Harvest and Processing


Seaweeds are fast growing crops that reach harvest size relatively quickly compared to other aquacultured organisms. While shellfish may take 18-24 months to mature and finfish can take 2-3 years, seaweed grow-out is roughly 6 months, but can be shorter or longer depending on the species. In Maine, once kelp crops reach maturity, the entire farm will be harvested in a period of a few weeks. (This is different from shellfish or finfish where fast growing individuals may be harvested before slower growing ones over the span of months or even years). After seaweed is harvested, the resulting product must be processed immediately to ensure it maintains its quality.  

Similarly to all other aspects of seaweed farming, determining when and how to harvest and process a seaweed crop depends on the species you are cultivating. Generally, though, seaweed harvest begins by identifying when the organisms have reached a target marketable maturity. Then manual or mechanical methods of stripping the seaweeds from their substrates are used to harvest the crop. Seaweed processing tends to occur within hours of harvest and may be done by the farmers or by processing companies. The most common methods of primary processing include drying, blanching, or freezing. 

In this section, we will discuss kelp harvest and processing in Maine as a relevant example of seaweed harvest and processing more generally. To learn more about harvest and processing of commercially important species around the world, visit the Seaweed Insights harvest and post-harvest pages.

Preparing to harvest

During the short cold days of winter and early spring in Maine, kelp thrives and can grow from just a few millimeters to as much as two meters in length. This rapid growth results in a crop that is fully mature and ready to harvest by late April or early May. At this point, it is ideal to begin harvesting and processing the kelp to prevent biofouling from occurring as the water begins to warm.

When you are farming kelp to turn a profit, the quality of your kelp at harvest time is key to the success of your business. There are a few factors to consider when deciding exactly when and how much to harvest: 

  • The intended use of the kelp
  • The desired quality of the kelp 
  • The requirements of the processor or contract

The intended use for your kelp crop and the desired quality of the kelp go hand in hand. For example, kelp blades that are destined for high end restaurant markets are often a full blade of a certain size, have few blemishes, and no epiphytes. This means that as a farmer, you will need to wait until the kelp reaches the desired size in order to harvest, but you cannot wait too long since the kelp will begin to deteriorate or collect epiphytes. With these constraints, you may have to selectively harvest kelp throughout the end of the season. Alternatively, kelp used for value-added food products will undergo significant processing (blanching, cutting, etc.) so the size and quality of the blades is less critical, and kelp harvest can happen in larger volumes, but  before the blades begin to grow epiphytes that would make them unsuitable for food use.

Generally speaking, the intended use of kelp post-harvest impacts how you harvest the crop in a couple ways. 

  1. First, as kelp blades lengthen later in the season, the quality of the seaweed begins to degrade as rising water temperatures bring increased biofouling pressure. Therefore, kelp destined for the highest food-grade markets has a window of time for harvest before it is no longer usable for its intended purpose and may be considered for lower-value markets like fertilizer or animal feed.
  2. Second, if you are growing kelp for high-grade food markets that require blades of a specific size and quality, you may decide to trim kelp periodically throughout the harvest season to optimize production. This targeted harvest may not be necessary for kelp destined for low-value markets.

When it comes to quality of the kelp, the most significant considerations are biofouling resulting in blemishes on the blade, particularly from kelp snails (Lacuna vincta) and epiphytes. In addition to grazing the kelp blades, kelp snails lay small, donut-shaped egg masses on the kelp blades that are difficult to remove and, in abundance, make the kelp unsuitable for food use. Biofouling tends to happen late during the grow-out period when the kelp is large and the water begins to warm. The best way to prevent the impacts of epiphytes on your crop is to harvest your crop before epiphytes, especially kelp snails and bryozoans, begin to proliferate. Farms will need to monitor the water temperature of the site, especially nearing the harvest season. Kelp should be harvested once temperatures reach approximately 50° F, or earlier if visual signs of biofouling (snail eggs, bryozoans) are increasingly apparent.

Blemishes and deterioration of kelp blades can also result from high growth density and the kelp blades reaching and passing maturity– both things that occur late in the farming season. Intentionally thinning crops and harvesting in a timely manner are methods that can be used to help mitigate blemishes or deterioration in your kelp crop.

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A final important consideration for determining when in the season to harvest your kelp crop comes down to constraints of the seaweed processor. Every hour that your seaweed spends out of the water before being processed decreases the quality and impacts the safety of your crop to consumers, particularly when the seaweed is destined for human consumption. 

As a farmer, you may decide to process your own crops or you may choose to sell your seaweed to another company for processing. If you process your own seaweed, it will be up to you to determine your capacity to process based on space and time constraints. If you are working with a secondary processor for your kelp, it is key to coordinate your harvest with them so they have the time, personnel, and space to process your product. Transporting your seaweed to a separate processor will also require coordination around pickup/dropoff timing, or they may handle these logistics as part of a contract. 

Once you have a general idea of when to harvest your kelp farm and what processing constraints exist, the final step in preparing to harvest is to look at weather and tides. As with any ocean industry, at the end of the day, natural forces will determine if it is possible for you to get out to your farm.

Wind and weather

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Tide cycle

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It’s April and you’ve finally reached the moment of truth: your baby kelp is all grown up, the weather is ideal for harvest, and you’ve got a mid-morning low tide. The first step in actually harvesting the kelp is to check and make sure you’ve got all the necessary equipment and have a well thought-out harvest plan in advance of this weather window. For most seaweed farms, basic harvest equipment includes:

Aquaculture Production and Employment by Region

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Total Production:

  • 2,354.3 thousand mt
  • 2,250.2 thousand mt aquatic animals
  • 104.1 thousand mt algae

Percent of Global Aquaculture Production: 1.92%

People Employed in Aquaculture: 634 thousand

Notable Regional Producers:

  • Egypt → 67.62% of production in Africa
  • Nigeria → 11.12% of production in Africa

AMERICAS (North and South)

Total Production:

  • 4,400.5 thousand mt
  • 4,375.2 thousand mt aquatic animals
  • 25.3 thousand mt algae

Percent of Global Aquaculture Production: 3.59%

People Employed in Aquaculture: 606 thousand

Notable Regional Producers:

  • Chile → 34.21%
  • North America → 14.08%


Total Production:

  • 112,293.3 thousand mt
  • 77,277 thousand mt aquatic animals
  • 34,916.3 thousand mt algae

Percent of Global Aquaculture Production: 91.61%

People Employed in Aquaculture: 19.3 million

Notable Regional Producers:

  • China → 62.77%
  • Indonesia → 13.22%


Total Production:

  • 3,291.7 thousand mt
  • 3270.0 thousand mt aquatic animals
  • 21.8 thousand mt algae

Percent of Global Aquaculture Production: 2.69%

People Employed in Aquaculture: 94 thousand

Notable Regional Producers:

  • Norway → 45.2%


Total Production:

  • 238.6 thousand mt
  • 228.5 thousand mt aquatic animals
  • 10.1 thousand mt algae

Percent of Global Aquaculture Production: 0.19%

People Employed in Aquaculture: 10 thousand

Depending on how large your harvest is, you may have to make alterations to this gear list. For example, a site where silt gathers on the kelp blades might want a hose and equipment to power wash the kelp with seawater before putting it into harvest bags.

Farms also need to plan for the volume of their harvest and the capacity of their boat. For example, it is important to consider how much weight can go on the deck and how much work space will be required to harvest the kelp once containers onboard begin to fill up. Some farms may even need additional equipment or vessels to harvest efficiently and safely– such as harvest platforms, small barges, or powerlifting equipment.

With the correct equipment and good weather conditions, the actual business of harvesting kelp can be fairly straightforward. Longlines under tension and with good kelp growth are heavy and will need to be carefully hoisted out of the water to a height where the kelp can be easily cut. Depending on the boat and the harvest strategy, the lines may be raised over the gunnel of the boat or run through the center of the boat using specialized pulley, winch, and/or hauler set-ups. Having the long line just about head height is most efficient, and once the long-line is lifted out of the water, the kelp is cut either where the meristem joins the stipe or just above the holdfast where the stipe begins. (The location of the cut depends on whether the stipe will be used for anything during processing or if it will be removed and discarded).

After it is cut, the kelp is placed into coolers or net bags for storage until it is processed. Depending on your end buyer(s), some harvests may have designated containers for different grades of kelp – setting aside the best kelp for high-quality food use and designating other containers for kelp that will go to fertilizers or non-food use. Once you have harvested one section of the long-line, you should move your boat down the line to harvest the next section. When a day’s harvest is complete, the seaweed is brought to shore where it can be offloaded and transferred immediately to a processing facility.

A video (3:15) about the role of aquatic food systems in global food security and trends for the future. Video Credit: Food and Agriculture Organization of the United Nations. Available on YouTube: https://www.youtube.com/watch?v=ZBZ-KzryyPI


Seaweed processing refers to the steps that must be completed to stabilize freshly harvested seaweed and transform it into a product for a consumer market. Various methods of processing exist for seaweed depending on what type of seaweed is being used and what the end use of that seaweed is. Seaweed processing methods can range from letting fresh seaweed air dry to using multi-step techniques to isolate and extract seaweed biomolecules known as polysaccharides that are used in pharmaceuticals.

Generally, seaweed farmers in Maine fall into a few categories: (a) farmers that process and produce their own seaweed products to sell to consumers, (b) farmers that sell their seaweed to processing companies who process it and market it to consumers, and (c) farmers that do some combination of both A and B. Farmers that process their own seaweed are responsible for creating a shelf-stable product that meets the requirements of their target market themselves. Meanwhile, farmers that contract with processing companies coordinate with those companies to get their seaweed to the processor in a way that meets the needs of the processor’s target market. For farmers that are growing seaweed for food-grade markets, the seaweed should be kept cold after harvest through the time when processing begins.

Processing your own seaweed often requires additional time, space, equipment, funding, and permitting, and these factors are usually dependent on the type of processing and scale of your operation. Drying and blanching are two common methods used by farmers to stabilize their kelp, because they can be accomplished relatively quickly and cost effectively. Freezing, fermenting and salting are additional options that can be used to process seaweed, though some of these methods will require additional infrastructure and costs. The Byron Lab at the University of New England has produced a useful infographic that can serve as a starting place for learning more about how to handle and stabilize harvested kelp.

In all cases, farmers who plan to process their own kelp should become familiar with permitting and state regulations through the Maine Department of Agriculture Conservation & Forestry (DACF). DACF is the state agency responsible for the human consumption of kelp, and while DACF doesn’t require specific permits for the sale of raw, unprocessed kelp to buyers, they do require facility permits if you plan on processing your own and selling it – including drying, blanching, and freezing. Beyond the state level, seaweed processing operations should be familiar with how their current or planned processing activities line up with FDA food safety regulation in order to ensure compliance and food safe production.

Additional resources for farmers who would like to process their own seaweed or develop their own products are listed below.