There are two important procedures which follow sporeling culture in the seedling-rearing station: (i) intermediate culture and (ii) transplantation. Though taking only 15–30 days to complete, these procedures are important for determining final yield of Laminaria at harvest. Good intermediate culture procedures result in rapid growth and development of young sporophytes. Early transplantation greatly improves harvest yield. The purpose of this chapter is to look at these two procedures in detail.
(1) Intermediate culture refers to the period when young sporelings attached to sporeling ropes are taken out of the seedling-rearing station and placed in seawater at the raft site before being transplanted to kelp ropes for the final grow-out period. This stage is therefore “intermediate” between seedling-rearing and grow-out. The purpose of the intermediate culture stage is to stimulate growth of young sporophytes to a length of 10–25 cm in preparation for transplantation.
(2) Transplantation refers to the procedure of removing young sporophytes from the sporeling ropes on which they have been raised in the seedling-rearing station and “transplanting” them to thicker kelp culture ropes which are suspended from kelp rafts for final grow-out.
In the seedling-rearing station young sporophytes are cultured to a length of 3–5 cm. Beyond this length conditions become too crowded in the culture tanks, however young sporelings are too small to be transplanted to culture ropes. Therefore sporeling ropes are transferred to rafts in order to stimulate further growth under natural seawater conditions. Transfer from the seedling station to the raft site takes place between mid-October and mid-November when seawater temperature has fallen below 20° C. Intermediate culture lasts about 15–30 days, during which time young sporophytes grow very rapidly to a length of 10– 25 cm.
Selecting a good seawater region for intermediate culture is very important in order to reduce “transfer stress” and to enhance sporophyte growth. The ideal location should:
Water current seems to be the most important factor. Constantly moving seawater provides young sporophytes with uniform illumination, maintains stable water temperature and inhibits the attachment of undesirable fouling weeds. However all factors listed influence the growth of young sporophytes during intermediate culture.
The floating rope raft facilities used for the grow-out stage are also used for intermediate culture. The most common types of rafts used are: (i) the single floating rope raft, (ii) the raft block, and (iii) the one-dragon kelp rope raft.
In seedling-rearing young sporophytes are attached to a long sporeling cord which is wound around a wooden frame to form the culture mat (as described in Chapter III). The sporelings grow until they are between 2–5 cm in length. At this length they become crowded on the culture mats and their continuing growth is stunted unless the culture mats are disassembled and the young sporophytes are transferred to rafts.
When disassembling the culture mats, the sporeling cord may either be: (i) cut into shorter sporeling cords, or (ii) left uncut as one long sporeling cord. This depends on which type of raft is to be used (single, block, one-dragon) for intermediate culture.
The sporeling cord is cut into shorter sections, each section about 50 cm long, when using either single line floating rafts or raft blocks. The sporeling cord is left uncut when using the one-dragon raft culture method. The former method of sectioning the sporeling rope is generally practiced in northern China and the latter method of leaving the sporeling rope intact is practiced in various places in southern China.
Intermediate Culture: Using Single Floating Rafts
In northern China the sporeling rope is unwound from the culture mat and cut into 50 cm lengths. These shorter seedling cords are then hung vertically from a floating raft rope, spaced about 25 cm apart. A small stone weighing about 150 gm is tied to the lower end of each seedling cord. Connecting ropes about 100 cm long are used to suspend the sectioned sporeling cords from the floating raft rope.
Intermediate Culture: Using Raft Blocks
The same method of sectioning the sporeling cord into shorter lengths is used for intermediate culture on raft blocks. A raft block consists of two or more floating raft lines placed parallel to one another and spaced about 5 m apart. Raft blocks are permanent facilities. When using raft blocks the density of cultivation may be increased during intermediate culture by adding more temporary floating raft ropes between the permanently anchored floating ropes. For example, if permanently stationed parallel raft ropes are spaced 5 m apart, adding a temporary raft rope will divide this space in half, so that spacing between floating raft ropes is reduced to 2.5 m. Sectioned sporeling ropes are then suspended from the raft ropes as described above.
This increase in the density of raft ropes has several benefits: (a) it provides better shelter for young sporelings; (b) it allows more efficient application of fertilizer over a smaller area; and (c) it decreases illumination. Points (a) and (b) are self-explanatory. Point (c) perhaps needs clarifying:
In early growth stages Laminaria plants require lower levels of illumination, both for good development of fronds and for healthy growth of holdfasts. Photophobic holdfasts move away from light in order to fasten. But in artificial culture on rafts sporelings hang upside-down on the sporeling ropes, reversing the natural growing condition. Plants hang down from the holdfasts, exposing the holdfasts to intense illumination. This tends to loosen the grip of holdfasts, causing sporelings to fall off the sporeling ropes. Increasing cultivation density during intermediate culture reduces light intensity by providing additional shading, thereby preventing this problem.
Intermediate Culture: Using One-Dragon Raft Ropes
In Fujian Province, where one-dragon rafts are used for intermediate culture, sporeling ropes wound around frames to form the culture mats are much shorter, about 10 m long. These are left uncut when disassembled from the culture mats. The 10 m long sections of sporeling rope are then suspended beneath a long floating raft rope, tied at 100 cm intervals to the floating raft rope with connecting ropes. Several 10 m long sporeling ropes may be suspended beneath a 50–70 m long floating raft rope. The result has the appearance (underwater) of an undulating dragon, whence the name of this raft culture method (Fig. 5.4).
All types of rafts used for intermediate culture, whether single, block or one-dragon, should, where possible, be positioned at right angles to the tidal current. Then currents spread the plants in the direction of current flow, thereby giving uniform levels of illumination to all the growing plants. The buoyancy of the rafts must be sufficient to hold the sporeling ropes at a constant water depth where they receive adequate but not excessive illumination.
During the intermediate culture of summer sporelings it is important to keep the plants at a suitable water depth in order to accelerate their growth. If the plants are cultured at too shallow a water depth they may suffer from exposure to high light intensity. In serious cases a spot-like white rot disease, or crinkle, results from such over-exposure. On the other hand, if plants are cultured at too deep a water depth, their growth will be retarded due to insuffcient light.
Care must be taken to place plants at the proper water depth based on local water transparency and tidal current conditions. In following the hanging kelp rope raft culture method in regions where there is high water transparency, the seedling cords should initially be suspended at a water depth of about 100 cm. As the young sporophytes increase in size, seedling cords may gradually be raised to a depth of 30–40 cm. A good method is to raise the seedling cords about 20 cm at a time at intervals of 3–5 days. When following the hanging kelp rope raft culture method in regions where there is low water transparency and high turbidity, then initially the seedling cords may be suspended at 50–80 cm in depth and thereafter raised 20–30 cm at 2–3 regular intervals.
Changes in weather patterns or in tidal periods should be observed when preparing to adjust the water depth of sporeling ropes. In order to be on the safe side, sporeling ropes should be raised on days when strong winds have subsided or during spring tides. In fine weather, after several sunny and calm days, it is best to postpone the raising operation in order to avoid excessive exposure of plants to high light intensity caused by increased water transparency which, in turn, is due to reduced turbidity.
Seawaters in the Yellow Sea area of northern China are very nutrient deficient, concentrations of nitrogen and phosphorous being well below requirements for Laminaria production. To solve this problem a number of methods for fertilizer application have been devised.
It is especially important that rapidly growing sporophytes receive adequate amounts of fertilizer. This enhances growth during the intermediate culture stage and therefore makes earlier transplatation possible. Fertilization during the intermediate culture stage also greatly enhances rate of growth in the early grow-out period immediately following transplantation. Therefore fertilizer should be applied generously throughout the intermediate culture stage until transplantation operations are fully completed.
Methods for Applying Fertilizers
Fertilizer is commonly applied using porous plastic bags which are attached to the connecting ropes just above the kelp ropes. Several pin holes are made in each bag, allowing fertilizer to be secreted gradually into the culture area. A large number of plastic bags, each containing a small amount of fertilizer, should be used to ensure even distribution without too high concentration in any areas.
Additional nutrient requirements may be applied by spraying fertilizers over the culture area. This is done using boats equipped with pumps and high pressure hoses. Loss of fertilizer can be reduced by spraying on fine days when there is little wave action. Spraying should be avoided on days when there are strong winds and high turbidity.
Another method of applying fertilizer is to use the so-called “soaking method”. This method is employed when young sporophytes are observed to be thin or slow-growing. Young plants are soaked in a fertilizer solution made with dissolved ammonium nitrate [NH4NO3] or ammonium sulphate [(NH4)2SO4]. The ratio of ammonium nitrate to seawater for use in the soaking method should be about 1:400. (I.e. 1 kg of ammonium nitrate is added to 400 kgs of water.) For ammonium sulphate the ratio is 1:300. Young sporophyte plants should be soaked for about 15 minutes at a time, repeating the soaking operation at 3–5 day intervals, depending on improvement in the condition of treated plants.
Quantity of Fertilizer Application
The quantity of fertilizer applied during the intermediate culture stage varies according to levels of naturally occurring nutrient salts in different sea regions, as well as size and condition of plants. Typically, the quantity of fertilizer applied during the short intermediate culture stage should account for 15–20% of total fertilizer application over the entire grow-out period. Intensive fertilization is critical during the intermediate culture stage when plant growth rate is very rapid.
Results of a survey of fertilizer use by mariculture farms in Rongcheng County, Shandong Province, are given in Table 4.1. The results indicate a strong correlation between amounts of fertilizer applied daily during the intermediate culture stage and the growth rate of young sporophytes. The data show that high fertilizer application during the intermediate culture stage results in more rapid growth of sporophytes to a length (15–20 cm) when they are ready for transplantation. Plants that were given higher daily amounts of fertilizer grew to lengths of 20–25 cm by the end of October, making them ready for transplantation earlier. Plants given lesser amounts of fertilizer reached lengths of only 4.5–10.0 cm in the same period, making them unsuitable for transplantation. Delayed transplantation of plants that had not yet reached the desired length had to be done between early and mid-November. In addition, plants given greater amounts of fertilizer showed healthier characteristics, being more robust with thicker dark brown fronds, whereas plants given lesser amounts of fertilizer had thin, narrow fronds, yellowish brown in colour, indicative of nutrient deficiency.
In turbid areas mud may settle on the kelp blades or seedling cords, resulting in rotting of the fronds or causing attachment of undesirable weeds. Where turbidity is very high settling mud and debris may be a main factor causing high sporophyte mortality. In order to keep young sporophytes robust and disease-free during intermediate culture, they should be washed frequently by shaking the hanging kelp ropes vigorously to remove mud and debris from the kelp fronds. Care should be taken not to exert sudden forces that will dislodge the plants.
Very early in intermediate culture young sporophytes should be washed 1–2 times daily. When plants reach a length of 3 cm washing can be reduced to 2–3 times weekly. When over 5 cm no further washing is required.
When young sporophytes reach a length of 15–25 cm after a successful period of intermediate culture, they should be detached from the seedling cords, where plants have grown dense, and transplanted to thicker culture ropes. This thinning and dispersal process is equivalent to transplanting rice shoots in paddy farming.
The best rope used for transplantation is rust-coloured coir rope made of palm-husk fibres and having a diameter of 3 cm. This kind of rope is durable and corrosion resistant. Its texture is such that sporelings don't easily become detached or shaken loose. Made of naturally occurring palm fibres, coir rope doesn't contain any chemical substances that would adversely affect sporophyte growth. Culture rope is made of three strands of coir rope twisted together with a rope-twisting machine. The twisting force must be moderate, enough to resist breaking but neither too tight nor too loose. If too tight, it is difficult to insert holdfasts; if too loose, holdfasts will detach.
Recently, because supplies of palm-fibre coir rope have fallen short of demand, a substitute rope is sometimes used, made of a blend of artificial chemical (polyethylene) and natural palm fibres. Types of ropes available are composed of various blended ratios of chemical-to-palm fibres (3:7, 1:1, 7:3). Pure polyethylene fibre rope may also be used, but with caution. When using pure polyethylene rope it is important that the colour of the rope not be white or bright yellow-coloured. This is because of the photophobic nature of kelp holdfasts which become detached from light-reflecting surfaces.
Prior to the transplantation operation the kelp rope should be soaked in seawater for some time. A new rope will need longer soaking in order to be soft enough that young sporophytes can be attached. Great care must be taken to prevent dry kelp rope from absorbing water contained in the stipe surface of young sporophytes, otherwise the sporophytes will wither or require time to recover after being transferred into the sea.
The length of kelp culture ropes used depends: (i) on sea conditions and (ii) on the method of culture practised.
Short kelp culture ropes, 1.0–1.5 m in length, are used for the hanging kelp rope raft culture method. The short length allows kelp plants on upper and lower parts of the rope to grow more evenly by being exposed to more uniform illumination. Use of longer kelp culture ropes is not recommended because plants on the lower ends of the ropes will be too shaded.
Longer kelp ropes are appropriate for horizontal kelp rope raft culture, a raft culture method used in seawater with high turbidity. This is because the longer kelp ropes are suspended horizontally, thereby providing adequate and uniform levels of illumination for all plants. The length of kelp ropes is determined by spacing between adjacent parallel rafts. If rafts are spaced 5 m apart, kelp ropes should be about 2.5 m long. The lower ends of pairs of kelp ropes hanging from adjacent rafts are then joined, thus lifting them into a horizontal position between the adjacent rafts.
Longest kelp ropes are used in the one-dragon kelp rope raft culture method. There are two variations of this culture technique: (a) using an uncut kelp rope, and (b) using shorter kelp ropes joined together with connecting ropes.
(a) One version of one-dragon kelp rope raft culture uses a single length of uncut kelp culture rope, 50–70 m long, suspended beneath a long floating raft rope. The submerged kelp rope is suspended from connecting ropes. This method is simple but suffers from a major drawback. It is difficult to alter the depth of the kelp culture rope by adjusting (shortening or lengthening) the connecting ropes. When the connecting ropes are shortened, the suspended kelp rope merely folds together rather than rising nearer the surface. Thus illumination of plants on the kelp culture rope is not improved and may in fact be worsened.
(b) Another version of one-dragon kelp rope raft culture uses short lengths of sectioned kelp culture rope, each about 2 m long, tied together with connecting ropes to form a long kelp rope of alternating connecting-and-culture ropes. This length of culture rope, 50–70 m long and consisting of 20–30 short sections joined together, is then suspended beneath the long floating raft rope with connecting ropes. When the depth of the long kelp rope is adjusted by shortening or lengthening the connecting ropes, the long kelp culture rope folds somewhat, but illumination is still increased because empty spaces (where connecting ropes between culture ropes have no sporelings attached) have been left between the sections of kelp ropes. In other words, kelp plants have been thinned and culture intensity reduced by inserting intervening connecting ropes. In this way overall kelp production is increased even though the number of plants attached along the length of the suspended culture rope has been decreased.
The drop in seawater temperature during early winter months stimulates sporophyte growth (Table 1.2). Tables 4.2 and 4.3 show scheduling of transplantation of batches of sporophytes in northern and southern China times so that early transplantation takes advantage of the correlation between falling seawater temperatures and increased rate of growth during the final growout period.
In order to take advantage of the positive correlation between falling seawater temperatures and sporophyte growth, early transplantion of sporophytes is critically important. This means that successful intermediate culture is also critical, because it determines the timing of transplantation. The faster that young sporophytes grow to a size when they can be transplanted (ideally 15–25 cm long), the better, since early transplantation will mean increased yield at harvest.
In 1976 an experiment was done by the Shandong Research Institute of Mariculture to study the relation between time of transplantation (early or late) and plant growth. Results of the study are given in Table 4.4. The earliest transplantation began on 12 November while the latest transplantation began on 26 November, with an inverval of only 14 days between. But the results of the experiment unexpectedly show that the dry weight yield from plants that had been transplanted earlier is 62.2% higher.
A second study relating transplantation time (early/late) to dry weight yield at harvest in different sea regions also confirms these findings (Table 4.5). Data presented in Tables 4.4 and 4.5 indicate that, under the same conditions and with an interval difference of transplantation time of only 14 days, the yield from plants transplanted earlier was 60–85% greater. From this it can be concluded that a very effective way of increasing kelp production is through early transplantation.
After 15–20 days of intermediate culture, a small percentage of rapidly growing sporophytes will have reached a length suitable for transplantation. If intermediate culture begins in mid-October, a first “batch” of “early-term sporophytes” (10–50% of total plants) may be ready for transplantation (i.e. around 10–15 cm long) between November 1–15. A second “batch” of “middle-term sporophytes” ready for transplantation (40–70% of total plants) may be gathered between November 16–25. A third “batch” of “late term sporophytes” ready for transplantation (10– 20% of total plants) may be gathered between November 26 and December 10. Any remaining sporophytes are then transplanted between December 11–31.
Thus sporophytes are collected in “batches” for transplantation in stages. Plants of suitable length for transplantation are removed from the sporeling ropes, leaving plants that have not reached adequate length to continue growing. With successful intermediate culture optimal sizes of sporophytes ready for transplant in early, middle and late batches seem to be 15, 20 and 25 cm respectively. About 1 month is the maximum time that should be allowed for completing the transplantation of all batches of sporophytes at the close of the intermediate culture stage. The earlier that transplantation can begin, based on successful completion of intermediate culture, the higher will be the yield at harvest. Data in Table 4.6 show the relation that exists between yield at harvest and time of transplantation.
|Transplantion time||% of total plants||batch||yield at harvest (kg dry weight per kelp rope, 22 plants per rope)|
|1–15 Nov||10–50||early term||2.0–3.5|
|16–25 Nov||40–70||middle term||1.5–2.5|
|26 Nov–10 Dec||10–20||late term||1.0–2.0|
|11 Dec–31 Dec||remaining||very late term||0.5–1.0|
Two factors are at odds in determining the best time for transplantation: (i) it takes time for sporophytes in intermediate culture to reach a length suitable for transplantation, and (ii) the earlier that transplantation takes place the higher will be yield at harvest. The problem, then, is to determine an optimum correlation between length of sporophytes and time of transplantation that gives highest yield at harvest. Results of such a study conducted by the Shandong Research Institute of Mariculture are given in Table 4.7.
Results shown in Table 4.7 clearly indicate that larger sporophytes used for transplantation grow faster. Sporophytes that were 19–23 cm in length at time of transplantation grew seven times more quickly (comparing slowest increases in daily growth) than sporophytes that were 6–8 cm in length at time of transplantation. Hence optimal size of sporophytes for0 transplantation purposes is 19–23 cm. In general, the larger that sporophytes are at transplantation time, the faster will be their rate of growth in the final grow-out stage of kelp production. Thus it is not advisable to transplant smaller sporophytes earlier, even though earlier transplantation time is positively correlated with increased rate of growth during growout. The best situation is to create conditions in intermediate culture that will produce sporophytes 15–20 cm in length ready for transplantation as early as possible.
The work of transplanting sporophytes from seedling ropes to culture ropes in preparation for raft culture is carried out indoors. Indoor work avoids exposing kelp plants to intense sunlight. (And, of course, indoor working conditions are better for persons involved in this operation.) The transplantation procedure can be divided into several tasks:
Detaching Sporophytes from the Seedling Ropes
This work is done outdoors at the raft site. The task involves removing a “batch” of longer sporophytes from the seedling ropes while leaving shorter sporophytes to continue growing. This is done by lifting a sporeling rope from the seawater and gripping the upper third of a bundle of sporophytes in one hand. By shaking back and forth lightly but firmly, the rhizoids of the longer plants will come loose. Care must be taken not to loosen smaller sporophytes that are not yet long enough for transplantation. The detached sporophyte plants are placed in baskets. Care should be taken not to damage the young plants while transferring them to the indoor work room.
Attaching the Sporophytes to Culture Ropes
Sporophytes are attached to culture ropes by stretching open the strands of the twisted palm-coir culture rope and inserting the holdfast. This work is usually done manually, two persons working in a team sitting on opposite sides of a round revolving table about 100 cm in diameter. One person selects sporophytes and arranges them so that blades point toward the centre and holdfasts hang over the outer edge of the revolving table. The other person opens the twisted strands of the culture rope and quickly but carefully inserts the holdfasts between the strands. The table is turned as the work progresses. If 34 plants are attached to 2 m long culture ropes, two skilled persons can complete 300–400 culture ropes a day using this method.
A second method is to use a “vice gripping board” (Fig. 4.1), a method now widely adopted in China. This simple machine is a labour saving device operated by one person. Its purpose is to hold the culture rope steady while plants are attached to culture ropes. It consists of two boards pivoted together to form a V-shaped vice grip. Several gripping boards can be installed along a work bench. One board is fixed while the other pivots. The pivoting board is levered by a rubber spring whose force creates the vice-grip action. A culture rope is threaded through the V-end of the vice grip formed by the boards and is this held steady. With one hand the operator turns the coir rope against the direction of its twist and thus opens its strands. With the other hand the operator selects a young sporophyte and inserts its holdfast between the opened strands. Releasing torque on the rope closes the strands and the holdfast is held tightly enmeshed in the rope. A new section of kelp rope is then pulled into the vice grip and the procedure is repeated. Using this simple machine one worker can finish 400 culture ropes a day.
Fig. 4.1 Gripping board used for transplanting.
1: anchoring nail 2: fixed board 3: pivoting board 4: pivot nail 5: rubber spring 6: vice grip 7: work table 8: culture rope
Spacing of the Sporophytes on Culture Ropes
Two methods for spacing plants on culture ropes are used during transplantation operations: (i) single plant spacing, and (ii) multiple or “bunch” plant spacing.
In the former case, single plants are spaced at intervals along the kelp culture ropes. Spacing between plants varies considerably from place to place. In northern China, spacing seems to follow the general practice of allowing 11 cm between plants for “early term” (large and rapidly growing), 10 cm for “middle term” and 8 cm for “late term” sporophytes. In addition, spacing should be somewhat wider between plants in the middle section of a kelp culture rope, thus increasing illumination. Illumination levels for plants spaced more closely at the ends of kelp culture ropes can be adjusted by reversing the kelp ropes periodically during grow-out.
In the latter case, two or three plants are attached in a “bunch” at about the same position on the kelp rope, with spacing intervals of 5–6 cm between bunches of plants. Holdfasts of several young sporophytes are inserted at the same position on the kelp rope. The method is no longer used in China because of difficulty inserting multiple holdfasts at one position and, more importantly, because shading and competition between bunched plants reduces output.
Transferring Kelp Ropes to the Rafts
Kelp culture ropes with attached sporophytes are coiled in baskets that have been lined with moist straw to prevent drying. While being transferred baskets should be covered with moistened burlap sacks and culture ropes should be kept moist by spraying them from time to time. The baskets are carried to the rafts in boats together with other raft culture equipment required for suspending the culture ropes from the rafts, such as already tied stone “pendulum” weights (to be attached to the bottom of each culture rope), connecting ropes (for use in connecting kelp ropes to the floating raft ropes) and other miscellaneous equipment (bamboo pole hooks, for example) required for the transfer procedure.
Hanging the Kelp Ropes
Three common methods are used for hanging the culture ropes during the early grow-out stage.
(1) Dense hanging culture: Here the kelp ropes, weighted with stones, are suspended close together on rafts. The number of kelp ropes that will eventually be hung on 2–3 rafts is hung from one raft. When plants reach a length of 1–1.5 m they are gathered for dispersal on several rafts and are spaced at wider intervals, using either the hanging kelp rope raft method or the horizontal kelp rope raft method of grow-out.
(2) Hanging culture followed by horizontal culture: Culture ropes are suspended vertically with normal spacing. When plants reach a length of about 90 cm, hanging culture ropes between adjacent parallel rafts are joined with connecting ropes, thus changing to the horizontal kelp rope raft culture method.
(3) Direct horizontal culture: Kelp ropes are suspended in the horizontal position by joining pairs of kelp ropes between adjacent parallel rafts with connecting ropes. Direct horizontal culture is used in deepwater sea regions with strong currents.