Mussels

At a global level the farmed mussel industry produced some two million tonnes of mussels in 2015, and almost all of this production relied on wild spat. Variability in spatfall has resulted in production fluctuations and business uncertainty problems in many mussel producing countries. With few measures being taken to overcome this fluctuation, it has generally been tolerated by growers¹.

However, to combat the spatfall variability a number of countries are now intervening to try to better understand their mussel settlement patterns and to optimise use of the resource through¹:

• Studies and surveys of mussel larvae and seed beds (Chile, Canada, USA, the Netherlands, UK, Ireland)
• Use of dedicated spat collection sites (Chile, Canada, USA, Netherlands, UK, Ireland, France, New Zealand)
• Novel collection devices (the Netherlands, Norway)
• Controlled reproduction in hatcheries (Canada, USA, France, the Netherlands, Australia, New Zealand

The process of mussel aquaculture starts with the collection of spat from the wild. This is undertaken either by natural or passive settlement on purpose-made spat collectors (rope, plastic mesh, etc. attached to a long line and hung in the water column), or active collection from large, naturally occurring accumulations of mussel seed, which is dredged from sub-tidal zones. An example of a well-managed and sustainable mussel dredge fishery to supply seed for on-growing is Bangor Mussel Producers Ltd. of North Wales, who have been Marine Stewardship Council (MSC) certified since 2010².

Whilst passive collection is more common globally and is not considered to have any negative impacts on wild stocks, active collection does have the potential to impact wild mussels and the environment due to seabed disruption. This can result in increased burial of organic material and anoxic seabed conditions, increased turbidity, as well as dislodging mussels and non-target organisms which may cause increased stress or mortality³.

Potential impacts of mussel dredging and the subsequent transfer of spat to on-growing sites can be habitat loss or degradation through physical damage, and the introduction and spread of pathogens, parasites, and/or non-native invasive species^{4, 5, 6, 7}. Dredging and relaying must be carried out in ways that minimise these risks.

Generally, wild seed collection only occurs in areas where the accumulation of mussel seed would not persist naturally; so called ‘ephemeral’ accumulations. Stable naturally occurring beds of mussels are considered to be biological reefs and thus are often protected features. Dredging of wild seed mussels for relaying and on-growing is subject to increased scrutiny in Europe under, for example, the requirements of both the Habitats Directive and the Birds Directives which are cornerstones of Europe’s nature conservation policy and which establish the EU Natura 2000 network of protected areas^{8, 9}. Access to any settlement of mussel seed, even from traditional areas, is no longer a foregone conclusion as a variety of management and scientific assessments have become part of the process¹⁰.

Although mussel culture affects the population dynamics of sub-tidal mussels in an area by relaying in specific plots, this activity results in a higher mussel biomass than on natural beds and enhances the total stock which favours ecosystem services, including the provision of biodiversity and a high-quality food source for birds¹¹. Also, when it comes to harvest, bottom cultivated mussels are generally harvested with dredges applying minimal force and penetration into the sea bed¹².

Another important safeguard is to have biosecurity management plans in place at individual farm and area level. The key elements of biosecurity include: practical and appropriate legislative controls; adequate diagnostic and detection methods for infectious diseases; disinfection and pathogen eradication methods; reliable high quality sources of stock; and best management practices^{13, 14}. Management measures include those that are regulatory (e.g. lease conditions and permit requirements) but also the use of voluntary agreements and codes of practice; the Bangor Mussel Producers Association Code of Good Practice for Mussel Seed Movements^{15, 16} is an excellent example of the latter.

Also important are designations to protect sensitive marine habitats. For example in the UK these include Marine Protected Areas (MPAs), Special Areas of Conservation (SACs), Special Protected Areas (SPAs), and intertidal areas identified as Sites of Special Scientific Interest (SSSIs)¹⁷.

Mussel seed hatchery production is now possible and although it is currently small-scale globally³, it may be a future mechanism by which a reliable supply of quality, disease-free and traceable spat can be supplied to mussel farmers^{1, 18}. However, these hatcheries would need to produce extremely large numbers of spat to sustain what are already significant industries in many countries.