floating Raft (SRFR) method
A long polypropylene rope of 10 mm diameter is attached to 2 wooden stakes with 2 synthetic fiber anchor cables of 1-2 m and kept afloat with synthetic floats. The length of the cable is twice the depth of the water column (2-4 m). Each raft is kept afloat by means of 25-30 floats. The cultivation rope (1 m long x 6 mm diameter polypropylene) is hung with the floating rope. A stone is attached to the lower end of the cultivation rope to keep it in a vertical position. Generally, 10 fragments of Gracilaria edulis are inserted on each rope. The rope is untwisted slowly and the fragments of seed materials is inserted inside the gap and the rope has to be released to tighten the seed material. The distance between two rafts is kept at 2 m.
Tube net method
In this type of culture method seaweeds are cultured using a net tube, which is attached to a bamboo raft. The net tube is made up of 3 m length and 1 m perimeter made of 25 mm mesh size. 30 numbers of such tubes are tied at an interval of 50 cm to a bamboo raft of 15x3 m size. 50 litre water cans are used as floating material to keep the units buoyant. 25 kg of seeding material can be planted in one such unit.
Four poles of bamboo with 3 m length are fixed andthe four sides are tied using a 6 mm rope and the polypropylene seeding rope are attached to this. One segment (36 m length and 6 m breadth) constitutes 10 monoline units. Seedlings are planted at a distance of 15 cm. 40 seedlings can be planted in one monoline. The total seed requirement is 60-80 kg. HDPE fishing nets can be used for making fencing for avoiding grazing fishes and drifting away of the seaweed seeded nets. Used PET bottles can be tied on each rope for increasing the buoyancy.
Care and maintenance
The growth of seaweed is observed twice in a week by taking random measurement of their length and biomass. The hydrological and environmental parameters such as water current, transparency, turbidity, light intensity, temperature, pH, DO, salinity, phosphate, silicate, nitrite, nitrate, sedimentation, fouling organisms (epiphytes and epifauna) and predators are monitored regularly. Netting is placed around the units to avoid grazing by herbivorous organisms. Remove undesirable algae, barnacles or attached sediments by periodic cleaning. Check and tighten loose ropes. Check for any signs of diseases.
In the presence of sunlight, on the surface water, they can grow 5 times more of their original weight within a culture period of 45 days. It is recorded that a growth of 30 fold increase in yield in 63 days during post monsoon period was obtained for Kappaphycus alverizii by adopting suspended nylon hook method at Thikkodi near Calicut, Kerala.
Harvesting can be done by handpicking or using scissor/knife. The harvested seaweeds should be thoroughly washed in clean seawater to remove the sand and other foreign materials Seaweeds can be sold wet or dry. Theclean seaweed sundried for 2-3 days and with moisture content of 35-39% fetches more price. The harvested seaweed must be kept in a cool dry and wellventilated place.
In the pond culture, the seed material has to be cut into small pieces and broadcasted uniformly on the bottom of the pond. Seed material introduced on long line ropes and nets can also be cultured in the pond water at subsurface level.In pond culture, water depth has to be monitored and maintained at 30-40 cm. The depth has to be increased to 60-80 cm during summer months to prevent a significant rise in water temperature. Frequent exchange of water (50-75%) is necessary to maintain the optimum temperature of water in the ponds. Fertilization with either organic or inorganic fertilizer can be done to enhance growth. For pond culture, the site should be located near seawater sources and the bottom of the pond should be at or near zero tidal level.
Aquaculture management can be done effectively by integrating seaweeds into aquaculture systems. It is done either by stocking seaweeds along with shrimps, fishes or mussels in optimum stocking density or by recycling the water through a pond supplemented with seaweeds. Integration of Kappaphycus alverazii, the carrageenan yielding red seaweed with green mussels (Perna viridis) at Padane, Kasaragod District, Kerala produced a maximum of 20.1 fold increase in yield in 80 days and a minimum of 13.2 fold increase in yield in 40 days. Gracilaria verrucosa is an ideal seaweed for integration with shrimp in brackish water pond which reduces stress on shrimp by utilizing excess nitrogenous wastes from the system andalso results in luxuriant growth of G.verrucosa.
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