FAO.org

Cultured filter feeders (e.g. bivalves, such as mussels and oysters, and some echinoderms, such as sea cucumbers) and algae do not need external feeds. They can live on carbon and other nutrients in the environment. Mussel farming can be done with no or minimal greenhouse gas emissions and environmental impacts. The impacts of mussel farming are most often related to the production of feces or pseudofeces, which can cause hypoxia (lack of oxygen) when they accumulate in sediments. However, well planned, well situated and appropriately sized shellfish culture, even when it occupies large areas, seems to have little effect on the community of organisms which live on, in, or near the seabed. On the other hand, the benefits of the biological absorption of nutrients to control the symptoms of eutrophication^vi  have been documented in many parts of the world. It is clear that the existence of significant filter feeder aquaculture (e.g. in China) has been instrumental in controlling coastal eutrophication, probably on a national scale (Ferreira et al., 2013). 

In some countries, China for example, mussel farming provides an important source of food and protein and supports livelihoods. In countries with lower seafood consumption, such as Chile, mussel farming provides jobs and livelihoods, with much of the production being exported. In general, the ecosystem services that extract nutrients and reduce eutrophication risks in the water column have not been properly evaluated. However, Bunting and Pretty (2007) build the case for the culture of mussels (Mytilus edulis) on rafts in Killary Harbour, Ireland, (as described by Rodhouse and Roden, 1987). They estimate that 10.8 tonnes of carbon per year would be assimilated^vii in mussel production, and that the removal rate of carbon during harvest was 0.008 tonnes of carbon m-2 per year 1, which is equivalent to 80 tonnes of carbon per ha per year. 

The great majority of mussel farming is carried out in floating or underwater holding systems that facilitate the mussels’ permanent filtration of phytoplankton from the water. These systems use two types of techniques: suspended lines from a floating tray or individual long lines tied to a weight on the bottom; and a floating system on the surface. Mussel seeds attach themselves to these lines and individuals grow by feeding on the available phytoplankton. Most bivalve farming can be carbon-friendly and is comparatively energy efficient. However, these farming systems are sensitive to several threats related to climate variability and climate change, the most common of which are changes in water quality (e.g. temperature and salinity) and the increased frequency and prevalence of red tides.

Mytilus galloprovincialis is the main species of mussel farmed in the Galicia Rias area of Spain. Mussel growth is very fast. It normally takes about 18 months for mussels to reach commercial size, but in many cases, mussels can grow this size in one year. The Ria Arousa is the most productive area for mussel farming. It benefits from an upwelling system that regularly contributes cold water and nutrients, which causes exceptional phytoplankton production that can sustain more than 2 000 floating rafts. The rafts or floating platforms are rectangular wood frames of about 500 square metres. Usually one raft represents one farm and is owned by a single family. The estimated annual yield for the Ria Arousa area is about 60 tonnes per ha without shell. The annual estimated production in Galicia is around 250 000 tonnes (with shell). Current preliminary estimates of employment indicate that about 10 000 people live directly or indirectly from mussel farming in Galicia.

In Chile, the most common farmed species is Mytilus chilensis. Farms can be of different size and operate at different scales of production. The farming is done using long lines. A ‘mother line’ normally about 100 metres in length is held by floats and tied to the bottom. Many vertical lines hang from the mother line. Farming density can range from 10 to 12 long lines per ha in protected bays, channels or fjords in a water column that can reach from 10 to 40 metres in depth. Estimated yield ranges from 60 and 70 tonnes per ha without shell. In 2011, Chile produced 221 000 tonnes of mussels. Although there is no current available quantitative information, it is claimed that mussel farming generates significant local direct and indirect employment, not only through production but also in processing, an economic activity that employs mostly women.

The sensitivity of mussel farming to climate change and its adaptation potential

Mussel farming in both southern Chile and Galicia is very sensitive to red tides, which have been increasingly associated with climate change and climatic variability. The best approach for reducing exposure to red tides is to implement permanent food safety monitoring programmes. These programmes lower health risks and improve preparedness. In both countries, aquaculture systems have such programmes in place. Mussel farming is also sensitive to extreme weather events. However, since it tends to be carried out in more protected coves and bays, where higher phytoplankton productivity can be ensured, it is generally less exposed to weather events than other aquaculture systems. Ocean acidification can be a major threat for all bivalve farming, as lower levels of pH in the water can interfere with the formation of the calcium-rich valves and other physiological processes. Adaptation to this threat may involve activities such as selecting resistant strains of mussels and increasing hatchery-produced larvae under more controlled conditions. Another potential threat is the lack of available wild seeds. Coastal oceanographic conditions and nutrient availability, which strongly affect the production of wild seed, are also influenced by climate variability and climate change. The production of larvae in hatcheries and adequate management of the brood stock are essential adaptation measures.