Mussels

Mussel Culture in British Columbia
Several species of mussel occur in BC’s coastal waters yet mussels have traditionally been more of a fouling problem to shellfish growers than a potential farmed animal.

For centuries, mussels, mainly Mytilus edulis and Mytilus galloprovincialis, have been cultured on the coasts of the Netherlands, France and Spain. Methods of growing them range from bottom culture with dredge harvesting in the Netherlands to intertidal “Bouchot” systems in France to off-bottom raft culture in Spain. New Zealand has recently developed an impressive farming system for the green lip mussel, Perna canaliculus, and now market them worldwide. Their system was originally based upon the Spanish methods but soon moved on from those.

Aquaculture of mussels in North America began in the 1970’s and is now a substantial industry in the northeast United States and Atlantic Canada where M. edulis is the main species grown. On the Pacific Coast, mussel culture is practiced around Whidbey Island and other areas in Puget Sound, Washington. In BC, efforts at mussel culture with the native Mytilus trosullus have been thwarted by both duck predation and massive natural summer mortality before reaching marketable size. A pilot project was carried out in the early 1980’s to determine the potential for growing the blue mussel, M. edulis. It met with limited success. Problems of summer mortality, weak byssal attachment and losses due to handling and shipping were the main obstacles to successful culture. Those problems are slowly being overcome and now M. edulis and M. galloprovincialis are being cultured on a small scale.

In other areas of the world, mussels are grown in bottom culture systems as well as off-bottom intertidal systems. At this point, those involved in mussel growing in British Columbia are all utilizing deep water suspended culture methods, so consideration will be given to those systems only. Using longlines or rafts, a grow-out period of 12-18 months appears to be attainable here. Growers are adapting techniques from the Atlantic coast and other parts of the world as well as developing their own innovations to deal with the unique conditions here. Mussel growing appears to be on the verge of developing into a large scale operation for some shellfish growers. Besides typical longline culture techniques, growers are experimenting with tray culture of mussels using the HighFlow tray system. This system will eliminate losses of stock due to duck predation and sloughing. However, significant challenges remain in increasing handling efficiency of mussels grown in trays. The high initial capital cost of the tray system is another consideration for a start-up operation.

Hatchery and Nursery
Because most of the natural populations of mussels in British Columbia are unsuited to culture collecting seed from natural sets is not practiced. Therefore the industry must be based on the reliable availability of hatchery-produced supply of seed. Several Pacific coast hatcheries produce seed and all but one (Island Scallops Ltd.)are in the United States.

Seed may be purchased by the grower after it has set and is about 1mm in size or after it has been nursed in upwellers for 3-6 additional weeks and is 2-3mm. If seed has not been set directly on material (e.g. ropes) that are to be used for grow-out, the seed must be transferred to a material with a suitable surface where nursery rearing will occur. However, a remote setting method is being developed in BC by Island Scallops Ltd. where advanced pediveliger larvae are brought to the nursery site and set there on framed screens. The screens are then placed in protected floating nursery systems. Once the mussel seed is hard set on the screens, they are removed from the frames and put into fine mesh bags and hung from longlines. These bags hung on strings are enclosed to protect them from duck predation. This nursery phase may take only three months. At this point, seed must be stripped from the bags and socked.

A somewhat different method involves a combination of land-based downwellers, upwellers and marine floating nursery systems. Taylor United, operating in Puget Sound, Washington, raise the mussel larvae from spawn to set in tanks. When they are ready to set, they are put in screened downwellers for a few days which then switch to upwellers. After 3-6 weeks when the mussel shell height measures 2-3mm, they are transferred to the nursery sea sites. This is done by allowing mussels to attach to screens (24 hours) and then transferring them in bundles to trays in floating nursery rafts. They are held there for 3-6 weeks. These are protected by netting to keep perch from grazing on the juvenile mussels. Ideally seed grows to about 6-15mm in the nursery rafts. At that size, the frames are dipped in fresh water to remove the flatworms and seed are stripped from the the frames in the trays, size graded and socked.

A recent development has been the production of triploid mussel seed. They may prove to have significant advantages over diploids. They do not spawn so it is possible to ship all year. Diploids stop or slow growth in winter and most available energy is diverted to gametogenesis. As they do so their condition is poor for marketing. Triploids continue to grow through the winter, although at a reduced rate. They appear to attach more firmly to the culture ropes reducing stock losses. Triploids are also reputed to have smoother, less grainy meat.

Socking
The rapid growth of mussels through the nursery phase means that after three months they are ready to be socked and hung in the grow-out system. Socking is the process in which the mussels are transferred to tubular mesh material forming sausage-like socks of specific length depending on bottom depth and the depth of food availability. Mussels are removed from nursery systems and transferred to socking tables or a socking machine. The tables have sides to contain the mussels and a hole at one end with a length of pipe attached. The entire length of the sock, which may be netted polyethylene or extruded vexar, is placed around the pipe, knotted with a half hitch at one end and mussels fed into it. As mussels fill the sock it is drawn off the pipe until it is filled and the other end knotted closed. A polypropylene line is sometimes threaded into the sock to give it added strength. There are also socking machines available that feed mussels into the socking material. The socks are then hung on longlines or rafts for grow-out. The mussels will soon migrate to the outside of the sock to maximize food capture. A more recently introduced method involves using a biodegradable socking material wrapped around a culture rope. When the material disintegrates, mussels have established a firm enough attachment to the rope. However, continued growth may lead to weakened attachment and growers take precautions to prevent mussels from sloughing off by inserting pins through the lay of the rope at regular intervals.

Growout and Predator Protection
A variety of systems are used for mussel grow-out but most of them fall under either raft or longline systems. One grower in BC uses salmon net pens to contain the culture and has installed barrel and timber floats inside the perimeter to hang the mussel socks. The net pen provides a barrier to duck predation. The principle concerns for growout systems are predation control and ensuring calm conditions prevail around the mussels. Too much agitation caused by rough water conditions can result in mussel detachment and significant stock losses.

Long lines are the most common system in use on the Atlantic coast. On the Pacific Coast, both longlines and raft systems are used. Where conditions are reasobly calm and protected, rafts may be the most suitable system. In the Puget Sound area, rafts are used more frequently than longlines.

Rafts have the advantage of offering the potential for very high stock densities, but there will be limitations depending on water flow and food availability. One grower in Washington reports that at specific sites, around 10,000 feet of grow-out socking is attached to each 30 by 34 foot raft producing about 18,000 pounds of mussels. Nets can be hung around rafts to prevent access by diving ducks.

For higher energy sites with greater wave and current action, longlines may be more suitable. They are better able to withstand such conditions and absorb the energy better than rafts, keeping in mind mussels can be lost as a result of excessive movement. However, predator control is generally more difficult with a longline system. The most common means to prevent access by ducks is to surround the socks or culture ropes with a lantern net.

Mussels are usually harvested when they reach a 50-60mm shell length, but this will vary depending on the market demands. Harvestability will also depend on the condition of the mussels and timing of gametogenesis and spawning. Retrieving the mussels from rafts or longlines is usually done by hoist and winch, sometimes with the aid of longline rollers. This is done carefully since the byssal attachment may be weak enough that the mussels may drop from the rope. Harvesting from rafts can also be done by winch and hoist. However, larger companies such as Taylor United are using sink floats that are maneuvered under the rafts. The culture ropes are cut and the mussels fall onto the sink float and can be transported to the processing point. Typically growers will be responsible for washing and declumping their stock prior to delivery. Some markets also prefer the mussels to be de-byssed which adds cost to processing. There are machines available for these processes.

Printed Sources to Consult about Mussel Culture:

  • Scarret, Dave. 1995. A Handbook of Northern Mussel Culture. (Montague, PEI: Island Press).
  • Skidmore, D. and Chew, K. 1985. Mussel Aquaculture in Puget Sound. (Seattle: Washington Seagrant Program).
  • Quayle, D.B. and Newkirk, G.F. 1989. Farming Bivalve Molluscs: Methods for Study and Development. (Baton Rouge: World Aquaculture Society).

Web sites to consult:

  • Fisheries and Oceans Canada:Species and Habitat of Shellfish.
  • Fisheries and Oceans Canada:Diseases and Parasites of Shellfish