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Bivalves boost biodiversity

Boosting biodiversity

Emma V. Sheehan, Danielle Bridger, Llucia Mascorda Cabre, Amy Cartwright, David Cox, Sian Rees, Luke Holmes and Simon Pittman of the University of Plymouth explain the ecological and social benefits of offshore bivalve farming.

The development potential for sustainable food production in the ocean is vast, with aquaculture capable of meeting global seafood demand using less than 0.015% of the total ocean area[1].

Marine bivalves, such as mussels, oysters and scallops, have become one of the fastest growing animal-food sectors and are increasingly sought after by consumers due to their taste, high nutritional value and perceived positive benefits to the environment. Global production of marine bivalves for human consumption has grown from less than 1m tonnes per year in 1950 to more than 16m tonnes per year in 2016, with 89% produced through aquaculture (Figure 1)[2].

In addition to food supply, there is also growing awareness of the potential wider ecosystem benefits of bivalve aquaculture, including regulating services such as nutrient remediation, carbon sequestration and coastal defence[3]. In fact, non-food ecosystem services provided by bivalve aquaculture globally have been estimated to be worth between $2.95bn to $9.99bn USD per year[3]. The urgent need for sustainable food production systems capable of meeting a growing global demand for animal protein whilst removing the negative environmental impacts associated with wild bivalve harvesting[4] and supporting a ‘Blue Growth’ agenda[5] has boosted interest in bivalve aquaculture. Traditionally, bivalve aquaculture had largely been established in inshore areas either on the seabed or on structures fixed or floating in shallow sheltered waters. In this environment there have been notable negative environmental impacts. For example, farming bivalves in sheltered, poorly flushed inshore waters can result in the accumulation of waste products that pollute the seabed and reduce local biodiversity[6].

Figure 1 Global bivalve fishery production 2000- 2016(2) (mussels, oysters and scallops)
Blue mussels, top right, on a rope in Lyme Bay

Offshore bivalve farming

Recently, however, there has been a significant global offshore expansion of the industry with many large-scale aquaculture farms now operational in deeper and more exposed offshore waters in the UK, China, Canada, USA, New Zealand, France and Japan. In contrast to inshore waters, the offshore aquaculture industry is perceived as a more space-efficient and lower impact method to produce seafood[7]. Offshore bivalve aquaculture has considerable growth potential especially in areas where planktonic food is plentiful, water conditions including depth are suitable and likelihood for conflicts over space use is low[7]. The rise of offshore aquaculture has been driven in part by technological innovation and also a desire to increase sustainable food production. In 2018, a global seafood consumer survey highlighted that consumer demand patterns are changing, with 83% of seafood consumers agreeing that there is a need for sustainable seafood and 72% wanting independent evidence to support claims for sustainability[8].

Offshore bivalve farming, such as cultivating rope-grown mussels, is a relatively new mode of bivalve aquaculture being promoted as one of the lowest environmental impact animal protein production methods available because the species feed naturally in the ocean and can be harvested with relatively low fuel requirements. Mussel farms in general have also been highlighted as a climate-friendly method of food production with greater potential as a carbon sink for large-scale offshore farms. While mitigating climate change through bivalve aquaculture is currently not the primary driver for industry growth, it will become an increasingly important consideration in food production. A recent report by the Scottish Aquaculture Research Forum[11] showed that mussel farms have the lowest carbon footprint in animal food production with high carbon sequestration in the shells of the mussels estimated at 218kg CO2-eq per tonne of mussels harvested.

Sustainable production

Understanding the complex spectrum of ecological and social consequences of aquaculture is key to determining the sustainability of the industry[10,12] and to secure its role in global food production. International organisations, such as the Global Aquaculture Alliance, support sustainable growth of food production by providing guidance on best-practice for responsible and sustainable aquaculture and a certification scheme for facilities that are managed in an environmentally, socially and economically responsible manner. The success of certification schemes hinges on robust and transparent evidence of sustainable practice. The future growth of offshore aquaculture as a sustainable low-impact production method will in part depend on addressing concerns over perceived environmental impacts, competition for ocean space and evaluation of benefits to inform policy in permitting and regulations.

Assessing sustainability for offshore aquaculture provides a wealth of new opportunities for scientists to collaborate with industry to help steer growth toward sustainable practice and support decision makers in managing ocean space for a balanced ‘blue growth’. Robust monitoring programmes capable of providing reliable evidence of the positive and negative impacts of offshore aquaculture to inform recommendations for best practice are crucially important to accomplish sustainable growth of marine bivalve production. To date very limited research has been conducted on environmental change associated with offshore aquaculture, despite ‘blue growth’ strategies calling for greater offshore development. The UK is currently one of the largest producers of aquaculture products within the EU, with farming of mussels (inshore and offshore) being a major contributor to the UK shellfish aquaculture sector accounting for 95% of the total shellfish tonnage in 2012 and 80% of the total income[13]. UK aquaculture rope-grown mussels are included on the Marine Conservation Society ‘Best Choice Top 10’ list recognised with the highest rating for sustainability[14].


In the UK, unique partnerships between scientists from University of Plymouth and mussel farm entrepreneurs and scallop ranchers are exploring whether offshore bivalve aquaculture can not only be an efficient producer of seafood but also capable of delivering a ‘win-win’ outcome with gains for marine biodiversity and spill-over benefits to neighbouring capture fisheries. The research team, which specialises in marine assessment and monitoring, has developed robust monitoring methods and analyses designed to inform the offshore aquaculture industry and marine managers of both positive and negative impacts associated with aquaculture practice.

Using a range of underwater survey vehicles and sampling techniques, the team has been measuring the ecosystem effects of an offshore mussel farm in Lyme Bay (South West England), since it was established in 2013, and a scallop farm in nearby Torbay since 2015. The mussel farm, founded by Offshore Shellfish Ltd, is located between three and six miles offshore. The farm has developed an innovative suspended rope technology to cultivate the native blue mussel, Mytilus edulis, the larvae of which live in the plankton and naturally settle and colonise the ropes. Once fully completed, the farm will be the largest of its type in European waters, covering a total area of 15.4 km² and growing 10,000 tonnes of mussels per year. The scallop farm, founded by Scallop Ranch Ltd, is an experimental project to determine whether a sustainable and commercially viable alternative to traditional scallop dredging can be created and replicated. The farm is 0.18 km² in size and the scallops are grown in lantern nets suspended from secure lines. The young scallops (known as ‘spat’) are bred in a nursery from local wild stock before being placed in the nets. Both the mussels and scallops then filter feed on the surrounding seawater requiring no additional food or chemicals.

Mussel farmers at work


Early observations by the University of Plymouth team demonstrate that the mussel ropes, home to millions of blue mussels, also attract a high diversity of other animals and plants that settle on and live around the ropes which, between harvests, form a unique and species rich ecosystem – a floating reef. Underneath the ropes, crabs and lobsters find shelter and feed on the mussel clumps that fall to the seabed. Similarly, the submerged structure of the scallop ranch is also attracting mobile predators. During intensive long-term monitoring of the farms, the team has used a range of underwater video techniques, collected mud samples from the seabed, and carried out visual surveys of seabirds, seals and dolphins to monitor a range of trophic levels in the ecosystem. One key finding to emerge is that the farms have increased the number of mobile predators in the area. Video images indicate that the vertically hanging ropes act as fish aggregation devices and large shoals of Atlantic horse mackerel (Trachurus trachurus) are frequently seen swimming around and feeding on the ropes. Larger predatory fish, including European bass (Dicentrarchus labrax) and grey mullet (Chelon labrosus), have also been observed around the ropes. Underwater filming using a remotely operated vehicle revealed that commercially important brown crab (Cancer pagurus) feed on the mussel clumps resulting in increasing numbers of crabs at the farms.

We now know that crabs, lobsters and predatory fish utilise these sites for food and shelter, but these observations are still only brief snapshots in time relative to the daily activities of these animals. We know very little about how these predators move around the farm, how long they remain resident, or if they move off out into fishing grounds or to the nearby MPA (Marine Protected Area). By offering a refuge function, the farm may function as a de-facto MPA by replenishment of local fished populations (referred to as a ‘spill over’ benefit). To investigate these important questions a new project, ROPE (Response of Predators to Protection and Enhancement), funded by the European Maritime and Fisheries Fund via the UK Marine Management Organisation, applies innovative acoustic tracking technology to understanding how the UK’s largest offshore rope-cultured mussel farm influences the movements of commercially important fish and crustaceans. Working in collaboration with the local fishermen and the mussel farm owners, the project will tag and track the movements of seabass, brown crab, lobsters and crawfish for up to two years. The tags attached to the animals emit a unique coded ‘ping’, which is recorded and monitored by static underwater listening devices, or acoustic receivers, placed around the farm, inside the nearby MPA and across fishing grounds in Lyme Bay. To supplement the acoustic tracking, we also use thin plastic coded tags that are reported back to the team when tagged crabs and lobsters are caught by fishermen. This unique project has received strong support from the seafood industry, marine conservation groups and government agencies with jurisdiction for effective marine planning and licencing of commercial aquaculture.

Vertically hanging ropes act as fish aggregation devices and large shoals of Atlantic horse mackerel (Trachurus trachurus) are frequently seen swimming around and feeding on the ropes.


All sectors support the view that robust scientific evidence will improve our understanding of marine ecosystems and this information is vital to underpin policies and strategic decisions for sustainable and responsible use of our oceans. Our initial observations suggest that offshore bivalve aquaculture can potentially have a wide range of beneficial effects for the environment and may enhance the catch of local fisheries while having lower negative impacts than other methods of marine food production.

The science-industry partnership in Lyme Bay is aiming to increase recognition of the potential for offshore aquaculture to play a significant role in achieving local conservation objectives, as well as demonstrating that offshore aquaculture can meet sustainable development goals. Early indications are that offshore bivalve aquaculture, particularly when strategically located over already degraded seabed or areas with low conservation value, has potential for achieving net environmental gains, supporting thriving oceans and achieving prosperous and healthy fisheries. As the blue economy continues to grow, more research will be needed to provide sound advice on the feasibility of co-locating offshore aquaculture with other interests, such as MPAs, offshore renewable energy installations and other fisheries.

University of Plymouth is developing and applying survey tools and scientific techniques through collaborative work with food producers to support progress in sustainable aquaculture in the UK and beyond. The bivalve aquaculture industry clearly has an important role in the blue economy as a sustainable food production method, where a healthy ocean is essential to economic success. As Dr Sandra Shumway, Editor of the Journal of Shellfish Research, wrote ‘shellfish growers are committed to water quality – quality of their product and quality of the environment – from the day the molluscs spawn to the day the finished product is eaten by the consumer’[12].

Emma V. Sheehan1*, Danielle Bridger1, Llucia Mascorda Cabre1, Amy Cartwright1, David Cox1, Sian Rees1,

Luke Holmes1 and Simon Pittman1

1 Marine Conservation Research Group, School of Biological and Marine Sciences, University of Plymouth, United Kingdom *corresponding author More information on the work in Lyme Bay can be found at: https:// projects/

Twitter @Dr_Emma_Sheehan


  1. Gentry, R.R., Froehlich, H.E., Grimm, D., Kareiva, P., Parke, M., Rust, M., Gaines, S.D., Halpern, B.S. 2017. Mapping the global potential for marine aquaculture. Nature Ecology & Evolution, 1:1317-1324.
  2. FAO. The State of World Fisheries and Aquaculture 2018. Food and Agriculture Organization of the United Nations, Rome, Italy.
  3. van der Schatte Olivier, A., Jones, L., Vay, L.L., Christie, M., Wilson, J. and Malham, S.K., 2018. A global review of the ecosystem services provided by bivalve aquaculture. Reviews in Aquaculture.
  4. Sciberras, M., Hiddink, J.G., Jennings, S., Szostek, C.L., Hughes, K.M., Kneafsey, B., Clarke, L.J., Ellis, N., Rijnsdorp, A.D., McConnaughey, R.A., Hilborn, R., Collie, J.S., Pitcher, R.C., Amoroso, R.O., Parma, A.M., Suuronen, P., Kaiser, M.J. 2018. Response of benthic fauna to experimental bottom fishing: A global meta-analysis. Fish and Fisheries, 19(4), 698–715.
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