Climate change effects

Human-made climate change due to increased emissions of carbon dioxide (CO₂) is known to induce a wide range of changes in the aquatic environment, triggering chain reactions with the potential to disrupt ecological balance. Its main effects include a measurable increase of water temperatures, altered rainfall, air movement and water circulation patterns as well as ocean acidification. Taken together, these phenomena can degrade the environmental conditions on which mussels depend to thrive. The European continent is warming faster than any other part of the world, with a particularly pronounced rise in temperatures around the Mediterranean Sea.

Seawater temperatures above 20°C for M. edulis (blue mussel) and 24°C for M. galloprovincialis (Mediterranean mussel) increase mortality risk in mussel farming, as shown by scientific studies. Temperatures above the seasonal average in both summer and winter have been observed in all mussel-producing countries. For instance, the exceptionally high mussel mortality rates recorded at the end of the summer in Italy in 2024, leading to a loss of 75-100 % of farmed mussels, resulted from a combination of winter temperatures 2-4°C above average and an exceptionally hot, windless summer that pushed water temperatures above the tolerance thresholds of 30-31°C. While increased temperatures are not necessarily lethal, they can stress farmed mussel species, resulting in reduced growth rates and greater susceptibility to diseases.

Moreover, higher water temperatures give rise to a number of other negative effects, such as lower levels of oxygen, which deteriorate the living conditions for many aquatic organisms. In addition, non-native warm-water-affinity species can spread more easily and displace indigenous species, with changes in ecosystems and disruptions of marine food webs as possible consequences.

Furthermore, considerable amounts of CO₂ released into the atmosphere are absorbed by the oceans. The gas dissolves in the seawater, where carbonic acid is formed. A more acidic water environment reduces the availability of calcium carbonate, thus making it more difficult for mussels to produce and maintain their calcareous body parts.

Nutrient availability

Mussels feed on phytoplankton, also known as microalgae, which in turn require nutrients, such as phosphorus and nitrogen, for growth. Nutrients can enter the sea via rivers or be released during microbial recycling processes, for example, from the sediment layer on the seabed. Reduced rainfall during the summer months, as has been observed in recent years, for instance in France and Italy, has decreased the inflow of nutrient-containing freshwater and thus the availability of phytoplankton on which mussels feed.⁷ Scientists also attribute weakened byssal threads to the lack of nutrients: mussels that lose their ability to attach to the substrate and instead float around in the water can be considered lost for producers.

Changing patterns in coastal upwelling have been identified as a further cause of nutrient deficiency.⁸ Coastal upwelling, a process that typically occurs in coastal regions, brings cold, nutrient-rich deep water to the surface. The high productivity of mussel farming along the Galician coast is particularly strongly associated with this phenomenon. A marked reduction in coastal upwelling was observed in 2023 and 2024 in the Spanish mussel-farming region of Galicia, and can, together with exceptionally high water temperatures, be linked to poor growth, low meat content and detachment from farming structures, leading to historically low production levels during these years.

However, nutrient levels must be kept in balance. Excessive nutrient inputs into aquatic environments can lead to the opposite effect, giving rise to a rapid and massive growth of algae.

Harmful algal blooms (HABs)

Certain types of microalgae produce and release toxins that can be harmful to the organisms living in the same aquatic environment. The rapid and excessive growth of algae causing HABs has become a major environmental problem worldwide over the past two decades and is likely to increase in geographical extent and frequency. Once released into the water, these toxins are taken up by the marine organisms and can accumulate in their flesh, particularly in filter feeders such as mussels. Thus, when harmful algal blooms occur and potentially threaten human health, local authorities often close shellfish harvesting areas as a measure to prevent further damage, resulting in significant economic losses for the farming business.

Furthermore, the decay of large quantities of algae requires oxygen, which is taken from the surrounding water, thereby decreasing the oxygen available for living organisms, and in extreme cases leading to 'dead zones', with little or no oxygen.

There is still some uncertainty as to the causes of algal blooms. While the link with over-availability of nutrients seems to be confirmed, there is no conclusive evidence of the connection with increased water temperatures. Other possible explanations include overfishing and its disruptive effect on the food chain: declines in larger predatory fish are followed by significant increases in their prey, namely smaller fish, and, in turn, by decreases in algae-consuming herbivorous grazers, which are prey for the smaller fish.

Predation

Natural predation by fish, seabirds and other species can lead to substantial losses for farmers. In particular, suspended culture seems to attract various predator species.

In the French regions of Normandy and Brittany, a significant abundance of spider crabs has caused considerable damage to bouchot farms since 2016.⁹ Although this phenomenon is not directly linked to climate change, it is assumed that certain environmental changes favourable to this species have contributed to its proliferation.¹⁰

Another major concern for mussel farmers are gilthead seabream and black seabream, which can cause severe losses, as experienced by some farms on the French Mediterranean coast.¹¹ The seabream also preys on baby mussels and is considered a major cause of the scarcity of spat in commercial mussel farms in recent years. Further predators feeding on mussels include the common starfish, which can consume large quantities of young mussels in particular. The impact of starfish may increase as average winter temperatures rise due to climate change.¹²

Pollution

Pollutants can have considerable impacts on marine ecosystems, affecting the natural environment of marine coastal areas in particular. Pollution source include both land and sea-based activities ranging from waste disposal, agricultural run-off, industrial activities, inadequately treated wastewater, intense maritime traffic, oil and gas extraction, to abandoned fishing gear. Pollutants comprise physical matter (in particular, plastic litter, including microplastic particles); chemical substances (e.g. heavy metals, pesticides, per- and polyfluoroalkyl substances (PFAS)) and biological substances (e.g. excessive amounts of nutrients such as phosphorus and nitrogen), as well as emerging pollutants such as nanoparticles and pharmaceuticals.

Given that mussels feed by filtering, they are particularly exposed to the substances present in the water they live in. For instance, they can accumulate heavy metals in their soft tissue and finally transfer it through the food chain to other species, including humans as the final consumer.¹³ The ingestion of microplastics by mussels and their transfer to other organisms along the food chain have been reported; however, the health implications for marine organisms and human consumers have not yet been fully determined.

Diseases and parasites

By filtering water from their environment, mussels easily take up and accumulate bacteria or viruses that possibly affect their own growth and life span as well as consumers' health. Pathogens are often derived from land-based activities and result, for instance, from insufficient wastewater treatment or diffuse agricultural run-off.

The mass mortality events that have affected mussel farms on the French Atlantic coast since 2014 are probably related to the presence of pathogenic bacteria and parasites, including novel ones. The presence of the norovirus has also repeatedly been reported. Mussels' immune response, which allows them to cope with pathogenic microorganisms, relies exclusively on innate immunity. Unlike vertebrates, mussels cannot develop acquired immunity. Consequently, vaccination is not possible, nor can disinfection treatments be applied in open-water environments.

The depuration technique, which allows mussels to purge contaminants, is not effective against viral contamination.¹⁴

Operational issues

Seafood farming involves compliance with a large body of legislation, ranging from rules on the use of space and water, animal health, food safety and environmental protection, to rules on organic production and marketing. Alongside the comprehensive legal framework at EU level, a large part of the rules for the aquaculture sector is established at the national, regional and local levels. Moreover, legislation is not always consistent across Member States or even among authorities within the same Member State. The high administrative burden for aquaculture businesses – including, for instance, the procedures for obtaining or extending licences – has repeatedly been highlighted as costly and burdensome.

Moreover, in a context of continued coastal development and the rise of the blue economy, aquaculture faces strong competition for space with other users. Mussel production in particular can be more space-demanding than other aquaculture activities and often loses out to other sectors of the blue economy that benefit from a higher priority at the national level (such as coastal tourism) or enjoy greater social acceptance.

While the different production types vary with regard to their cost structures, labour and energy are the main cost components for most mussel-farming businesses. As in fisheries, personal and family-owned businesses rely to a large extent on unpaid work done by family members. In raft culture, the imputed value of unpaid labour is 43 %, while in the large and professionalised companies informal labour is rare. Energy costs equally vary depending on the production technique and are highest for longline production. Moreover, a sudden rise in energy prices, caused by geopolitical crises, can increase financial burdens for businesses.

On the other hand, none of the mussel-farming techniques involve any feed costs. However, the relatively simple production technology gives producers limited control over the production process, making them dependent on good environmental conditions. The traditional production system offers limited opportunities for economically viable innovation and productivity growth.

Market-related changes

In addition to the inherent challenges, the EU mussel sector is facing pressure from international market dynamics. Over the past two decades, Chile has emerged as a strong competitor on the world market, having seen an 18-fold increase in its mussel production (Mytilus chilensis) between 2000 and 2022 (from 24 000 tonnes to more than 428 000 tonnes). This country, which is the world's second-largest mussel producer, exports more than 90 % of its production and is displacing, in particular, the Galician mussel from the EU market for frozen and processed mussels.

Between 2020 and 2024, EU mussel imports increased by 39 % in volume and by 45 % in value, mostly due to the increase in imports of prepared/preserved mussels. EU imports from third countries came almost exclusively from Chile, accounting for 86 % of import volume in 2024. Over the same period, EU export of mussels decreased by 23 % in volume, while slightly increasing in value (6 %).

Moreover, following the United Kingdom's (UK) withdrawal from the EU, exports of live mussels from EU producers to the UK declined sharply due to stricter sanitary and classification rules. By 2024, UK-bound trade had recovered only partially, with exporters seeking to avoid new sanitary and customs rules by shifting trade towards EU markets.

Innovation

Current technological and research projects aim to make mussel farming more resilient to climate change and other challenges. For instance, the EU co-funded research project ShellFishBoost focuses on the development of advanced tools for genetic selection targeting key traits such as growth, and resistance to heatwaves, salinity stress and harmful algal toxins. Other projects aim to assess the availability of mussel seed, to improve natural recruitment or enhance mussels' immune mechanisms. Projects with a broader focus seek to find innovative solutions for climate change adaptation in the aquaculture sector.

Another option for innovation in the mussel sector could be moving to offshore sites. Areas further away from the coast can reduce the competition for space, provide cleaner water and enable operation on a larger scale. However, this option poses new risks such as harder offshore conditions or additional costs due to the long distance from the shore. The integration of aquaculture activities within an offshore wind park is another approach, which has already been tested in countries such as Belgium and the Netherlands.

Consumer demand

Following growing consumer demand for local and/or organic production, a shift towards even more sustainable production might set the tone for the future. This could, for instance, include an expansion of off-bottom culture systems or the deployment of Integrated Multi-Trophic Aquaculture (IMTA)-arrangements, whereby different species – for example, seaweed and mussels – are co-cultured for mutual benefit. This could go hand in hand with improved consumer information in the form of certification and labelling.

Political awareness

In a 2026 resolution, the European Parliament called on the European Commission to better integrate aquaculture into maritime spatial planning and to improve the sector's operational conditions. In a 2022 resolution, Parliament highlighted the role of aquaculture in the transition to a sustainable food system in Europe and called, among other things, for greater investments to mitigate the impact of disasters and extreme weather. In another 2022 resolution, Members of Parliament stressed the importance of aquaculture within the blue economy. In its answers to recommendations by the Aquaculture Advisory Council, the European Commission refers to a number of recent initiatives at the EU level aimed at addressing climate-related challenges in particular.