Productivity of a seaweed farm is measured by the number of harvests a year and the biomass increase in weight (yield). Some seaweeds are harvested fully, meaning the entire biomass is removed from the site, while others are harvested partially, with some biomass remaining for further grow out. Yield is often stated in kilograms (kg) per metre (m) of line, since metric tonnes (MT) per hectare (ha) is not always a common measure among farmers.
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Harvesting only takes place a few months a year, between February and April, and is therefore an intensive and crucial period for the farmers.
In January, some farmers start partially harvesting to adjust density, however main harvesting will usually begin in February and can go on until the end of April. The Undaria blades are harvested when they reach approximately 1.5 to 2 metres in length.
In Dalian, China, farmers harvest the blades when they reach 1 metre and sometimes harvest a second time a couple of months later. Before 2010 Undaria would be harvested three to four times a year in Dalian.
The harvesting period can go on until the end of April, depending on the sea temperature and crop conditions. Since in many regions Undaria is farmed for abalone feed, farmers try to extend the harvesting period as much as possible, sometimes until May. At this point, the warming water temperatures cause the tissue of the blades to erode, but this is not problematic when the seaweed is used as abalone feed.
The eroded tissues, including lower stipes are usually cut off if the seaweed is destined for human consumption. In Iwate and Miyagi Prefectures in Japan, the long stipes are not included, while in South Korea the cutting point is just above the Sporophyll. Such differences should be taken into account when comparing yields.
In South Korea, different harvesting methods can gather between 600 and 3,000 metres per hour. Yet it is important to note that the fastest method, stripping the entire lines with a semi-automated continuous process, significantly affects the quality of the harvested seaweed.
In Japan, a fishery cooperative typically owns one large harvesting boat, where all member farmers work together during harvesting season. With this method approximately 150 metres of grow line can be harvested per hour.
In China, harvesting boats are typically smaller and manned by two people.The cutting is done manually with the support of a crane on board. One boat with two people can harvest about 200 metres of culture line per hour.
Culture lines are typically removed from site after harvest and maintained ashore.