The purpose of this chapter is to introduce raft culture methods. Raft culture is used during the grow-out period, i.e. the period between transplantation and harvest. The chapter covers a range of topics, including ocean site selection, materials and methods for constructing rafts, positioning of rafts and management of mariculture plantation areas.
Laminaria seafarming requires suitable ecological conditions. Selection of a seafarming plantation site depends on the following criteria:
A flat sea floor composed of mud or silt-sand is best. A harder mud, sand or sludge-covered substratum is also acceptable. Floating kelp rafts must be firmly anchored with long wooden stakes pounded into the sea floor. The sea floor must be soft enough that anchoring stakes can be driven in deeply. A hard rocky substratum is therefore not acceptable.
Suitable water depth for Laminaria seafarming depends: (i) on the length of kelp ropes used, and (ii) on the raft culture method employed. In general, sea regions should be selected where the minimum water depth during neap tides is 5 m.
In recent years Laminaria cultivation has been undertaken in deeper seawater areas of 20–30 m, where strong tidal currents stimulate Laminaria growth. Deeper waters have the disadvantage of exposing rafts to strong winds, waves and currents which, if precautions are not taken, may damage raft facilities.
Tidal Currents and Wave Action
Floating raft culture is best suited for sea areas where there are moderately but not excessively strong tidal currents and where wave action is relatively calm. The best rate of current flow for raft culture is between 25–40 m/min. At this rate of flow, nutrient exchange and dissolved O2 and CO2 are maintained at constant levels. However, tidal currents beneficial for kelp culture are often accompanied by strong wave action. In deeper water areas with strong currents or wave action, special measures must be taken to protect the floating rafts.
Water Temperature / Turbidity / Transparency
Turbidity and diaphaneity (water transparency) are inversely related. The higher the turbidity, the lower the water transparency. If turbidity fluctuates seasonally, water transparency will also exhibit seasonal variations. Seawater regions with high transparency and constant turbidity normally have good levels of illumination throughout the year, providing best conditions for photosynthesis and thus for production of commercial seaweeds.
Kelp production is best in shallow nearshore or deeper offshore regions where turbidity is relatively stable year-round and where water transparency is at least 1–3 m. Hanging kelp rope raft culture, using longer culture ropes suspended at greater depth, is practiced in regions that have constant turbidity and high transparency. Horizontal kelp rope raft culture may be practiced in areas where transparency is less than one metre, because this method enhances illumination.
Special care must be taken to avoid regions polluted by industrial wastes. Water carrying industrial wastes usually contains many harmful substances, such as sulfides, acidic or alkaline compounds and various heavy metal ions. These pollutants are seriously detrimental to Laminaria growth and kelp plants harvested from polluted regions cannot be processed into foodstuffs.
However, organic or human wastes are good for seafarming. Waste water from municipal sewage is especially good for kelp growth and should be utilized, where available, as long as it doesn't contain industrial pollutants.
Freshwater flowing into the raft site area will dilute the seawater to a certain extent, resulting in brackish water conditions. Too much runoff from inland water sources may lower the salinity and specific gravity of seawater, with adverse effects on kelp culture. A moderate volume of freshwater flowing into the seafarming area is acceptable and may enrich the seawater because it carries beneficial nutrients. Special care should be guarded against runoff that contains agricultural pesticide pollutants.
Nitrogen and phosphorous are necessary for Laminaria growth. Therefore careful analysis of nutrient salts should be undertaken during plantation site selection.
In China, levels of nutrient salts in seawater show marked seasonal variation. Higher concentrations are usually observable between November and January, whereas nutrient salts decrease in the summer months, falling to lowest levels in August.
In southern China, along the coastal regions of Fujian, Zhejiang and Jiangsu Provinces, concentrations of nutrient salts are quite adequate for kelp production. However, in northern China levels of nutrient salts vary considerably between different locations.
Experiments have shown that during the rapid grow-out period, kelp plants 1–2 m long absorb 6 mg/m3 of nitrogen (nitrate-N) per day. Therefore, in sea regions where dissolved N-nitrogen-N and N-nitrates are lower than 10 mg/m3, artificial fertilization of cultured kelp plants may be required.
Methods Used for Analysis of Nutrient Salt Levels
Nutrient levels may be analysed either by laboratory techniques or by field observations of natural biological indicators. For example, nutrient salt levels may be judged by observing the colouration of a number of different marine algal species growing in the region. Observations should be made during seasons when seaweeds are growing most vigorously, i.e. when nutrient requirements are highest.
If the fronds of Ulva lactata are a very deep dark green in colour, this means that the region is rich in nutrients. Whereas if the colour is yellowish green, the area is nutrient deficient. Similarly, if Porphyra fronds are dark purple with a healthy gloss or shine, the region is nutrient rich. Or, if Sargassum fronds are dark brown in colour, again the region is nutrient rich. When these colourations are not observed, then it can be inferred that the seawater region will definitely suffer from a deficiency of nutrients during summer months when nutrient salts fall to their lowest levels.
It is important to be able to refer, systematically, to different marine conditions that critically affect management of commercial seafarming operations. The following classification system provides standard criteria that are widely used to differentiate between three types of seawater regions:
Deeper coldwater regions with strong tidal and upwelling currents but not affected by coastal currents. During spring tides current flow is between 30–50 m/min. Water depth is more than 20 m during neap tides. Substratum is mud or sandy mud. For more than 200 days during the year water temperature is between 1–13° C. These regions have good levels of illumination. Turbidity is steady, with water transparency in the range of 1–3 m. Seawater is rich in nutrients, containing sufficient CO2 and nitrogen/nitrate levels are greater than 20 mg/m3. Kelp yield from seafarming in such regions will be between 30–37.5 tons/ha.
More inshore coldwater regions with weaker currents in areas which are affected by coastal currents. Current flow is between 10–20 m/min during spring tides and around 15 m during neap tides. About 180 days during the year seawater temperature is between 1–13° C. Transparency fluctuates between 1–5 m with wide variations in illumination. Nitrogen/nitrate levels usually between 5–10 mg/m3 throughout the growing season. Dissolved CO2 levels somewhat lower than for Type I seawater. Kelp yield from raft farming will be between 15–25 tons/ha.
Seawater regions which are less than 10 m deep during neap tides. Since these regions are not affected by coastal currents or upwelling water masses, water circulation is poor. Current flow is less than 10 m/min and, in those areas where rafts can be positioned, current flow may be only 2–5 m/min. For about 150 days during the year water temperature is between 1–13° C. These regions are seriously affected by winds that stir up particle matter, causing wide variations in turbidity. Transparency fluctuates between 0–5 m. On windy days seawater is very muddy. On calm days clarity may be high with the sea bottom being visible. In some places mud particles may be suspended in the water year-round. These areas may be seriously affected by spring and neap tides. During the former, seawater is clearer; during the latter, seawater becomes more turbid. Poor nutrient content, with nitrogen/nitrate levels less than 5 mg/m3, sometimes undetectable. Yield from kelp farming will be less than 7.5–15 tons/ha. Hence this type of seawater region is unsuitable for Laminaria seafarming.
Productivity and economic performance in Laminaria seafarming depend on very efficient utilization of plantation areas. Different management techniques employed must be appropriate to the different types of sea regions mentioned:
Utilization of Seawater Region Type I
Special measures must be taken to safeguard raft facilities because of strong tidal currents and wind forces in this type of seawater region. Floating rafts should be positioned parallel to, i.e. in the same direction as, prevailing ocean currents. Some single-line hanging kelp rope rafts, shorter in length and well-anchored, can be positioned as breakwaters against tidal forces in the more exposed areas. Longer single-line rafts can be positioned in areas inside the shorter rafts. And horizontal kelp rope rafts can be stationed within the sheltered areas created by the breakwater effect of the outer single-line rafts.
Utilization of Seawater Region Type II
Currents and wave action are more moderate, but some sheltering of inner plantation areas may still be required. Firmly anchored, shorter horizontal kelp rope rafts may be positioned around the outer periphery of the plantation area, acting as a breakwater. In inner areas, larger blocks of rafts with longer floating raft ropes can be stationed at right angles to prevailing currents. Polyculture rafts, with hanging and horizontal kelp ropes for growing mussels and kelp or scallops and kelp, may also be established in the more sheltered areas.
Floating Raft Ropes
The basic raft unit for Laminaria cultivation consists of a long floating rope buoyed with numerous floats and anchored with fixed wooden stakes at both ends (Figs. 2.1a and 2.5). The rope, itself, must be very strong and is usually made of durable, noncorrosive synthetic fibre, such as nylon, about 2.0–2.5 cm in diameter. A floating raft rope is typically 45–55 m long. In less exposed areas raft ropes may be 55–65 or even 70 m in length, whereas in very exposed regions length may be reduced to 30–45 m.
Kelp or Culture Ropes
Kelp ropes or culture ropes are the actual ropes to which Laminaria seedling plants are attached for grow-out (Figs. 2.1b and 2.2b). They are equivalent to furrows in a field or paddy into which young seeding plants are transplanted in land-based agriculture. Culture ropes are usually between 1.5–3.0 m in length and are made of strands of palm fibre coir rope. Three fibre strands are twisted together to make kelp culture rope having a diameter of about 3 cm by using a special manually driven rope-twisting machine.
Kelp sporelings are attached to culture ropes by splicing their holdfasts between the twisted strands of the kelp rope. A kelp rope 2 m long can hold about 34 young plants spaced 3–4 cm apart, actual spacing depending on growing conditions. The kelp culture ropes with attached sporelings are then hung from the floating raft ropes, spaced 50–100 cm apart and joined to the floating raft ropes with adjustable connecting ropes. Culture ropes are suspended either vertically, in “hanging kelp rope raft culture” (Fig. 2.1), or horizontally, in “horizontal kelp rope raft culture” (Fig 2.2). They are stabilized by tying stones weighing about .5 kg to their lower ends, so that they sway with a pendulum-like motion in the seawater.
Fig. 2.1 Hanging kelp rope raft culture method.
a: two floating raft lines with hanging culture ropes
b: single culture rope with maturing kelp plants
1: wooden anchor stake 2: anchor rope 3: glass ball float 4: floating raft line 5: empty connecting rope 6: top of kelp culture rope
Fig. 2.2 Horizontal kelp rope raft culture method.
a: numerous horizontal culture ropes suspended between three parallel floating raft ropes. Current direction indicated
b: pair of culture ropes hanging from parallel raft ropes and joined to form a single horizontal kelp culture rope
1: wooden anchor stake 2: anchor rope 3: glass ball float 4: floating raft line 5: empty connecting ropes joining two culture ropes hanging from parallel rafts 6: kelp plants hanging from a horizontal culture rope 7: link or knot joining culture ropes
Use of light-coloured (white or yellow) polyester rope is unsuitable for making kelp culture ropes because Laminaria holdfasts are photophobic and therefore the rope's light-reflecting surface will tend to cause them to loosen and fall off.
Connecting ropes are, as their name indicates, used to connect culture ropes to a floating raft rope. Also, in horizontal kelp rope raft culture, they are used to tie together pairs of kelp culture ropes hanging from parallel floating raft ropes. Connecting ropes are made of polyester rope about 3–4 mm in diameter. They are not used for culture purposes and hence are sometimes referred to as “empty ropes” or “empty connecting ropes”. Polyester ropes are used precisely to discourage attachment of organisms such as alien seaweeds, mussels, barnacles or tunicates.
The purpose of connecting ropes is to allow adjustments to be made to the depth of kelp culture ropes when using either in hanging kelp rope raft culture (Fig. 2.1b) or in horizontal kelp rope raft culture (Fig. 2.2b). Frequent adjustments to the depth of culture ropes are necessary to compensate for changes in water transparency and light intensity due to changing climatic conditions, as well as to adjust for changing levels of illuminaation required by plants during different stages of their growth and development. Connecting ropes are also used for such purposes as hanging net cages or mussel culture ropes to floating raft ropes in polyculture.
Cement Anchor Weights
Solid cement anchor weights are used to anchor the floating rafts. Cement weights are made of poured concrete, with a steel anchor ring (diameter 20–22 mm) imbedded for attaching anchor ropes (Fig. 2.3a). Cement anchors are cube-shaped and weigh at least 1,000 kg. They are proportioned so that the ratio of height to width is approximately 1:3, giving a low centre of gravity and thus improving their stabilizing force.
Cement anchor weights are transported to the raft site and sunk in place by using two wide-bottom wooden boats joined together with two solid crossbeams (Fig. 2.3b). A space about one metre wide is left between the two boats and between the two crossbeams. The boats are beached at low tide and cement anchor weights are fastened to the sturdy crossbars using a short lever-pole. Then, when the incoming tide lifts the boats in the water, the cement weights are floated to the raft site where they are let loose at the desired location. The procedure is repeated until all anchor weights are in place.
Fig. 2.3 Equipment for anchoring floating raft ropes.
a: poured concrete anchor weight
b: wooden boats joined with crossbeam
c: wooden anchor stakes
1: iron ring 2: shackle connector 3: anchor line 4: crossbeam joining boats 5: boat 6: anchor rope attached to crossbar
Wooden Anchor Stakes
Wooden stakes are driven into the sea floor to anchor the culture rafts. The wooden stakes are made from either softwood or hardwood (pine, willow, poplar, fir). Stakes should be about 2.0–2.5 m in length and 20 cm in diameter (Fig. 2.3c). Where there are strong currents and wave action, longer stakes should be used, since shorter stakes will be pulled up by the enormous vertical forces exerted by wave action during stormy weather.
The lower end of each stake should be tapered to a point. A deep notch should be cut around the upper end or a hole drilled about 20 cm from the top of each stake so that anchor ropes can be securely attached. Sometimes a rope ring is fashioned near the upper end so that anchoring raft ropes can be slipped easily through them. The stakes are pounded into the substratum by using a manual-powered pile driver. The pile driver is floated to the raft site on a platform built over a pair of wooden boats that have been joined together with crossbars.
Anchor lines attached to the wooden stakes should be long enough to allow for good elasticity when stretched and relaxed by wave action during stormy weather. If anchor lines are too short they will either break or submerge the floating raft rope when stretched under great tension during foul weather. On the other hand, care should be taken that anchor lines are not too loose, because this will allow too much free play of the raft ropes. Then changing currents may shift the positions of floating raft lines, causing them to intertwine and tangle with potentially serious production and equipment losses.
A good rule is to make anchor lines twice as long as water depth. If water depth is 10 m, anchor lines should be 20 m long. This will give about a 30° angle of declination between the water surface and the descending anchor line. In areas with much wave action the ratio of anchor line length to depth may be increased to 3:1. In more protected areas the ratio may be decreased to 1.5:1.
Anchor Lines and Floating Raft Lines
Anchor lines and floating raft lines are made of synthetic fibres (polyethylene, polypropylene or polyvinyl chloride) which are strong and durable, normally lasting 8–10 years. Safety of rafts is a major consideration, therefore all raft lines should be made of good quality materials having high tensile strength and with some degree of elasticity. Diameter of the floating raft ropes may vary according to conditions at the raft site. In areas where there are strong currents and wave action, rope diameter should be 1.5–2.5 cm. In more sheltered sea regions, rope with a diameter of 1.0–1.5 cm may suffice. Every rope must be tied securely to prevent their coming loose and letting rafts go adrift.
When properly installed, raft lines should be neither too tense nor too relaxed. If too tense they will break under the stress of waves and currents. If too relaxed they will twist and snag on adjacent raft lines. The best situation is where floating raft ropes are relaxed but steady, with enough elasticity to allow for some movement during spring tides.
Glass ball floats 25–30 cm in diameter are used to buoy the floating raft lines. Each glass float is wrapped tightly within a rope net which both holds the glass float securely and allows it to be easily attached to the raft ropes. Glass floats are inexpensive to manufacture and are readily available in China. However, they are heavy and their buoyancy is uneven. They break relatively easily and they frequently break loose.
Large plastic floats may be used instead. These normally have “earholes” on either side, making it easy to fasten them securely to floating raft lines. Polyester rope 0.2–0.7 mm in diameter is used to tie the plastic floats to the floating raft lines. Plastic floats are sturdy and durable and are not easily shaken loose from the raft lines. Each plastic float weighing 1.6 kg gives buoyancy of 12.5 kg. In spite of the extra cost, use of plastic floats together with strong polyester lines is generally recommended to maximize the security of expensive raft installations.
Bamboo rods may be used as secondary floatation devices (Fig. 2.5). Cut into 2 m long sections 10 cm in diameter, bamboo rod floats give evenly distributed buoyancy at water level and thus help prevent sagging of the main raft ropes. A single floating raft line about 60 m long may require about 20 such bamboo floats. Bamboo floats have the two advantages of low cost and high availability in China.
The greater the number of floats, the greater will be the wind and wave resistances against the raft facilities. In early stages of Laminaria grow-out, only 50% of the floats need to be attached to buoy the light weight of young sporophytes hanging from culture ropes. This helps lower the raft rope's resistance to wind and wave forces. As kelp plants mature, their hanging weight acts as a stabilizing balance which can be buoyed up by attaching glass and/or bamboo floats as required. Buoyancy is usually judged sufficient when plastic or glass floats are only half submerged.
Single Floating Rope Raft
Single floating rope rafts are independently positioned raft units, i.e. they are not joined to other floating raft ropes but are anchored separately (Fig. 2.4). The basic raft structure has already been described in section 4 above. Raft length is determined by environmental conditions at the raft site. Longer raft ropes are less stable and more difficult to manage in rough seas. Therefore in regions with strong tidal current or wave action the length of the floating raft must be shorter.
Because of its stability - each floating rope is anchored individually - the single floating rope raft structure is well-suited for growing kelp in more exposed outer regions of a seafarming plantation area, where currents and wave action are strongest. Positioned in the outer areas, these stable rafts act as breakwaters, protecting the inner areas of a seafarming operation.
Fig. 2.4 Single floating rope raft.
1: floating rope 2: glass ball float 3: tying rope 4: anchor rope 5: wooden anchor stake 6: connecting rope 7: young kelp sporophytes 8: hanging culture rope 9: stone weight
Fig. 2.5 Square block of floating rafts.
1: floating line 2: bamboo float 3: anchor line 4: wooden anchor stake 5: hanging kelp culture rope
Block of Floating Rafts
A block of rafts is composed of between 10–40 floating kelp rope rafts joined together (Fig. 2.5). Floating raft ropes in parallel series are positioned 3–5 m apart so that water circulation is not impeded and so that kelp plants at maturity do not tangle.
Large cement weights and very solid wooden stakes are used to anchor the main corners of a block of rafts. The main floating raft ropes are buoyed with large glass floats. Buoyancy of the main raft ropes may be increased by attaching additional floating bamboo rods. Actual buoyancy required depends on seasonal conditions (current force, wave action) at the raft site.
Main floating raft lines are joined with cross-ropes, the length of the cross-ropes determining the shape (square or rectangular) of the block. A number of parallel floating raft lines are then positioned in series within the block, their ends tied to the cross-ropes (Fig. 2.5).
A main advantage of raft blocks is that they allow adjustments to be made to many floating raft ropes at once. When main anchor ropes are shortened, tension increases the separation between all raft lines at once, thereby raising all horizontal culture ropes closer to the surface. When main anchor ropes are lengthened, relaxed tension causes raft lines to drift closer together, thereby lowering horizontal culture ropes in the seawater. In this way spacing between all parallel raft lines within a block can be varied, between 3–5 m, in one overall adjustment operation, thereby raising or lowering all culture ropes suspended from the raft lines depending on whether adjustments are intended to increase or decrease light intensity. This greatly improves efficiency of adjustment operations.
A main disadvantage of raft blocks is that they are structurally weaker and more unstable than individually anchored raft lines, since the number of anchors used to hold raft lines in place is comparatively low. Twisting of raft lines is difficult to prevent. Blocks of rafts are best located in sheltered inshore bay areas protected from strong currents, heavy winds and wave action. Blocks of raft lines are well-suited for horizontal kelp rope culture in nearshore regions where water transparency is low (Fig. 2.6).
Raft mariculture techniques are used not only for growing Laminaria but also for growing many other species of commercial seafoods in China. As early as the 1950's seafarmers began culturing Undaria using rafts. Raft seafarming of Undaria was done even earlier in Japan. Other species of commercial seaweeds, such as Gelidium and Gracillaria, have also been cultured successfully in China using raft culture methods. Presently, other sea organisms are being cultivated in this way, such as scallops, mussels and abalone. Polyculture of several species at once (Laminaria and scallops, Laminaria and mussels) is also a rapidly developing practice which uses raft culture techniques.
Orientation of Rafts
The positioning of rafts should be determined by directions of tidal currents and waves. If wave action is the dominant factor at a seafarming site then rafts should be positioned parallel to the prevailing direction of waves. Otherwise rafts should be positioned parallel to the direction of prevailing tidal currents, i.e downcurrent.
Horizontal rope raft culture has become the most popular Laminaria seafarming method in China. When using this method floating raft lines are best positioned downcurrent so that culture ropes hang at cross-currents (Fig. 2.2a). Whereas when using either single floating raft lines or a variation on this culture method called “one-dragon kelp rope raft culture”, it is best, for reasons which will be described in Chapter V, to set the rafts at cross-currents, i.e. at right angles to prevailing currents.
The basic raft unit is the single floating rope raft (Fig. 2.4). When two raft units are positioned parallel to one another 3–5 m apart they form a double raft which is the basic block unit used in Laminaria seafarming (Fig. 2.1a). Raft lines making up a block unit can be used for either “hanging kelp rope raft culture” (where culture ropes are suspended vertically from each floating raft rope, as in Figs. 2.1a and 2.4) or “horizontal kelp rope raft culture” (where pairs of culture ropes are joined between parallel floating raft ropes, as in Figs. 2.2a and 2.6).
Between 10–40 raft lines may be anchored in parallel series or joined together with cross-ropes into larger blocks of rafts to form a plantation unit. Several such plantation units may be tended in one seafarming operation, depending on the size of sea area being farmed and resources available.
Planning is required for good management of a large scale seafarming operation. Planning the layout of a raft plantation area is based on two desired criteria: (a) high yield resulting from favourable growth conditions, and (b) raft safety.
The plantation area is subdivided into raft blocks, spaced so that channels 30–40 m wide are left between the blocks. About 10–40 rafts are normally linked together in each raft block. Generally rafts are arranged in long rectangular or square blocks, care being taken to keep individual rope rafts parallel to one another but separated at 3–5 m distances. The raft block plantation method permits easy seafarming management.
It is important that the density of rafts not cause seawater stagnation. Steady current flow is essential for healthy growth of Laminaria and other sea organisms. Thus spacing of rafts is a critical consideration.
The safety of rafts is improved by stationing certain kinds of rafts (“single floating rope rafts” or “one-dragon rope rafts”) in outer more exposed areas of the seafarming plantation. In a typical plantation, 20 such individually anchored rafts may be positioned in outer exposed areas. These act as a breakwater. Another 30–40 rafts joined into blocks may be positioned in the inner more sheltered plantation areas (Fig. 2.6).
In this way the plantation is subdivided into two ecological regions: (i) exposed, and (ii) sheltered:
(i) The outer ecological area, exposed to wave action and currents, is well-suited for Laminaria raft culture. Here single and one-dragon rafts should be shorter in length and they should be positioned in the direction of prevailing currents, thereby lowering stresses that might damage raft facilities. Laminaria plants grow well in such regions where moderately strong currents provide higher and more stable levels of nutrients and where kelp blades are spread by current flows, providing even illumination.
(ii) The inner ecological area, which is more sheltered from currents and wave action by the breakwater effect of the outer rafts, is well-suited for culturing Laminaria in combination with other sea organisms (mussels, scallops, Undaria). Here Laminaria may be grown using the “horizontal kelp rope raft” method, which improves illumination for kelp plants grown under more sheltered conditions where turbidity may be higher. On the same rafts, other hanging culture ropes and net cages may be attached for growing mussels or scallops in a polyculture system.
By planning the layout of the seafarming plantation in this way, the two criteria for successful management of large-scale mariculture operations are attained. Both (a) harvest yield and (b) raft safety are maximized.
Fig. 2.6. Outlay of rafts in a seafarming plantation area.
Showing arrangement of one-dragon rafts in outer exposed areas and blocks of rafts in inshore sheltered areas.