Mussels

Site selection for cultivating mussels is extremely important and factors include: substrate and/or depth of water; salinity; temperature; exposure to air, wind and currents; sedimentation rates; and food availability.

Buyers should seek assurances that all national and local laws are adhered to. All farms should have the required licences, permits and registrations in regards to their site and its operations accompanied by documentary evidence to demonstrate this compliance. Mussel farming may be managed to minimise site operations during peak sensitive periods or in times of low water quality episodes. Farm leases and permits can stipulate sustainable management practises.

In considering the environmental impacts of mussel aquaculture, it is important to view both the scale of the sector as well as the diversity of production systems¹. The impacts of mussel aquaculture on the environment are often considered less than those of finfish and warm water prawn aquaculture.

Bivalves are considered keystone species in the ecosystem and therefore they can affect the surrounding environment in various ways². At all scales of bivalve production, their culture and the physical on-growing structures used to support them may produce changes in water movement and sediment dynamics that can affect both planktonic and seabed communities. However, the on-growing structures may also act as new habitat and nursery areas for fish, crustaceans and molluscs³.

Operations associated with the growing and harvesting of mussels are relatively low impact in terms of activities that might lead to amenity and wildlife disturbance. One of the greatest potential impacts of cultivating filter feeders such as mussels is the net loss of energy (i.e. phytoplankton) from the ecosystem. Large monocultures, particularly in enclosed bays with limited water exchange, may exceed the carrying capacity in that area (e.g. the supply of planktonic food) and thus affect all aquatic organisms including the farmed mussels themselves. Conversely, as mussels are primary consumers they can potentially mitigate impacts of nutrient enrichment (e.g. from land-based discharges and run-off) which can often lead to eutrophication of coastal waters.

Coastal areas and estuaries where mussels are farmed are often sites of ecological and high amenity use and therefore any large-scale cultivation may have impacts such as disturbing shore bird feeding sites, or on local navigation. Marine aquaculture operations may also have an aesthetic impact.

In many countries, siting a mussel farm would be restricted in areas with key biological or ecological functions. In the absence of such restrictions, the farmer should implement an environmental management plan to ensure no adverse effects on the ecological integrity of the area, and demonstrate there is no harm to threatened or endangered species and/or habitats. In relation to the carrying capacity of the ecosystem and phytoplankton availability for other aquatic animals, farms should consider stocking at appropriate densities.

Best Management Plans (BMP’s), codes of good practice (often developed by industry groups)⁴, and certification has been used as a means of prevention for unacceptable environmental interactions. In 2017, Loch Fyne Oysters Ltd. in Scotland received the Aquaculture Stewardship Council (ASC) certification for its responsible farming of blue mussels, the first company worldwide to do so⁵. Whilst in 2018, Offshore Shellfish Ltd. on the English south coast was the first European mussel farm to earn Global Aquaculture Alliance Best Aquaculture Practices (GAA BAP) certification⁶.

Different countries regulate aquaculture and enforce policies differently, but often with the same goal of minimising environmental impact. Overall, the content of habitat regulations surrounding mussel culture takes into account environmental impacts and ecosystem services. Similarly, enforcement organisations should be identifiable, permitting and licensing process transparent and based on zoning or planning⁷. It is not clear if such regulations are as effective or well-enforced in all locations (e.g. in some Asia-Pacific countries)⁸.