Management of the grow-out stage of Laminaria begins after transplantation of young sporophytes to culture ropes and continues until harvest of mature plants. In both northern and southern China, management of grow-out lasts between 5 and 7 months. During this time Laminaria sporophytes grow from about 15–20 cm in length, at time of transplantation, to a mature length of 2–3 m. In the grow-out period, seawater transparency, temperature and nutrient levels may show considerable variation. The growth of Laminaria causes significant changes in seawater currents and other environmental factors at the raft site.
Most factors (currents, illumination, nutrients) can be adjusted through various artificial forms of intervention during the management of grow-out. The purpose of management of growout is to utilize, adjust and improve natural conditions by using various seafarming methods to meet the needs of Laminaria growth in different stages of development. Management of grow-out is therefore very important in Laminaria seafarming. The purpose of this chapter is to examine some of these management techniques.
Because of the high investment in raft installations, Laminaria mariculture must make intensive use of facilities and seafarming regions. Close planting, as in agriculture, is necessary in order to maximize production output. However, in many ways seafarming is much more complicated than landfarming. For example, in order to obtain maximum yield the density of young sporelings attached to culture ropes must be closely related to such factors as distance between culture ropes, distance between adjacent rafts, distance between blocks of rafts, the method of raft culture being employed and general sea conditions.
Density of Kelp Plants on Culture Ropes
Kelp plants are attached to culture ropes at varying intervals. Spacing between plants on culture ropes is mainly determined by factors such as illumination, transparency and tidal currents at the raft site.
The best density of plants on culture ropes varies from one location to another. In China, culture density varies between 20 and 90 young sporelings per culture rope, the length of culture ropes varying between 2.0 and 3.5 m, depending on water transparency at different locations. High density of plants on culture ropes, however, does not necessarily yield highest biomass per culture rope at harvest. High planting density reduces illumination, nutrient flows and tidal currents, factors that greatly affect kelp growth. A balance must be established, through trial and error at any seafarming location, between intensity of production and biomass yield per culture rope. There are no definite guidelines to this complex question because of multiple factors involved at any particular seafarming site.
Density of plants on culture ropes is also determined by the time of transplantation. The “first batch” of transplanted sporelings show rapid growth and therefore require more space for grow-out on culture ropes, not only because they grow faster but also because their grow-out season is lengthened by early transplantation. The “second batch” of transplanted sporelings require less space for growth and so intervals between plants on culture ropes can be shortened. The same is true for the “third batch” of transplanted sporelings, which can be attached even more densely to culture ropes. Generally speaking, the appropriate density for sporeling batches 1, 2 and 3 is about 25–30, 30–40 and more than 40 sporelings per culture rope, respectively.
Distance between Culture Ropes
The overall objective of seafarming management is to maximize use of raft facilities while optimizing growth conditions for kelp plants. Hence the number of culture ropes per raft should be as high as possible, within given ecological limits. Too high a density of culture ropes per raft will reduce illumination, current flows and nutrient levels, thereby having adverse effects on kelp growth. On the other hand, too few culture ropes per raft, i.e. under-use of raft facilities, is a loss of potential returns on investment that could be obtained from more intensive use of expensive raft installations. Though many of the ecological problems mentioned above would be overcome by reducing planting density, this would mean uneconomic use of seafarming space and of productive materials. Again a balance must be established between (a) maximizing raft cultivation intensity to obtain highest output and (b) operating within regional environmental constraints that limit carrying capacity and yield.
Data in Table 6.1, from an experiment on density of culture ropes conducted in 1974 by the Rongcheng Mariculture Farm in Shandong Province, shows that under some circumstances increasing density of culture ropes may lead to increased yield by enhancing ecological conditions for good grow-out. Sporelings were attached to 2 m long culture ropes on October 31. Vertical cultivation (using the hanging kelp rope method) was followed by horizontal cultivation. The results show that culture ropes spaced 100 cm apart on rafts had lower biomass yields than culture ropes spaced 80 cm apart. This shows that the normally competing factors of cultivation density and environmental carrying capacity are, in this case, mutually beneficial.
At present, in China, kelp ropes are commonly spaced 100 cm apart when using the horizontal kelp rope raft culture method of grow-out in moderately strong tidal currents. Whereas kelp ropes are spaced 50–60 cm apart when using the vertical or hanging kelp rope raft culture method in more sheltered coastal waters. Thus, again, spacing of culture ropes must be determined through trial and error at any specific raft site, depending on factors such as transparency, water current and method of raft culture used.
Distance between Adjacent Rafts and Blocks of Rafts
In some coastal regions areas with good quality seawater for mariculture are becoming scarce. Therefore mariculture companies have tried to intensify their seafarming operations at these locations. Intensive use means that spacing is minimized between rafts and blocks of rafts. In some regions in China, seafarming companies have over-intensified density of rafts, blocks of rafts and numbers of culture ropes attached to each raft, with the result that density of culture surpasses the productive carrying capacity of the region. Consequently, production output drops sharply.
The problem of over-intensive production is found, for example, in the Weihai mariculture region of Shandong Province. From Table 6.2 we see that 4,200 mu of seafarming area can yield 6,000 tons of Laminaria with a production rate of 1,403.5 kg/mu. With more intensive seafarming practices yield decreases rather than increases. This is because high density raft blocks impede current flows, with resulting nutrient deficiency.
Plant Density on Culture Ropes and Raft Grow-out Methods
We have noted that the two most commonly used raft culture methods in China are: (i) hanging kelp rope raft culture, and (ii) horizontal kelp rope raft culture. Generally the latter is the chosen method because it gives plants higher and more even exposure to light. In 1962 the Shandong Mariculture Research Institute conducted an experiment comparing growth of densely planted culture ropes using vertical and horizontal culture methods (Table 6.3). Increased density of plants on horizontal culture ropes yielded a better harvest than increased density on vertical culture ropes. Using the horizontal raft culture method, 16.6% more sporeling plants could be attached to culture ropes compared with using the vertical raft culture method, without decreasing rate of growth per plant. Average fresh weight of individual kelp plants grown on horizontal culture ropes was 23.32 gm higher than for plants grown on hanging kelp ropes, with dry weight of individual kelp plants 18.2% higher. In conclusion, horizontal kelp rope raft culture allows increased density of cultivation with increased production levels. Horizontal kelp rope raft culture is the preferred method for intensifying production and for increasing output.
Experiments on the effects of water depth during grow-out have shown that young sporophytes grow best when attached at higher levels on culture ropes. One experiment compared growth of young sporophytes at depths of 60, 90, 120 and 150 cm. At harvest, on May 30, lengths of plants were compared (Table 6.4).
Table 6.4. Comparison of growth of kelp plants at different depths (grow-out period Dec. 2 to May 30). (SR#32)
Table 6.4 shows that kelp plants grown at a depth of 60 cm had lengths 12%, 17% and 28% greater than plants grown at 90, 120 and 150 cm, respectively. In another experiment, daily growth rates for kelp plants at depths of 60, 90, 120 and 150 cm were compared. Fresh weight samples were taken at about two week intervals from April 1 until harvest on June 7. Results of the experiment (Table 6.5) show that average rate of daily growth of kelp plants at the shallower depth of 60 cm is much faster than for kelp plants grown at deeper water levels. Blade thickening is also greater for plants grown at the shallower level. All plants began to lose fresh weight when water temperatures rose to 14o C on May 17, but there was less fresh weight loss between May 17 and June 7 (in the temperature range of 14 to 18.30 C) for those plants cultured at 60 cm depth compared with those grown at 90, 120 and 150 cm depth. Two conclusions may be reached from the experimental data: (i) the greater the water depth at which young sporophytes are cultured, the slower will be the rate of increase in fresh weight over the growing season, and (ii) the greater the water depth at which kelp plants are cultured, the faster will be loss of fresh weight when water temperatures rise in summer months.
Table 6.6 shows data from yet another experiment comparing Laminaria growth and product quality at different depths. The data indicate that shallower depths (20 to 40 cm) yield higher dry weight per culture rope and that the fresh/dry weight conversion ratio is lower than for plants grown at greater depths of 60, 100 and 140 cm. Also, the proportion of Class 1 product is greater from kelp grown at shallower depths.
Shallower water depth stimulates kelp growth because of enhanced light conditions which enable increased photosynthesis and therefore increased production of photosynthates. Light intensity, in turn, depends on water transparency and water currents. To maximize production yield, therefore, it is necessary to adjust the depth of culture ropes so that kelp plants receive optimum illumination. Various adjustments to the depth of kelp ropes should be made during the different stages of kelp culture.
Adjusting Water Depth to Decrease Illumination Immediately After Transplantation
Since sporeling holdfasts are photophobic, therefore water depth should be increased for a period of time immediately following transplantation so that holdfasts can firmly attach to culture ropes. In northern China, culture ropes are suspended at a depth of 80 to 120 cm following transplantation. In southern China, where waters are more turbid, culture ropes are suspended at 60 to 80 cm depth following transplantation. A shallower post-transplantation depth of 30 to 40 cm may be used in highly turbid seawaters.
Also, during the period following transplantation kelp plant density may be temporarily increased to reduce illumination. The density of culture ropes on certain rafts may be increased and, after holdfasts have attached, the dense kelp ropes may then be dispersed to other rafts for final grow-out.
Thinning the Density of Culture Ropes and Adjusting Water Depth to Decrease Illumination
After young sporelings have been transplanted to culture ropes, the culture ropes may be suspended from floating raft ropes in greater density than normal. This reduces light intensity. Then, as the plants grow, the culture ropes must be thinned to give greater illumination to plants. Some of the culture ropes are removed and shifted to other raft facilities. Appropriate timing for thinning depends on many factors, such as seawater temperature, water transparency, time of transplantation and sporeling density on culture ropes.
In general, when sporelings with well-developed fronds reach a length of 120 cm, the thinning and dispersing of culture ropes should begin. Culture ropes with largest kelp plants (those transplanted earliest) should be dispersed first. The culture ropes are gathered taking care not to damage fronds and holdfasts. Plants must be kept moist and shaded from direct light during this operation. Thinning exposes plants to a sudden increase in light intensity. All kelp ropes should at first be lowered to a depth of between 80–100 cm in order to reduce the sudden increase in light intensity that would otherwise occur because of thinning of plant density.
Adjusting Water Depth to Increase Illumination in the Middle Period of Grow-out
Typically, the growth of kelp on rafts gradually reduces illumination and blocks water currents. In order to increase illumination, culture ropes must be raised during the middle grow-out period. In southern China, where illumination is an especial consideration, water depth for middle grow-out is usually adjusted to between 40 and 60 cm, or even shallower, to between 20 and 30 cm, in muddier waters.
Adjusting Water Depth to Give Even Illumination by Reversing Culture Ropes
Another effective way of redistributing exposure of plants to light, especially during the middle period of grow-out, is by reversing the culture ropes. Immediately following transplantation, young sporelings grow faster on the lower ends of culture ropes because of their negative response to light. As growth progresses, however, the depth most favourable to rapid growth gradually shifts to the middle and upper parts of the culture ropes which receive higher illumination. This results in uneven growth between plants at the top and bottom ends of culture ropes. When the colour of plants on the lower ends of kelp ropes shows signs of fading (changing from dark brown to light yellow-brown), the culture ropes should be reversed. Or if uneven growth occur, where plants nearer the surface are observed to be larger than those on the lower ends of the culture ropes, again reversal of culture ropes is recommended.
A number of reversals of culture ropes may be needed during the entire grow-out period, depending on raft culture methods being used. In vertical or hanging kelp rope cultivation, culture ropes should be reversed 4 to 6 times during grow-out. For the combination vertical-horizontal raft culture method, culture ropes should also be reversed about 4 times, 2 reversals during the vertical culture stage and 2 during the horizontal culture stage. Reversal of culture ropes is generally not required when using the horizontal kelp rope raft culture method in moderately strong tidal currents.
Adjusting Water Depth by Using Different Raft Culture Methods
Raft culture methods, described in detail in Chapter V, are very important for the management of grow-out. In deeper waters where there are moderate but not overly strong tidal currents, the most effective grow-out system is the horizontal kelp rope raft culture method. Management of this grow-out method requires adjustments to alter levels of illumination in different grow-out stages. Adjustments are made by raising or lowering culture ropes, changing the lengths of connecting ropes, and increasing or decreasing the buoyancy of the floating raft ropes. In moderate currents transparency is usually stable. Fast flowing water lifts and spreads the middle and upper parts of fronds, giving even illumination to all plants, while stipes and holdfasts receive less illumination. These conditions best simulate Laminaria's natural ecological requirements. Reversal of culture ropes is not necessary when using this raft culture method.
In waters with slower currents, hanging or vertical kelp rope raft culture may be used initially. The vertical method is especially useful during intermediate culture where increased density of culture ropes is desired to provide shading. At the end of intermediate culture, plants can be redistributed on horizontal kelp rope rafts. Or vertical and horizontal methods can be combined. First the vertical hanging method is used, providing shading in the initial period of grow-out after transplantation. When plants reach a length of 1.2 to 1.5 m and shading becomes a problem, the hanging kelp ropes can be changed into horizontal position by joining pairs of culture ropes between adjacent rafts with connecting ropes. The change to a horizontal position increases illumination suddenly, a shock which must be buffered somewhat. Therefore the horizontally positioned kelp ropes should at first be suspended at an angle of 15–200 with the surface, then gradually lifted by shortening the connecting ropes.
Adjusting Water Depth to Increase Illumination in Late Grow-out
In the late period of grow-out, the rate of daily growth of Laminaria, both in length and in breadth, gradually slows and then ceases. However increase in biomass (both fresh weight and dry weight) continues, with significant accumulation of photosynthates occurring in this period. Thus it is very important that plants be given sufficient light for photosynthesis during the late grow-out period. We have seen, already, that plants at shallower levels tend to accumulate dry weight biomass more quickly than plants at deeper water levels. Raising kelp ropes to a shallower level by adjusting connecting ropes, blade tip cutting and interval harvesting - these are ways of improving illumination in late grow-out.
Table 6.7 reconfirms evidence presented in tables 6.3 to 6.5. Results are from an experiment correlating fresh/dry weight conversion ratios for plants grown at different depths (20, 100 and 200 cm) with water temperatures. Warming seawater in late grow-out causes loss of blade tips and frond length, but also stimulates the production of photosynthates (the storage products resulting from photosynthesis, such as mannitol and alginate) and thus decreases the fresh/dry weight conversion ratio. As in other experiments, the data also show that plants at shallower depths, with best illumination, have lowest fresh/dry weight conversion ratios, which means that, for these plants, photosynthesis stimulates production of the highest quantity of photosynthates in proportion to water content. Thus the object of farm management in late grow-out of Laminaria is to provide conditions which evenly distribute high light exposure for kelp plants. When kelp growth in length has stopped, the culture ropes should be raised closer to the surface to maximize illumination during this critical late grow-out period when biomass accumulation is so important.
Adjustments to the kelp culture cycle are made in response to many interrelated factors at the raft site: stage of kelp growth, current flow, turbidity, water temperature. These, in turn, affect illumination, which is the fundamental factor that must be controlled by using some of the seafarming management methods described in this chapter. The factors mentioned also determine which raft culture method is best suited to the specific ecological conditions at the plantation site.
In northern China, many deeper coastal water areas with moderate currents typically have an annual mean nitrogen/nitrate level of about 20 mg/m3, rising to 30–40 mg/m3 or even higher in some months. Some areas with moderate current action have poor nitrogen/nitrate levels of only 7–8 mg/m3 but, because of strong current flows which result in rapid gaseous exchange, are still able to support normal Laminaria seafarming with production of up to 30 tons/ha.
Thus fertilizer requirements depend on: (i) Laminaria's minimum nutirent demand level of about 20 mg of nitrogen/nitrates per cubic metre, and (ii) seawater conditions affecting the rate of distribution of nitrogen/nitrates, such as current velocity, temperature and water transparency. Research conducted by the Shandong Mariculture Research Institute has shown that a level of dissolved nitrogen/nitrates of 6–10 mg/m3, under conditions where currents promote rapid water exchange, can support normal development of Laminaria with high production yields. In general, however, the higher the level of dissolved nitrogen/nitrates in seawater the better will be the growth of Laminaria. In areas with deficient levels of nitrogen/nitrates (below 20 mg/m3) or where current action is insufficient to distribute lower levels of dissolved nitrogen/nitrates, fertilization must be undertaken.
Types of Fertilizers Used
Inorganic nitrogen-based fertilizers are used because they dissolve quickly in seawater. Table 6.8 shows some of the main types of nitrogen-based fertilizers used in Laminaria seafarming in China, listing nitrogen content, descriptive properties and precautions for use of each type.
Application of Fertilizer
There are six main stages during grow-out of the Laminaria sporophyte: (i) the young sporeling period, (ii) the young sporophyte period, (iii) the tender sporophyte period, (iv) the robust sporophyte period, (v) the mature thickening sporophyte period and (vi) the mature decaying period. Laminaria plants require different amounts of fertilizer in each of these growth stages.
In the nutrient deficient seawaters of northern China, the total annual amount of fertilizer required for the entire Laminaria growth cycle is about 2,250 kg/ha. Of this, 15–20% of the total annual fertilizer applied, or about 450 kg/ha, is required for the very rapid growth which takes place in the intermediate culture stage. During the young sporophyte period, which lasts from the time of transplantation to late February, 35% of total fertilizer applied, or about 787.5 kg/ha, is required to promote growth and accelerate formation of flat fronds. During the tender sporophyte period of growth, which occurs in March and April, another 35% of total fertilizer applied is required. In this stage of grow-out photosynthesis is very active with high accumulation of photosynthates. Finally, in the mature thickening sporophyte period, 10–15% of total fertilizer applied is used to promote blade thickening, to prevent diseases that occur as summer seawater temperatures rise and to delay yellowing of blade edges in late maturity.
Table 6.9 gives results from an experiment comparing blade tip deterioration and weight reduction between fertile waters containing adequate levels of dissolved nitrogen/nitrates and infertile nitrogen-deficient waters. The results show that there is less blade tip and fresh weight loss per kelp rope for plants cultivated in the more fertile seawater. Also, the colour of kelp fronds is much healthier for plants grown in the more fertile seawater, indicating less biomass and blade tip deterioration. These results suggest that application of fertilizer in the late period of Laminaria grow-out may prevent early yellowing and fraying of blade edges.
Furthermore, the production of sporangial sori in the late grow-out period is directly related to nutrient levels in seawater. Mature Laminaria plants grown in fertile waters not only have larger blades but also produce proportionally larger areas of sproangial sori on their blades. Whereas mature plants grown in nutrient deficient waters produce small and dispersed patches of sporangial sori on their smaller blades. Fertilization in the end stages of grow-out produces larger blades with a larger area of sporangial sori per blade area. Fertilization is especially important for producing healthy parent Laminaria from which zoospores can be collected. Fewer parent Laminaria plants grown in fertile waters are needed for collecting zoospores, because the blade size and the area covered with sporangial sori is much greater than for parent plants grown in infertile waters.
In Chapter I we briefly introduced various methods used for fertilization in Laminaria seafarming: (i) porous clay bottles, (ii) porous plastic bags, (iii) soaking of young sporelings, (iv) splashing or sprinkling, (v) spraying, and (vi) natural fertilization through polyculture. The following information provides additional details on these fertilization methods.
Fertilization Using Plastic Bags
Nitrogen-based fertilizer is placed in 20 × 12 cm plastic bags. Two small pin-holes are punched in the bottoms of the bottoms of the plastic bags to allow slow diffusion of the fertilizer into the seawater. Each bag should contain only an amount of fertilizer that will be completely dissolved and diffused into the seawater within 3–4 days. Otherwise there will be large fluctuations in fertilizer concentrations in the seawater and high waste of expensive fertilizer. To prevent uneven diffusion and fluctuation in fertilizer, concentration levels, the plastic bags should be filled with small amounts of fertilizer and constantly changed at intervals of a few days. Each day some of the bags should be replenished with fertilizer so that overall concentration and diffusion of fertilizer in the seawater is maintained at constant levels. The plastic bags should be tied to the tops of culture ropes so that diffusion of fertilizer takes place directly into the seawater environment where kelp plants are growing.
Fertilization Using the Splashing Method
The amount of fertilizer to be applied each day can be calculated from the total amount of fertilizer that should be used during each growth stage. The technology is simple. A boat carries tanks or barrels containing fertilizer in solution to the raft site and and pails are used to splash the fertilizer solution evenly over surface of the growing-area. The process is repeated several times each day, using a low concentration fertilizer solution. If the fertilizer concentration is increased and the number of times that splashing is done each day is reduced, there may be high loss of fertilizer.
The splashing process is labour intensive. Frequency of splashing, though optimum when high and using low fertilizer concentration, will depend on costs of labour and materials involved. An excellent procedure for fertilizer application is to combine use of plastic bags with splashing, apportioning the required amount of fertilizer each day between these two methods.
Fertilization Using the Spraying Method
This method is commonly used for large scale fertilizer application by seafarming companies. Fertilizer is sprayed over the seafarming area, in low concentrations several times each day, using motor boats equipped with tanks for holding the fertilizer solution, powerful pumps and spray hoses with high-pressure nozzles. Spraying can be carried out during rough seas and bad weather. High equipment costs can be distributed over several seafarming operations. This method is very efficient for fertilizing large areas in a short time at relatively low cost per area covered.
Important Points to Remember
Fertilization should be carried out frequently and comprehensively. Care must be taken to distribute fertilizer to all parts of the growing area, including outer, middle and inner areas.
Fertilization must take into consideration conditions at the seafarming site. For example, the direction of tidal currents will affect fertilizer distribution. Higher amounts should be applied where currents will carry the fertilizer solution over the growing area.
Whenever possible, fertilizer should be applied on clear days during calm weather, because calm seas signal low diffusion of nutrients. In seawaters which are already nutrient deficient, calm conditions mean that additional fertilization is required.
On windy days when waters are muddier the higher turbidity indicates that wave action is stirring up the sea bottom, lifting nutrient salts in the currents and thus providing adequate nutrient requirements. After heavy winds and surging seas, the natural level of dissolved nitrogen/nitrates in the seawater may be several to dozens of times higher than normal. Fertilization is then not required.
Kelp plants reach late maturity during the summer season when seawater temperatures rise. The mature blades begin to shed and deteriorate because of the high seawater temperature, with loss of biomass. This loss (Table 6.10) may amount to as much as 25–30% of total seasonal biomass production and translates into an equivalent loss at harvest. Furthermore, during late maturity the nutrient matter in kelp fronds is transported (through the trumpet cells) from the outer and upper edges of the fronds downwards to the thicker blade sections nearer the stipe. Thus accumulation of nutrient matter is greatest in the basal parts of the fronds that are farthest from the blade tips.
Because of the downward movement of photosynthates to the basal parts of the fronds, and because of the natural shedding of blade tips that occurs in late maturity, the practice of blade tip cutting has been introduced: i.e. the upper parts of kelp blades are harvested before biomass deterioration takes place, thus increasing overall kelp production by harvesting what would otherwise be lost.
In the blade tip cutting procedure between 25–40% of the blade is cut and harvested from the upper ends of kelp fronds. The cutting of blade tips is done between late March and early April in northern China. The removed blade tips, which are gathered and dried, may amount to between 12–17% of total dry weight production (Table 6.11), or as much as 6.75 dry weight tons per hectare.
Blade tip cutting helps to stimulate growth of the remaining kelp plants by improving light penetration, water flow and diffusion of nutrient salts, thereby greatly stimulating further accumulation of photosynthates and additional blade thickening. Blade tip cutting also decreases the incidence of diseases caused by rotting of fronds during natural shedding.
Though labour intensive, blade tip cutting can be a low-cost way of improving production by increasing both total yield and quality of harvested product. Experimental studies indicate that blade tip cutting has no adverse effects on the continued growth of kelp plants. In fact the reverse is true. Blade tip cutting stimulates growth, blade thickening and accumulation of photosynthates in mature kelp plants during the late grow-out period, resulting in much improved production output levels.
Timing of Blade tip Cutting
To increase yield blade tip cutting must be done at the right time of year. Table 6.11 shows results from a large scale experimental study of the relationship between time of blade tip cutting and production rate per mu.
From Table 6.11, we see that cutting of blade tips in mid-March, when water temperature is between 3.5–4.0o C, yields low dry weight per mu because blades at this time of year have a high proportion of water content in relation to accumulated nutrient matter. Poor total final harvest will result from decreased photosynthesis caused by too early blade tip cutting.
On the other hand delayed blade tip cutting, conducted in early-April when seawater temperature has risen to between 7.0– 8.0o C, also results in lower final total yield at harvest. Late cutting decreases blade thickening because over-crowding of mature fronds results in shading and poor illumination, thereby reducing photosynthesis and accumulation of photosynthates.
The best yields per mu occur when blade tip cutting is done in late March when seawater temperature is between 5–6o C. Blade tip cutting at this time yields more accumulated nutrient matter in the dried blade tips and also gives remaining fronds greater illumination for continued growth.
Experimental evidence indicates that before blade tips are cut the average fresh weight of plants per kelp culture rope, measured at the end of March, is usually about 17 kg and the average blade length per plant is about 300 cm. After cutting off 40% of blade tips, total fresh weight per kelp culture rope decreases to about 12 kg (i.e. an average of 5 kg is removed from each culture rope) and average blade length of individual kelp plants is reduced to about 180 cm (i.e. 120 cm of the blade is removed from each plant during the cutting operation). About 46 days later, around May 2, the average fresh weight per kelp rope will have increased to about 18 kg and the average length of blade per kelp plant will have increased to about 310 cm.
Blade tip Cutting Methods
It is important that blade tips are cut at the right position in order to obtain maximum increases in total harvest yield. The best cutting procedure is to remove about 40% of the distal part of the blade (the part of the blade farthest from the holdfast) in the mid-to-late grow-out period. Too much cutting will slow the growth of the remaining part of the frond, whereas too little cutting will result in deterioration of the edges of the blades and shedding or fraying of blade tips.
The best time for cutting of blade tips is in late March or early April in northern China. When blade tip cutting is done in late March, as a general rule 40% of the distal parts of blades should be cut from kelp plants having blades 3 m or more in in length, while 30% should be cut from blades between 2.5 and 3.0 m in length. If cutting is done in early April or at an even later date, then less than 30% of the distal parts of blades should be removed.
Cutting of blade tips is done at sea from small boats. The cutting procedure is simple. Kelp ropes are lifted and plants are quickly bunched together with their ends in the boat. A fine polyester cord is wrapped around the blade tips to be removed.
Cutting is done with a scythe. Or the thin polyester cord may be used as the cutting tool. Removed blade tips are shipped ashore where they are dried or preserved with salt, following the usual post-harvest processing methods.
Cutting not only harvests parts of kelp plants that would otherwise be shed naturally in late maturity, but also improves illumination, current flow and nutrition levels for the continued growth of remaining fronds. Cutting is especially effective in muddy or slow-flowing seawaters where silt deposits gather on maturing blades, weighing them down, decreasing light penetration and photosynthesis and thus increasing disease and deterioration. Heavy uncut fronds also weigh on raft buoyancy, making rafts more vulnerable to potential damage caused by tidal surges or strong wave action. For all of these reasons, blade tip cutting is a very important management procedure in the late grow-out stages of Laminaria production.