![]() From the initial seeding of mussels onto farms until harvesting takes from 12 to 24 months. Harvesting is achieved using specially designed vessels which allow the dropper ropes to be pulled on board to strip the mussels from the dropper rope. Growing mussels are removed from the dropper ropes and reseeded once and sometimes twice before reaching harvesting size of around 100 millimetres (4 in). Steps to reduce these stressors on the spat during transport could potentially improve retention rates. A 2007 study identified two stressors that reduce the retention of mussels on the rope desiccation and starvation (both of which are experienced on the journey from where the spat are harvested to where they are farmed). This loss of spat from mussel farms is a significant problem for the industry. This loss is partly due to the secondary settlement behaviour of mussels, whereby the spat can release their point of attachment to the growing rope and exude a mucous “parachute” to help move to an alternative settlement site using water currents. Subsequent loss of spat from the dropper ropes is typically high, generally over 50% and as high as 95%. Soon after, the stocking and seaweed rots away, leaving only the rope for the mussels to attach. Once the spat have been transported from the beach to mussel farms around the country, they are transferred into a stocking that holds the spat-covered seaweed material around a “dropper rope” which is suspended in the water column hanging at regular intervals off the backbone ropes. This adaptation of the Japanese longline method consists of a series of large plastic buoys connected by two ropes forming a backbone which is held in place by concrete anchor blocks or steel anchors screwed into the seabed. An adaption of the Japanese longline shellfish aquaculture system led to the methods used today for commercial greenshell aquaculture and facilitated the transition to large-scale production by incorporating mechanized harvesting. Initial farms were based on the 700-year-old European floating raft method of mussel cultivation which was suitable at small scales however, methods to support larger scale production were soon needed. New Zealand greenshell mussel cultivation began in the 1970s and has since undergone massive expansion, with production growth of 708% from 1988 to 2000 (an average annual growth of 18%). ![]() ‘Spatfall’ events are also affected by El Niño periods and can result in delays in mussel farm production due to the insufficient seed landing on Ninety Mile Beach. This uncertainty of supply has resulted in major production problems for the industry which must endure periods up to a year without the arrival of any spat. Furthermore, the amount of mussel spat that lands on Ninety Mile Beach is highly variable. Even with this industry’s heavy dependency on wild spat, the biological and environmental processes by which the spat arrives on Ninety Mile Beach and on spat collection ropes are largely unknown. The remaining 20% is caught using fibrous ropes which are suspended in the sea near mussel farms. This single beach provides around 80% of the seed mussels required for this aquaculture industry. The density of spat varies from 200 to 2,000,000 per kilogram (91 to 907,185 per pound) of seaweed. Nowhere else in the country are such large quantities of mussel-covered seaweed washed ashore. Around 270 tonnes of wild spat which is attached to beach-cast seaweed are collected from Ninety Mile Beach in northern New Zealand each year to supply the aquaculture industry. The aquaculture of the New Zealand greenshell mussel relies heavily on the production of mussel seed, or spat, by wild mussel populations. This industry produces over 140,000 metric tons (150,000 short tons) annually and in 2009 was valued in excess of NZ$250 million. ![]() When grown for aquaculture there, it is marketed under the trademark name Greenshell.
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