Sources, Quantities and Cultivation Methods
Sources and Quantities
Mussel farming extends across many countries in temperate and tropical regions and includes nine main species1, as shown in the table opposite. There are three species in what is known as the blue mussel complex (Mytilus edulis, Mytilus galloprovincialis and Mytilus trossulus) and they show varying levels of hybridisation wherever they occur within overlapping geographical areas. The UK for example farms M. edulis, M. galloprovincialis and their naturally occurring hybrids2.
Mussels are bivalve molluscs, meaning that they have a two-part hinged shell. They feed by filtering mainly microscopic algae (phytoplankton), but also some organic detritus in sea water. Farmed mussel aquaculture operations are located in intertidal and shallow sub-tidal environments, as well as in deeper, offshore waters (20m or deeper)3. The two-hinged shells are generally thicker in mussels grown inter-tidally as they protect the mussel from the more dynamic nature of this environment (the wave and tidal action), and mussels grown inter-tidally may exhibit a longer shelf-life than those which are grown sub-tidally and suspended in the water column.
As the map shows, Mussels are farmed across the globe, and was worth US$3.8 billion from a 2016 production total of some 2 million tonnes1. China produced almost half of this volume. Other important producing countries are Chile and Spain, which produce over 200,000 tonnes each. Some 10% of world mussel production enters international trade4.Less than 5% of total world bivalve production enters international markets; one of the lowest proportions in seafood trade. This is due to the fact that the majority of bivalves are eaten in the countries where they are grown, and because they are a highly perishable product. The EU is one of the main markets for bivalves taking over one third of the total bivalve trade4.
China produces over 80% of the world's bivalves, but domestically consumes almost all of this production. Around 200,000 tonnes of mussels are internationally traded per year, with Spain and the US generally the main markets4.
European production of mussels (some 500,000 tonnes in 2015), is divided between bottom culture and rope grown; northern countries, especially the Netherlands, concentrate on the former, whilst Spain and Italy the latter1, 4. Spain and the Netherlands are the main exporters of European mussels, but Europe also imports significant quantities of farmed Chilean mussels5.
Mussels are available all year round, and technologies such as vacuum packaging make them even more accessible to the consumer. Large mussels are packed in bags of 5 or 10 kg, while smaller bags of 1 kg are produced for distribution to retail.
Mussels are processed and sold in various ways; whole, cleaned, cooked (with or without the shell i.e. pure mussel meat), and sometimes presented in their half shell6.
Domestic Market Information7, 8
From 2008 to 2018 mussels have been in growth in Great British retail (i.e. in England, Scotland and Wales); increasing in value and volume by 10.4% and 8.8% respectively from a base of £19.6 million and 3,222 tonnes in 2008.
In 2018, UK retail sales of mussels were worth £26.5 million (+0.5% compared to 2017) with a volume of 4,731 tonnes (+8.8%), average price £5.59 per kg; ranking as the 18th most popular species by value (in the 52 weeks up to 16/63/2018 (including discounters).
In 2018, the UK imported 2,744 tonnes of mussels (Mytilus species).
Note: differences between the volume of mussels sold in UK retail and that which is imported is due to its use in the foodservice industry (e.g. restaurants) (no data available) and that which is re-exported.
Mussel farming is undertaken using a variety of methods based on the prevailing hydrographic, social, and economic conditions. The two main types of culture are either carried out on the seabed or by suspended culture3, 9, as shown in the schematic opposite. On-growing of mussels is sea-based, and the type of rearing method depends on both the environment (tidal range, water depth, and so on) and tradition. No feed is supplied and no chemicals or medicines are administered.
Seabed culture involves locating and dredging seed mussel of around 10mm shell length from offshore beds and relaying them in more productive, protected locations; in the UK such areas are often termed a ‘lay’10. Across the UK, in Wales, N Ireland, The Wash, North Norfolk and Poole Harbour in England, mussel production is mostly derived from bottom culture and within restricted Several or Regulating Order fisheries (i.e. where a person or company is granted legal ownership of the mussels in a given area of seabed to enable their cultivation)11, 12. In 2016, 6,136 tonnes of UK mussels where produced “on bottom”13.
In suspended or “off bottom” culture mussels are farmed on systems of rafts, ropes and floats, where they grow until harvest 18-24 months later. The UK produced 8,549 tonnes of off-bottom mussels in 201613, and these came predominantly from Shetland and elsewhere in Scotland. Suspended mussel production is increasing in Wales and England.
Mussel juveniles or ‘spat’ used for farming are either collected from the wild or settle naturally on purpose-made collectors. However, predicting the occurrence, time, location and size of any spat-fall is difficult, and there are many environmental variables to be taken into account. This introduces uncertainty and unreliability in seed supply to growers and often results in fluctuations in the supply of mussels to processors, retailers and eventually consumers14. Hatchery production of spat is possible and may be a method by which a reliable mussel spat supply can be supplied to farmers15, 16. Research into and the development of a mussel hatchery is currently going on in Scotland in order to address the issues of reliability and quality of Scottish spat17, 18.