Shetland and Scottish Mainland Rope Grown mussel Enhanced fishery
MSC status
Certified as sustainable in June 2012.
Summary
Species: Blue Mussel (mytilus edulis)
Location: North East Atlantic in Local Fisheries Management areas ICES areas IVa, VIa, FAO Statistical area 27.
Fishing methods: Ropes
Number of fisheries: 1
More about blue mussel.
Blue mussels (mytilus edulis) can tolerate wide variation in salinity, desiccation, and temperature and oxygen concentration, characteristics that result in the ability to occupy a large variety of microhabitats. Blue mussels are anchored to a secure substrate, which include; rocks, stones, gravel, shingle and dead shells. These characteristics make mussel an ideal species to grow on ropes.
The life cycle can be divided into the free swimming larval phase and the largely sedentary juvenile and adult phase. The blue mussel is a filter feeder, drawing in seawater, which is filtered through the gills. The blue mussel is dioecious, though rare instances of hermaphrodism have been reported. Generally the potential spawning season vary according to location, but the main spat-fall is generally in early summer.
Mussels generally produce gametes and are ready to spawn by the time they are one year old. During spawning eggs and sperm are released to the water column and fertilisation occurs externally. After fertilization occurs, the fertilised zygotes undergo several metamorphoses before settlement. Mussels settle after the sixth larval stage, the planktonic life of Mytilus edulis varies from 2-4 weeks depending on temperature, food supply and availability of suitable settlement substratum. The growth rate mussel depends largely on the availability of food.
More about the fishing methods
Mussel farming in Scotland mainland and Shetlands Islands collects their own stock from the wild spat-fall and settled spat is grown on ropes suspended from longlines. A typical longline would consist of either a single or double head-rope supported by grey plastic floats at regular intervals. The length of the line is generally between 200– 400 m. The spacing of the plastic floats (buoys) depends upon their buoyancy and the expected load upon the line.
Generally, they are spaced at up to 3 m apart. Rope of between 20 – 32 mm diameters is commonly used for the headlines. The rope droppers, on which the mussels are grown, are usually 12 mm in diameter. Plastic-pegs or discs may be inserted through the twist of the rope to provide additional support for the mussels. Droppers are generally between 6 – 10 m in length, depending on the depth of the water. Ropes-grown mussel cultivators collect their own stock from the wild spat-fall. The rope droppers are coiled so that they remain in the top 2-3 m of the water column and they are placed on the line in time to collect the natural spat. Spat settlement occurs generally in April-May in Scotland mainland and Shetlands Islands (although earlier and later spawning also may take place).
Each dropper is raised from the water and the mussels removed either by hand or by machine. They may then be transferred to a shore-based facility or the next stages may take place on-board the harvesting vessel. The mussels are separated, washed and graded, again by hand or automated line. Each dropper may yield between 5 – 7 kg of marketable mussels. Small mussels may be re-tubed and returned to the sea for further growth.
Fishery tonnage
Shetland: 2008 = 3,506 tonnes: 2009 = 3,698 tonnes
SSMG Scottish mainland: 2008 = 1,000 tonnes: 2009 = 1,000 tonnes
Commercial market
The vast majority of the combined Shetland and SSMG mainland tonnage is marketed in the UK High Street retailers.
Actual eligibility date
26 June 2012
