The blue mussel, Mytilus edulis, is a well-known bivalve shellfish in Maine and elsewhere; it occurs naturally between Labrador and North Carolina on the US East Coast, but also occurs in northern latitudes in the Pacific and Arctic Oceans as well. Most commonly, blue mussels are found in the intertidal and shallow subtidal zone, though they have been found well below 100 feet in depth.
Photo: Female (top) and male mussel, in spawning condition
In Maine, this species is often confused with the Baltic Mussel, Mytilus trossolus, given that similar size and shape of the two species, although M. trossolus is more prevalent in the eastern portion of the state. Blue mussels can withstand very low salinities for a time, but prefer salinities above 20 parts per thousand, and they are quite capable of thriving in fully oceanic salinities ~34 ppt. Thermal tolerance extends from 0 degrees Farenheit up to over 70 degrees F, although in most areas, byssal attachments become weakened above 65 degrees F. The preferred range of temperatures for successful growth and reproduction is generally between 45 and 60 degrees F.
Blue mussels are very efficient filter feeders, capable of retaining particles upwards of 3 microns in diameter: 60 microns is about the diameter of a human hair. Their main source of food comes from phytoplankton, and some detrital grazing as well; pumping rates can be as high as 2 gallons per hour, depending on factors such as the size of the mussel, amount of food in the water, temperature, and rate of flow. Spawning can occur over a wide time range in Maine, between April and September, depending on location and local conditions. Generally speaking, localized aggregations of mussels will spawn at the same time, called synchronous spawning. Larval development through settlement is highly dependent on temperature and food availability, and can range from 14 days to 30 days. Larvae go through several stages as metamorphosis occurs, and at settlement, mussels are approximately 500 microns - 1/2 millimeter - in size.
Photo: larval mussel, courtesy of Dr. Phil Yund.
In the eastern United States, blue mussels are principally sold into the market as live product. Elsewhere in the world, mussels are packaged into a variety of other products: fresh frozen, cooked and frozen (heat-and-eat), breaded for frying, and with different sauces. As growers in the US expand their production, it is likely that value-added products will be developed here as well, with a time-limited consumer in mind. The last two or three decades have seen an increase in demand for blue mussels in the US, and though the principal demand has come from suspension-grown production, there is a smaller but dedicated following for bottom-grown mussels too. According to the National Marine Fisheries Service's Fisheries Statistics and Economics Division, 2010 imports of all mussel products were valued at $4.8million USD, and 2011 import values were in excess of $6.1million USD. Imports come principally from Canda and New Zealand. US Exports of mussel products over those two years were valued at only $266,453 (2010) and $209,271 (2011). Consequently, there exists an opportunity for domestic producers to tap into US demand, and to improve export capacity.
Production can be characterized generally in two categories: bottom culture and suspension culture, and this page focuses on the latter. Suspension culture in Maine is accomplished principally using floating raft systems, adapted from designs originating in Ireland and Spain, although longline production is present in the state as well. Both methods have their strong points and liabilities, and it is up to the individual producer to evaluate their relative strengths and weaknesses, and to make a decision on which to use according to their own situation. Generally though, the processes for production follow similar paths: seed procurement, nursery production, socking for final growout, harvesting, processing, and packaging. These activities and others relating to farm production are addressed only briefly below; a more full description is easily available via documents listed at the bottom of this page. In general however, these activities can be summarized as follows:
Seed is collected in two ways; by using a bottom drag to collect juveniles, and wild spat collection. The bottom technique can be efficient, but the grower may run into problems in stocking bottom-grown seed to a suspension approach, and experience will play a large role in grower success.
Photo: seed collector with good set
When collecting seed from the bottom, the target size is usually about 0.75" to 1.25 inches. When collecting wild spat, collectors are set out when competent larvae are available, and larval sampling during the season is helpful in determining when larval abundance is highest. Spat collectors may be old lobster-pot rope, specialized 'fuzzy' rope designed for the purpose, mussel growout line, or similar materials with high surface area. Larvae that settle on the lines in spring may be large enough to harvest for final growout by fall, or it may need to be overwintered before it's large enough for socking. When appropriate, small mussels are stripped from the spat collection lines, cleaned and graded, and prepared for 'socking' or attachment to the final growout lines.
In recent years, there has been interest in the use of hatcheries to produce mussel seed, similar to West Coast and European approaches. Locally, the Downeast Institute has been experimenting in this vein, and will hopefully advance knowledge about the feasibility of hatchery production for blue mussels in Maine and the northeast.
Photo: mussels emerging from a socked line
The target size for socking mussels onto growout lines is generally in the 1.0-1.25"range. Socking may be accomplished in any of several styles. The Canadian method involves plastic socking that comes in a tube: mussels are sent into the sock much like making sausage, and the mussels migrate their way out of the sock mesh, where they can feed and grow more effectively. This approach is best for a longline setup, with dropper lengths of 10' or less, and very calm sites. The New Zealand-style socking is continuous, with mussels being aggregated along the longline, and held in place with a biodegradable cotton socking; these longlines can reach 50 feet deep, and can be several hundreds or thousands of feet in length. This type of longline uses loops of socked mussels that hang from a single horizontal longline; these can be adapted for surface use (calm sites) or submerged, in cases where the farm is on a more exposed site. Socking for raft production occurs with a cotton wrap around a central line; the wrap holds the small mussels to the growout line and also degrades, but is wrapped around the mussels as opposed to surrounding it as a tube.
Growout and Maintenance
In many cases, growout time to market size (approximately 3") takes 18 to 20 months. During this time, growers may have to deal with fouling organisms and predators. Predators like diving ducks may have to be chased off the farm, in the case of longline
producers, while raft growers will have to deploy and maintain predator protection nets.
Photo: fouled mussel line, courtesy MAIC and Carter Newell
Other maintenance activities will include monitoring for sets of starfish, or overset of small mussels. These oversets are sometimes difficult to deal with, and may require the grower to remove the mussels, grade, and re-sock; all of which is labor- and equipment-intensive. Harvesting is done by stripping the mussels off the lines, and bringing the product to a processing line, with undersized mussels being restocked to the farm site.
Processing and Packaging
The major steps in processing are washing, declumping, debyssing, grading, and quality control. Separate machines accomplish each task, with conveyors bringing products through the system. At the end, mussels should be separated to individuals, should be of uniform size, should have the byssal threads removed, and should be clean. A quality control step will minimize poor quality (ie: broken shells) product making it into packaging, and is a good step to make, to evaluate the breakage/loss rate of the processing equipment. Depending on the eventual purchaser - consumer or retail/wholesale - mussels are most commonly packaged in net bags from 2 to 20 pounds, or boxed for the larger quantities. Mussels must be kept in refrigerated conditions and preferably on ice as well. Buyers of live mussels should inspect the product; live animals should have tightly closed shells. Individual shells that have gaped a little bit can be checked by inserting a knife or other clean object into the cavity - the mussel should respond by closing the shell. If no response is observed, it should be discarded.
►Disease & Health
Although there are some diseases which the blue mussel is subject to, there is little information to indicate that cultured mussels in the Northeast have been subject to disease outbreaks, or transmissions from wild populations. The biggest potential issue with respect to human health is the mussels' ability to accumulated biotoxins during harmful algal blooms. However, since controls and monitoring of growing waters are strongly overseen by state agencies, such as the Dept. of Marine Resources in Maine, the risk of illness by the consumer is extremely small. Members of the public are highly cautioned to avoid harvesting wild mussels without knowing what the background water quality details are for the area being harvested; every year, there appear a number of news stories of individuals who consume wild mussels during so-called red tide events, and become seriously ill.
►Documents and Resources
- 'Mussel Aquaculture in the Northeast', Morse and Rice (2010), fact sheet for the Northeast Regional Aquaculture Center, NRAC Publication # 211-2010
- 'Cultured Mussels of the Northeast' by M. Rice (2010), fact sheet for the Northeast Regional Aquaculture Center, NRAC Publication # 210-2010
- The Maine Guide to Mussel Raft Culture, Island Institute, Rockland, Maine, 1999
- Mussel Seed Collection Strategies for Maine Mussel Raft Culture. 2008. By Carter Newell, final report to the Maine Aquaculture Innovation Center. (1.2 MB)
- 'Business Planning Handbook for the Ocean Aquaculture of Blue Mussels', by P. Hoagland, H.K. Powell end D. Jin, 2003. Wood's Hole Oceanographic Institution.
- An excellent workshop on mussel production from 2011, led by Scott Lindell of WHOI, with support from NRAC, the Marine Biological Laboratory and the Northeast Sea Grant Consortium. There are many useful and downloadable presentations here.
- Guide to Mussel Quality Control, by Carter Newell. Maine Sea Grant publication E-MSG-90-1
- Details from the UNH project on Open Ocean Aquaculture, specific to mussel production.
- A Practical Guideline for Mussel Aquaculture in Newfoundland. Marine Institute, Memorial University, Newfoundland, 2000. 16MB