Harvest of Laminaria requires sound organizational planning. It usually takes about 40 days and often requires employment of additional temporary manpower. Timing must be accurate to prevent loss of biomass as summer water temperatures rise.
If harvest is too early there will be a decline in yield and in quality of Laminaria because the firm and light brown fronds will contain too high water content. On the other hand if harvest is too late, Laminaria fronds will deteriorate and plants will be invaded by many different species of invertebrates, such as bryozoa and barnacles. Also, late harvest increases the danger of rafts being damaged by storms during the typhoon season. Furthermore, late harvest may cause problems with the drying of Laminaria because of the increased possibility of high humidity and rainy weather, thereby lowering product quality. Harvest time must be accurately timed to obtain highest yield and best quality product.
Harvest time is mainly based on the thickness of Laminaria fronds. Thickening, in turn, is directly related to the time of transplantation and to light conditions at the raft site. Early transplantation allows a longer grow-out period, resulting in good rate of blade thickening. Late transplantation delays blade thickening.
During the mature sporophyte stage, growth in length of the blades stops and the tips of blades may even deteriorate, causing blades to shorten somewhat. Even though plants become shorter in length, however, they continue thickening and thus adding biomass. Length is not a criterion for timing of harvests. The main criterion is blade thickening.
Also, fresh weight of Laminaria is not as important a criterion as dry weight, since fresh weight plants may contain high amounts of water. Blade thickening, which adds net biomass, must be measured in dry weight.
Fresh Weight : Dry Weight Ratio
Harvest should begin when blade thickening is completed. A ratio of fresh weight to dry weight of 6.5:1 is considered a good indication that blade thickening has reached an optimum point. If this ratio is set higher, for example at 7:1 or 7.5:1, anticipation of greater yields may delay harvest beyond the peak period of highest potential dry weight yield. If the ratio is set too high, blades may be quite firm (turgid) and dark green at harvest, indicating excessive water content that will result in poor dry weight yield.
Table 7.1 shows that Laminaria blades grow to greatest length about May 10. But peak time for highest dry weight yield occurs a month later, sometime after June 11. When the seawater temperature is about 15.2o C, around June 11, the ratio of fresh weight to dry weight lowers to 6.5:1, indicating a good time for harvest. The fresh:dry weight ratio is comparatively high before June 11. Around July 14, when seawater temperature is about 21° C, the fresh:dry weight ratio is lower, but production yield is decreased since fronds deteriorate, decreasing in length and in fresh weight per plant because of the warmer sea waters. In other words, harvest time should be neither too early nor too late and is determined by two factors: (a) increased thickening which results in increased dry weight yield, and (b) deterioration caused by rising summer water temperatures.
Product quality is an important consideration. Harvest on May 26, when fresh weight is at a maximum, will normally yield about 21.5% Class 1 product and 45.3% Class 3 product as a % of total yield (Table 7.1). If harvest is delayed and grow-out is allowed to continue, the proportion of Class 1 quality product will rise and Class 3 quality product will be proportionally less. Therefore it is best to harvest kelp when the fresh:dry weight ratio reaches about 6.5, which occurs about 15 days after maximum fresh weight is reached.
Yet another factor to be taken into consideration is the overall time it takes to complete harvest. Harvest of Laminaria usually takes about 40 days. Therefore advance planning is required so that the time when kelp plants are at maximum dry weight will be about midway through harvest.
Seasonal seawater temperature can also be used as a guide for deciding when to harvest. Table 7.1 shows that on May 26, when water temperature is 13.2o C, the fresh:dry weight ratio is still quite high, at 7.9:1. When the water temperature rises to 15.2o C, on June 11, the fresh:dry weight ratio decreases to 6.6:1. Evidently blade thickening doesn't occur until seawater temperature rises.
M. H. Ji (1958) has reported that Laminaria does not deposit large amounts of carbohydrates in its tissues during the rapid growing period, but only late in the grow-out season. In order to obtain higher dry weight, it is best to allow a period of time after rapid growth in length of fronds is completed before beginning harvest.
The deposit of carbohydrates seems to be correlated with rise in seawater temperature. In Table 7.2 we see that the “first crop”, harvested on May 15, with average 15—day water temperature preceding harvest of 12.5° C, had a high fresh:dry weight conversion ratio of 8.2, far exceeding the standard 6.5:1.
Only plants at the shallower depth of 60 cm approached the desired standard 6.5:1 conversion ratio. The “second crop”, harvested on May 30, with average 15-day water temperature preceding harvest of 14.12° C, reached a stage of blade thickening which produced a fresh:dry weight conversion ratio of between 6.73 and 6.26. The “third crop”, harvested on June 20, with average 15-day water temperature preceeding harvest of 19.7° C, completely reached and surpassed the conversion ratio of 6.5:1, as a result of blade thickening. Based on these experimental results, two conclusions may be reached:
In seawater with lower transparency, for example in southern China, optimum seawater temperature for Laminaria blade thickening is slightly higher. In some waters of Fujian Province, only Laminaria plants growing on upper parts of culture ropes reach the desired standard conversion ratio of 6.5:1, because plants nearer the surface receive higher illumination. Neverthess, because summer seawater temperatures rise earlier in southern China, harvest must take place even if all plants have not reached the standard ratio.
In some farming locations with high turbidity, such as in Zhejiang Province, kelp must be harvested when the seawater temperature rises above 17o C (to prevent loss of biomass) despite a high fresh:dry weight conversion ratio of 8–9:1. This very high ratio is the result of low illumination caused by high turbidity, resulting in slower blade thickening.
When thickening of mature sporophytes is completed, the blades will be soft, stout and pliable, not able to be broken by grasping or folding. (I.e. not overly turgid, a condition of the robust stage caused by high water content.) The base part of the frond gradually becomes round-shaped, instead of V-shaped, and the stipe becomes flat and develops longitudinal grooves. Fronds are a healthy dark colour and do not fade when dried in the sun.
In summary, Laminaria is generally harvested between late June and mid-July in northern China. Harvest should, if possible, be completed before the end of July. In southern China, harvest may begin in late-to-mid April and be completed before mid-June, depending on location.
Kelp plants thicken at different rates depending on their depth on culture ropes. Lower plants, which receive less illumination, thicken more slowly. If all plants on culture ropes are harvested at once, lower plants will not have reached their optimum thickness and thus potential yield will be reduced. Table 7.3 shows a significant difference in blade thickening and dry weight yield from kelp plants grown at different depths on culture ropes. Upper plants reach the critical fresh:dry weight conversion ratio earlier.
Therefore, “interval harvesting” is recommended. Interval harvesting is a selective method of harvesting where kelp plants on the upper parts of culture ropes are harvested first. Plants that were transplanted first should also be harvested first, since they reach the blade-thickening stage earliest.
This thinning process increases illumination for lower plants and thus permits their continued thickening. In selective harvesting, farmers crop only mature plants that have reached optimum desired thickness, plant by plant and rope by rope. Typically 5–6 interval harvests may give highest yield, each harvest usually taking 5–6 days to complete. About 50% of all plants should be harvested during the several interval harvests. Unharvested kelp plants left on culture ropes should be raised so that they receive more light.
Where horizontal kelp rope raft culture is employed, kelp blades may be equally thickened all along the culture ropes. In this case, interval cropping methods are still used, by selecting well-thickened plants and plants with yellow edges (indicating deterioration and loss of biomass).
There are two main kelp processing methods: drying and salting. Choice of which method to use depends on availability of manpower and weather conditions. Weather is the critical consideration. Rainy or cloudy days will delay drying and cause deterioration in product quality. In order to prevent delays in harvest, salting may be required to preserve kelp product so that it can be dried when weather conditions improve.
Selection of the Drying Site
A large drying area is required. Size of the drying area depends on the size of the seafarming operation. The drying area should be large enough so that harvest operations are not delayed by the drying process. Highest quality product is obtained from fresh drying, without salting. If the drying area is too small, then salting will have to be used, lowering product quality.
In order to keep the seaweed free of debris, drying should preferably take place on grass or gravel-covered areas. A less preferable option is drying on sandy gravel areas, which may result in adhesion of sand and other debris to the dried Laminaria fronds. This will add cleaning costs to processing and may reduce product quality.
Drying Method: Drying Fresh Laminaria
Both fresh-drying and salt-drying of Laminaria are done by laying out fronds in the sunlight on the drying-ground area. Fresh-drying is low cost, produces good quality product having dark colour, and is a simple work procedure. However, it depends on good weather conditions. If weather is rainy, salt must be added to prevent plant decay.
In northern China, harvested kelp fronds are arranged on the drying area so that all plants are parallel to one another, with holdfasts pointing in the same direction. Holdfasts are cut off either when plants are fresh or after they have dried. Kelp fronds must be placed close together in order not to waste any of the drying area. Plants should not overlap each other because this will prevent exposure to the sun and will delay drying.
During drying, plants should be turned over once, when they are semi-dry, i.e. after 3–4 hours in ordinary weather or after 2–3 hours on sunny days. In sunny weather, drying may be completed in 5–6 hours.
After drying, kelp plants should be bundled and stacked in a sheltered storage depot or other building used for storage purposes. If plants are not completely dried at the end of one day, they should be gathered for overnight storage and then rearranged on the drying area for additional sunning on the following morning.
In southern China, harvested kelp culture ropes are laid across bamboo poles so that fronds can hang in the air for drying. This method yields a clean product, free of debris, but is somewhat slower in drying plants than the method of laying plants flat on the ground. The method cannot be used in northern China where weather is not as hot for drying.
Drying Methods: Drying Salted Laminaria
Salting is done by two methods: (a) harvested plants may be soaked in a brine solution for a few minutes, or (b) harvested plants may be stored by packing them in layers with salt scattered between the layers. The salted and stored plants can then be dried after harvest is completed. Whenever possible, fresh plants should be dried first. Salted plants can then be dried as time and weather conditions permit.
When weather conditions are bad, kelp plants must be temporarily preserved. This is done be creating stacks of salted kelp plants for temporary storage until sun-drying becomes possible. Salt is scattered on the ground and a layer of kelp plants is arranged over the salted area. Holdfasts are thrown towards the centre of the stack and blades are piled up to form a rain-shedding roof which keeps plants dry. Salt is scattered between layers of plants added to the stack. Additional salt must be added to the stacks after rainy weather, when salt dissolves away. In this manner, stacks of about 50 tons of plants are made for temporary storage. When weather conditions turn fair, salted stacks of plants can be spread out for sundrying.
The problem with the salt-drying method, however, is that salting produces a lower quality product, by destroying some of the nutrient value in kelp blades. Salted fronds are not suitable for extracting iodine, mannitol and algin, which are the main products desired from processing. Also, cost of processing is increased because of the additional labour required for the salting procedures. Salted product can be processed into products for human consumption but cannot be made into lameal (Laminaria meal) for livestock fodder.
Classifying and packing are the final two procedures in Laminaria processing. Laminaria product is classified according to standards regulating the industry. The criteria used for classifying Laminaria are as folows (see Tables 7.4 and 7.5):
Fresh Dried Laminaria
Length of frond (holdfast removed) greater than 100 cm. Length of the flat-thick blade part of dried fronds is greater than 70 cm. Width of dried fronds is greater than 12.5 cm. The 100 cm long flat-thick blade area is dark brown in colour, without yellowing edges or tips, and the rest of the frond is brown or brownish-green. Yellowish patches occur on less than 25% of the frond surface excluding the flat blade area. Water content is less than 22% and impurities are less than 2%.
Length of frond (holdfast removed) greater than 80 cm. Length of the flat-thick blade part of dried fronds is greater than 50 cm, including broken thalli with flat-thick part of the blade over 50 cm. Width of dried fronds is greater than 9.5 cm. The 50 cm long flat-thick blade area is dark brown in colour, without yellowing edges or tips, and the rest of the frond is brown or brownish-green. Yellowish patches occur on less than 50% of the frond surface excluding the flat blade area. Water content is less than 22% and impurities are less than 2%
Length of frond (holdfast removed) greater than 60 cm. Length of the flat-thick blade part of dried fronds is greater than 40 cm, including broken thalli with flat-thick part of the blade over 40 cm. Width of dried fronds is greater than 6 cm. The 40 cm long flat-thick blade area is brown or yellowish-brown in colour. There may be yellowing of tips and edges. Water content is less than 22% and impurities are less than 2%.
No Class: Substandard:
Broken blades less than 40 cm long, with flat part of the blade narrower than 15 cm. Water content less than 22% and impurity levels may be as high as 2–4%.
Fresh Salted Laminaria
Length of frond (holdfast removed) greater than 100 cm. Length of the flat-thick blade part of dried fronds is greater than 70 cm., including broken thalli with flat-thick part of the blade over 70 cm. Width of dried fronds is greater than 14 cm. The 70 cm long flat-thick blade area is dark brown in colour, without yellowish edges, and the rest of the frond is brown or brownish-green. Yellowish patches must not cover more than 25% of the frond surface excluding the flat blade area. Water content is less than 32% and impurities are less than 4%.
Length of frond (holdfast removed) greater than 80 cm. Length of the flat-thick blade part of dried fronds is greater than 50 cm., including broken thalli with flat-thick part of the blade over 50 cm. Width of dried fronds is greater than 11 cm. The 50 cm long flat-thick blade area is dark brown in colour, without yellowish edges or tips, and the rest of the frond is brown or brownish-green. Yellowish patches must not cover more than 50% of the frond surface excluding the flat blade area. Water content is less than 32% and impurities are less than 4%.
Length of frond (holdfast removed) greater than 60 cm. Length of the flat-thick blade part of dried fronds is greater than 40 cm., including broken thalli with flat-thick part of the blade over 40 cm. Width of dried fronds is greater than 7 cm. The 40 cm long flat-thick blade area is dark-to-yellowish brown in colour, allowing for yellowing edges or tips. Water content is less than 32% and impurities are less than 4%.
No Class: Substandard
Broken blades less than 40 cm long, with flat part of the blade narrower than 15 cm. Water content less than 32% and impurity levels may be as high as 4–6%.
All fresh-dried and salted products are packaged. Fresh weight fronds are tied into 25 kg bundles. Salted fronds are tied into 50 kg bundles.
Generally, the standards for kelp production are as stated, with maximum water content for fresh-dried plants being 22% and maximum water content for salt-dried plants being 32%.
Thus, for fresh-dried plants, 100 kg of product is allowed to have 22 kg of water and 78 kg of net biomass. If water content is 23%, then kelp product equivalency to the standard 22% water content is calculated as follows:
That is, 100 kg of fresh-dried product having 23% water content = 98.71 kg of fresh-dried product having 22% water content. Thus 1.30 (rounded from 1.29) kg should be deducted from each 100 kg of fresh-dried product for each 1% of water content above the standard 22% allowed.
For salt-dried product, 100 kg is allowed to have 32 kg of water and 68 kg of net biomass. This means that 68 kg net biomass is found in 100 kg of salt-dried kelp product. If water content is 33%, then kelp product equivalency to the standard 32% content is calculated as follows:
That is, 100 kg of salt-dried product having 33% water content = 98.53 kg salt-dried product containing 32% water. Thus 1.47 kg should be deducted from each 100 kg of salt-dried kelp product containing 33% water. In general, 1.47 kg should be deducted from each 100 kg of salt-dried kelp product for each 1% water content above the standard 32%