Green-lipped mussels are New Zealand’s major aquaculture species. Explore how green-lipped mussels are farmed in New Zealand, the way that the industry has developed and the challenges that mussel farmers face.
How mussels are farmed in New Zealand
Most green-lipped mussels in New Zealand are farmed in the same way. Spat (juvenile mussels) are collected from Ninety Mile Beach and elsewhere in New Zealand, where they wash up in their millions attached to clumps of seaweed. After arriving at a mussel farm, spat are transferred to nursery ropes and grown on the ropes in seawater until about 6 months of age. At this point, they are removed and reseeded onto longlines (stretches of rope up to several kilometres long) that are suspended between buoys.
Mussels are grown for a further 9–12 months before they are harvested. Mussel barges, which harvest the mussels, are highly mechanised and contain equipment for removing mussels from lines, then declumping, washing, sorting and packing them.
You can learn more about how green-lipped mussels are farmed in New Zealand by exploring the Mussel farming in New Zealand interactive below.
Farmed mussels vs wild mussels
Because most farmed green-lipped mussels are grown from wild spat, farmed and wild mussels are not distinct populations. Many aspects of their life cycle and living conditions are identical – both farmed and wild mussels feed on phytoplankton and mate by means of broadcast spawning. Both are susceptible to predation, infestation by parasites and accumulation of toxins.
Where mussels are farmed
Most of New Zealand’s mussel farms are located in the same areas of the country where wild mussels thrive – in the waters around the coastline of the North Island, the top of the South Island and Stewart Island. Mussel farms also require relatively sheltered water (so that farming and harvesting are not disrupted by rough seas). For this reason, the Marlborough Sounds and the Coromandel/Hauraki Gulf are key mussel farming areas. There are over 600 mussel farms in New Zealand, and they cover thousands of hectares of marine space.
The importance of mussel farming for New Zealand
Birth of an industry
Mussel farming in New Zealand developed to meet demand from consumers after the supply of wild mussels collapsed in the 1960s. Before that time, wild green-lipped mussels were harvested by dredges (boats that scraped beds of mussels from the seafloor, particularly in the Hauraki Gulf and Marlborough Sounds) as well as by hand (from intertidal rocks at the shore).
Over about 50 years, dredging removed most of the accessible mussel beds from the seafloor, and the mussels did not regrow – by the early 1960s, mussel beds in the Hauraki Gulf were ‘fished out’. Even now, most mussel beds have not recovered. Virtually all green-lipped mussels eaten worldwide are now sourced from farms.
Key developments in New Zealand mussel farming
Since its beginnings in the late 1960s, New Zealand’s mussel farming industry has grown rapidly. Several key developments have driven the New Zealand mussel farming industry forward. Some of the most important advances are described below.
|Development||When||How things had been done before||Advantages of the new approach|
Using spat from Ninety Mile Beach (Kaitaia spat) to seed mussel farms throughout New Zealand.
Individual farmers had collected spat or put out ropes to encourage spat to seed onto them.
Plentiful supply of spat for mussel farming. High-density supply (up to 2 million spat per kilogram of seaweed).
Using longlines attached to buoys as support structures for mussel ropes (droppers).
Square concrete rafts had been moored in the water and used to hang mussel lines, but there were concerns about collision with boats. Also, mussels hanging from droppers in the centre of rafts tended to have limited access to phytoplankton and therefore to grow more slowly.
Cheaper and safer way to farm mussels. Additional buoys can be added as the mussels on the lines become heavier.
Using ‘mussock’ (cotton stocking) to seed spat onto ropes.
Spat seeding was laborious. One approach was to wrap Kaitaia spat (on seaweed) around ropes by hand then wrap a lacy bandage around it.
Spat can be seeded very rapidly by placing it inside mussock along with a length of rope. The filled mussock is placed in the water, where spat transfers to the rope. Mussock degrades over time, leaving a mussel rope covered in spat.
Mussels seeded onto continuous rope.
Mussels were seeded onto individual (short) dropper ropes, which were hung off longlines.
Seeding onto continuous rope (a kilometre or more) meant mussels could be stripped from ropes by machine.
Highly mechanised barges for harvesting.
Stripping mussels from lines, declumping, cleaning and packaging was often done by hand or on shore.
More rapid and efficient harvest of mussels from lines. Mussels are stripped, declumped, washed and sorted on board the barge.
Challenges in mussel farming
Like all farmers, mussel farmers in New Zealand face challenges at several steps of the farming process. Some of the most significant challenges include:
- unpredictable spat supply
- resettlement of spat away from nursery lines
- predation of spat by fish
- organisms other than mussels (seaweeds, sea squirts and so on) fouling longlines
- accumulation of toxins within mussels.
Recently, Oliver Trottier and his colleagues from Leigh Marine Laboratory have identified an additional threat to mussel farming. Oliver has shown that the New Zealand pea crab, which inhabits green-lipped mussels, can decrease the growth of mussels on farms significantly. Oliver estimates that this decrease in mussel size costs New Zealand’s mussel industry over $2 million per year. In addition, some countries refuse shipments of green-lipped mussels that are found to contain pea crabs.
Learn more: Testing how pea crabs affect mussel farming
Not all farmed mussels make it to the supermarket. Read why 3.5 million odd-sized adults were returned to the sea in the article Mussels.