Setting up & Tending the Farm


You’ve picked a seaweed species to cultivate and done your research on where to put your seaweed farm. You’ve applied for a license or lease to farm seaweed at your selected site and the application has been approved by the Maine Department of Marine Resources (DMR). And most recently, you’ve created a plan for obtaining your own seed for the upcoming season. With all of this taken care of a few months before grow-out begins, it’s time to get out on the water to prepare your site for its first crop of seaweed.

Setting up a kelp farm

In the world of aquaculture equipment and farm construction, seaweed farming systems tend to be on the simpler side of things. Although there are numerous designs for seaweed farms depending on the species being cultivated, the harvest method, the size of the crop, etc., seaweed farms come down to a basic design premise: seaweed grows on a suitable substrate that is suspended several feet below the surface of the water and anchored in place. Whether the substrate used is single or multiple long-lines wrapped with seed string, a net directly inoculated with spores, or a wooden raft with seed string fastened to it, the concept remains the same: create an artificial, easily seeded and easily harvestable attachment place for seaweed to grow.

The techniques and structures for farming seaweeds come directly from seaweed biology. 

As benthic organisms, most seaweeds evolved to have a frond, a stipe, and a holdfast that connects them to their substrate. Farm systems for seaweed provide an artificial substrate for seaweed to grow on.

Since seaweeds have complex life cycles, we require a controlled nursery environment to farm them. Direct or indirect inoculation of the substrate in a nursery setting is necessary for seaweed to be planted out on a farm.

Image credit: Jaclyn Robidoux, Maine Sea Grant

In Maine, the common setup for all farmed kelp is to use single long lines or an array of long lines suspended several feet under the surface, held in place by moorings or anchors on either end. It’s important to understand that there are many variations on farm designs, and no two farms will look identical, since farm design is unique to farm sites and operations. Going forward, we will provide a general overview of how to deploy a long-line kelp farm. 

Before we get too much further, let’s define what we mean by a kelp ‘long-line’: long-lines are horizontal lengths of poly line, often between 400 and 1,000 ft, with seed string wrapped around them. Each end of the horizontal longline is attached to mooring systems held in place by anchors or concrete blocks. Systems with multiple long-lines, sometimes referred to as arraysAdd a Tooltip Text can be attached to some form of spacer or configuration to maintain the shape at either end before being attached to the mooring lines. Depending on the configuration, horizontal longlines may be continuous lines or they may be broken into segments of 200 ft and attached end to end to form a single line that parallels the ocean surface. Between each 200 ft line segment (or otherwise spaced across the length of the longline) are droppers– depth maintenance systems frequently configured out of buoys, pvc pipe, and weights– that are, used to maintain the long line at an optimal depth of around 7 ft (~2 m).

Image credit: Jaclyn Robidoux, Maine Sea Grant

When you begin preparing your site to farm seaweed, phase one of the process starts with setting up the moorings. Mooring systems are used at either end of each kelp longline, and moorings should be set well in advance of the planned seeding date. The mooring buoys should be observed over several tide cycles to determine if the moorings are correctly spaced and aligned. In Maine, all buoys associated with your farm, including mooring buoys, are required to have “SEA FARM” painted on them with three inch letters.

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When setting up multiple parallel kelp lines, there should be enough space between the moorings that their associated long lines are not in danger of tangling. Typically people have ten feet at a minimum between kelp lines for sugar kelp and winged kelp. Skinny kelp can grow over ten feet and more space between lines may be required.

Image credit: Jaclyn Robidoux, Maine Sea Grant

While most aquaculture crops in Maine take over a year to reach harvest size, seaweed grow-out only takes 6 months between late fall and early spring. This shorter growth season means that aquaculture equipment does not need to be in place year-round to grow seaweed. Most lines and equipment used on a seaweed farm can be differently employed during the summer and early fall. Additionally, farms can be partially or entirely removed in the off season to open up waterways for expanded recreational and commercial use. Depending on how mobile your farm needs to be, you may choose between different mooring systems for keeping your long lines in place. Two different types of anchors are commonly used for seaweed aquaculture mooring systems:

Anchor options for kelp farms

In addition to those listed above, there are additional anchor types that are used on seaweed farms (helical anchors, mushroom anchors, etc.). What moorings or anchors a seaweed farm uses often depend on the bottom type and other factors like cost, deployment, maintenance, scale, and site conditions. When deciding which anchors to use in your mooring system, you should consider the pros and cons of multiple types of systems. For most seaweed farms, moorings/anchors will be the highest gear cost, so it is important to have a well thought out plan both for your wallet and to ensure your farm stays where it is permitted.

Pros and cons of different anchors

Seeding a kelp farm

The second stage of setup for a kelp farm is deploying long-lines. This aspect of farm construction often happens at the same time as seeding because of the technique that is used to transfer inoculated seed string to the poly line. Because long-line setup and seeding happen simultaneously, it is key to prepare gear such as long-line segments and droppers ahead of time to ensure a smooth and efficient seeding process.

Prepping long line and droppers

Making long lines

Long line segments are fairly easy to make. Starting with a spool of 7/16 inch poly rope (or similar), measure out 200 ft segments and cut the line into pieces. Then fuse the ends of each line segment with heat to prevent fraying and make knot tying easy. Finally, coil the line segments so they will be easy to transport and won’t tangle.

Making dropper buoys

Making depth control droppers, or depth maintenance systems, is also fairly straightforward. Droppers are used to keep kelp lines 2 meters (7 ft) below the surface, which is the optimal depth for growing sugar kelp. Droppers consist of a buoy, a 7ft PVC spacer, and a weight. When the kelp on the long line is small, the dropper buoy keeps the line from sinking too deep. As the seaweed grows and the kelp stipes fill with air and begin to float, the weights keeps the long line from coming to the surface. Droppers are placed in 200 ft intervals along the long line when new line segments are started.

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Buoy used to keep the kelp longline afloat if it begins to get heavy.

Seven feet of line threaded through a PVC tube to keep the long line at the appropriate depth.

Attachment point where the dropper buoy is spliced into the longline.

Weight on the dropper to keep the kelp longline from floating to the surface as the kelp holdfasts become more buoyant.

Once all of the farm equipment has been prepared and seed spools are ready to be transferred to their grow-out sites, it is time to begin seeding and deploying kelp lines. Seeding happens during November and early December as this is when nursery grown seed is mature enough to be moved without impacting its survival. Seeding should be done around a low tide on a calm day– preferably overcast but without rain. 

Seeding begins with the transfer of seed spools from the nursery to the farm. Whether you grow your own kelp seed or purchase seed spools from another source, you have to take care when transporting seed not to shock the juvenile kelp by significantly altering their ambient water temperature or leaving them exposed to the open air for too long. Reusing the settling tubes from the inoculation stage of nursery growth is a good system for careful transport of seed spools.

Making dropper buoys

Making depth control droppers, or depth maintenance systems, is also fairly straightforward. Droppers are used to keep kelp lines 2 meters (7 ft) below the surface, which is the optimal depth for growing sugar kelp. Droppers consist of a buoy, a 7ft PVC spacer, and a weight. When the kelp on the long line is small, the dropper buoy keeps the line from sinking too deep. As the seaweed grows and the kelp stipes fill with air and begin to float, the weights keeps the long line from coming to the surface. Droppers are placed in 200 ft intervals along the long line when new line segments are started.

Before heading out to the farm you should make sure to have all of the gear you will need to deploy long lines, including:

  • your seed spools
  • your pre-cut 200 ft segments of poly line
  • the appropriate number of droppers
  • any other lines, buoys, or anchors that you may need for your chosen mooring system

You will also want to have two or three people on the boat to handle both seeding and steering the vessel at the same time. 

On your way out to the farm site, you and your crew should determine which way the wind is blowing, which way the current is flowing, and what the direction of your boat’s net movement is without the motor. If possible, you should begin seeding from the upwind/up-current side of the farm so the boat will drift naturally toward the location of the opposite mooring.

Once you are in position at the first mooring, long line deployment begins with threading the first segment of polyline through one of the seed spools and attaching it to the mooring system. Then you take one end of the seed string and tie it securely to the long line. At this point, you should be holding the seed spool with locking pliers to control the rate it unravels onto the long line while your other crew member slowly begins backing the boat toward the corresponding mooring at the other end of the farm. The seed string will begin to unwind and wrap around the long line as it is deployed. As the long line passes through the seed spool, you should keep tension on the line to achieve tight, even wraps of the seed string. As the seed string is unraveling, be aware of prolonged exposure of the seed to cold or windy conditions that may freeze or dry the delicate blades, particularly if seeding late or in winter conditions. Occasionally submerging the spool or gently pouring seawater over it can help prevent this.

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:

When the seed string from one spool is about to run out, you must tie it to the long line to prevent unraveling. Ideally this occurs near the end of the first 200 ft long line segment. This is the point at which the second seed spool can be threaded with a new line segment and the two line segments can be joined together securely. A dropper is also spliced into the long-line where the two segments connect to maintain proper depth.

Once the next line segment is ready to go and threaded with the second seed spool, the new seed string is fastened to the long line and the process of unwinding the seed spool while maintaining tension on the long-line begins again. This process is repeated until you reach the mooring system at the opposite side of your lease and the final long-line segment is attached to the mooring. It is common for long-lines to be a total of 1,000 ft long– containing 5 line segments and 3 droppers.

Once all of the long lines on a farm are deployed and tied off at their moorings, the last task is to tension the lines to prevent tangling. This is easiest to do at low tide on a calm day and will depend on which mooring system you use. For moorings with deadweight anchors, you will need to use block and tackle to tension the lines. Meanwhile, for moorings with claw anchors, you can drag the anchor line on the mooring to reposition the anchor and tension the long-line.

Tending to the seaweed farm

Kelp grow-out on the farm takes about six months. During this time, there is still some need to handle crop, though time spent tending the farm is usually limited by weather. There are two farm maintenance tasks which are critical to stay on top of during grow-out: 

  1. Observing and correcting crossed or tangled long-lines
  2. Monitoring and maintaining proper depth of long-lines

The farm should be monitored on a weekly or biweekly basis between the time you seed the lines to the time you harvest to make sure everything is in order. Additional checks on long-lines are also advised after storms, high wind events, and cold snaps where ice may become a factor. It’s important to keep records of the growth on your farm beginning at seeding and over the course of the season, especially if you plan to farm seaweed year-after-year or are hoping to grow and sell a certain volume. Simple photo logs can help visually capture growth and color, monthly measurements of blade length or weight can help understand the biomass, and short notes about each long-line’s productivity (“good growth,” “bald spot in the middle of the line, 200ft-400ft”) can help farms identify areas that grew well or pin-point where things might have gone wrong.