Aquaculture Methods

Those traveling the waterfront or the coastal waters of Maine will likely know what lobster boats and lobster traps are all about: what they look like and how they are used. On the other hand, with the increase in marine aquaculture in the state,
visitors (and residents) may encounter gear that doesn’t look like anything they’ve seen before. The descriptions below are written to give an idea of what sea farming looks like in Maine, and to take some of the mystery out of these new types of equipment.

mussel longlines | mussel rafts | oysters [surface] | oysters [bottom culture]
clams [hard] | clams [soft-shell] | ocean pens for finfish | kelp

Mussel Longlines Mussel culture begins by collecting mussel larvae or “spat” from local waters when wild mussels are spawning, usually in late spring. Collector ropes and other materials are placed in the water to catch the mussels as they settle toward the bottom. Once mussels set they typically remain on the spat lines until they grow to half-to-one-inch long “seed,” at which time they are stripped from the spat lines and transferred to new lines in a step called “socking.” Long tubes of cotton are pulled over a length of rope, and filled with young mussels; alternately, ropes are wrapped with biodegradabe cotton sheeting. The mussels attach to the ropes with their byssal threads. The lines are either strung between buoys or hung from rafts (see below). Longline systems consist of a main horizontal line, anchored at both ends, with flotation along the center segment. Dropper lines are then suspended from the main line into the water column. Longline systems have the advantages of being adaptable to deeper waters, or more exposed sites, and can support high capacity and high efficiency.
Mussel Rafts Mussel rafts are constructed with steel I-beam main frames, wooden cross beams, and large polyethylene floats. Most measure 40 feet square and are capable of supporting up to 400 vertical lines, each roughly 45 feet long. As much as 75,000 pounds of mussels can be grown in an 18-month cycle from seeding to harvest. Rafts are used in sheltered, shallow sites.



Shellfish bags are among the
most common aquaculture gear observed. Oyster farmers purchase baby or “seed” oysters from a shellfish hatchery and place them in plastic mesh cages that float on the water surface. The oysters are allowed to “grow out” for two to three years until they reach market size. Oysters need flowing water in order to filter out food particles, so the cages are rotated by hand to keep marine organisms from “fouling” the surface. Shellfish bags come in many shapes and sizes, but most often are hung on the surface in long paired lines. Some oyster farms keep their oysters in floating shellfish bags all the way through until market size. In the wintertime, oysters are either set in cages on the bottom, kept in refrigerated storage until the next growing season, held in floating rafts in an ice-free area or, if they are large enough, they are seeded directly to the bottom for final growout.

Occasionally, other species of shellfish are grown in these floating bags; such as bay scallops or surf clams.

Floating/Sinking Cages

Float/sink cages are used for oyster farming, and are a relatively new type of equipment. The basic approach is to combine a cage for holding shellfish bags with a set of floats. When in the ‘grow’ position, the cage is submerged in the water and held just under the surface by the floats, or pontoon. This allows the shellfish to feed. When the cage is flipped over, the cage is exposed to air and this air-drying allows the farm to kill off any biofouling that might have begun to form, such as algae, barnacles or mussels. Air exposure usually lasts from 24 to 48 hours, and does not harm the oysters. The floats themselves have caps at either end; in late Fall, farmers remove the caps and allow the floats to fill with seawater. The cages are then lowered to the bottom, which keeps the cages free of damage by ice. Cages are raised in the spring, the floats are drained and capped off, and the unit is once again placed in the growing position.


Bottom Culture Bottom culture is used for both mussels and oysters. For mussels, the goal is to harvest a dense bed of seed, and then to spread it out on the seabed at lower density, so each mussel has access to better feed. For oysters, the seed are taken out of the nursery at about two inches, and then spread out on the bottom for final growout. Mussels are generally harvested with a drag pulled behind the harvest vessel, while oysters can be dragged, raked, or gathered by divers.
Clams, hard Hard clams, or quahogs, are sourced from the hatchery or nursery at half to inch-and-a-half, and then set out on leased grounds in the intertidal zone and covered with plastic mesh netting to keep out predators. Clam farmers watch their crop carefully to keep the netting clean, and to remove any predators that make it into the covered area. Grown clams are harvested by hand raking.
Clams, soft-shell (seed stock) Many communities acquire soft-shell clams from a nursery to enhance their flats. The clams are planted in the intertidal mud during spring at a size of one-quarter to half-inch. The beds are often covered with netting to deter crabs, gulls, and other predators from eating the small shellfish. These nets are removed by early winter. Commercial harvesters use a hand rake when the clams have reached a minimum size of two inches.
Ocean Net Pens for Finfish Ocean net pens are used to raise finfish species such as Atlantic salmon and Atlantic cod. A net pen consists of a floating circular frame that is moored to the ocean floor with anchors. Young, hatchery-raised fish are placed in the pens. A mesh net suspended from the frame keeps the fish inside the pen. Food pellets are distributed to the fish by hand or with mechanized spreaders. The fish are harvested when they attain a desired market size, typically within two years.


Kelp Kelp culture begins in a laboratory setting, where string is wrapped around PVC tubes and soaked in a saltwater solution that contains reproductive cells (spores) taken from wild or cultured seaweed. The cells attach to the string and begin growing into tiny plants. The “seeded” string is then wrapped around larger ropes that are strung between buoys at an ocean aquaculture lease site. Within six months, the ropes are retrieved, along with the full-size blades of kelp attached.