Production Systems
All GreenshellTM mussels are grown suspended in the water column using a system known as long-line farming.
The mussels are grown on a series of ‘dropper’ ropes that hang down through the water column. These ropes are suspended from a ‘backbone’, that is held up by a row of buoys.
This suspended rope culture method, adapted from the Japanese long-line method, has lower environmental impact than the bottom culture methods used in some countries. The suspended rope culture method also has less of a visual and navigational impact than the Spanish raft systems previously used by New Zealand farmers in the early developmental stages of the industry.
Most existing mussel farms occupy about three to five hectares of water space.
GreenshellTM mussels are farmed in the following New Zealand regions:
Source of Stock
Most seed stock for GreenshellTM mussel farming is collected from beach-cast seaweed on Ninety Mile Beach in the northern North Island. Here, the spat (juvenile mussels) are found attached to pieces of seaweed that wash up naturally on the beach.
This beach-cast mussel spat ‘fishery’ is managed by the Ministry of Fisheries under the Quota Management System (a system that uses individual, transferable catch entitlements). Once collected, these spat are carefully transported to growing areas around the country.
The other main source of seed-stock are the wild spat that settle naturally onto ropes left hanging in the water. This ‘spat catching’ is done at special sites near existing marine farm areas - particularly in the Marlborough Sounds, Tasman Bay and Golden Bay in the northern South Island.
Mussel farmers seed the spat onto farm lines using a light cotton tubular stocking that surrounds the nursery rope and holds the spat until they attach themselves to the rope using their byssus thread (or ‘beard’). The stocking mesh biodegrades naturally over time.
Hatchery production of juvenile mussels is still in its pre-commercial, pilot stages, so most spat is still collected from the wild. Hatchery production systems need to be compliant with New Zealand’s environmental management legislation, the Resource Management Act (1991), and with other building and operating codes to ensure any environmental effects are avoided, remedied or mitigated.
Stocking Densities
When young mussels reach at least 1 cm in length, they are stripped and reseeded onto continuous culture rope, using an automatic seeding system. This system controls mussel densities along the culture rope, to optimise the growth and quality of the end product.
GreenshellTM mussels grow at high densities in the wild, so while they are stocked at high densities in aquaculture operations, this is not unnatural or problematic. Using an appropriately high stocking density is actually an important part of effective farm management, as it reduces the crop rope space that would otherwise be available to competing organisms. Using the right stocking density also helps avoid unwanted drop-off of mussels and loss of product to the seafloor. Depending on the farm location and farming practices utilised, there are generally about 100-170 mussels per metre of crop rope at harvest size.
Growing and Harvesting
Mussels are filter feeders, which means they filter natural plankton (small free-floating plants and animals) out of the water column. Feed is not added at any stage of GreenshellTM mussel farming.
Mussel growth rates depend mainly on natural plankton availability. This can vary between sites, and can even change from year to year at the same site (depending on large-scale climatic events, including El Nino-Southern Oscillation). But usually, GreenshellTM mussels take 12 to 18 months to reach suitable harvest size (generally 90 mm to 120 mm).
As the crop grows, additional floats are added to the mussel long-line to support the weight of the growing crop.
Mussel farmers harvest their crop after they check the mussel size and condition (or fatness).
Only crop from growing areas with clear sanitation status and where there is no evidence of biotoxin accumulation is considered for harvest (see Food Safety section for regulations and details).
Specially designed, highly mechanized harvesting vessels are used to lift lines out of the water and then strip, de-clump, clean, and bag the harvested product.
