Blue mussel

Wild blue mussel populations in Dorset are data limited. There is concern for biomass of mussels due to the lack of information. However, there is no concern for fishing pressure. Mussel cultivation involves collecting juvenile, or ‘seed’ mussels from natural beds and relaying them in another area. There, they are left on the seabed until they are large enough to be removed and sold to the market. Usually, dredges are used for both seed collection and final harvesting. Between 2016 and 2020, most wild UK mussels were cultivated using this process, accounting for 92% of all mussel landings. 15% of the landings were from Poole Harbour in Dorset. This rating covers mussels cultivated in Poole Harbour from wild seed. Mussel dredging in this area has taken place since 1991.  Route 2 (data limited) scoring has been applied to this rating owing to the lack of a reference point for biomass and uncertainty in the data. Blue mussels are considered to have moderate vulnerability to fishing (scoring 36 out of 100). The structure of blue mussel stocks is complicated. There appears to be no clear genetic separation over the species’ range, suggesting no clear stock structure. Following a period as larvae in the water column, they settle in beds. While some beds are permanent, and likely to be a source of larvae, others change location depending on water conditions and may disappear entirely each winter. These are challenging to monitor because locations and sizes change. Stock scoring for this rating is focused on seed mussel beds. In the Southern IFCA district, there appears to have been no assessment or survey of seed mussel beds since 2015. The bed disappeared entirely during the winter of 2014-2015, which was attributed to storms, although there is no published research available to confirm the cause. Information relating to the beds since then has been anecdotal, but indicates that the bed is now the same size as it was prior to winter 2014/15. As there is no actual data to indicate the current extent and density of the mussel bed, and no indication of what would be considered a sustainable size, we consider there to be insufficient data relating to stock/bed size in this area. We therefore consider there to be concern for biomass. There is limited available data about fishing pressure on mussels in the Southern IFCA district. The IFCA reports that average annual removals from 2001-2010 were around 2,000 tonnes. In 2011 an annual catch limit of 2,000t was introduced. From 2011-2013, surveys indicated that fishing activity had not caused changes in density or population structure. Catches appear to have declined after this, with a total of 70t of mussels landed between 2016 and 2020 in the Southern IFCA area. In recent years catches have increased to around 400t, and the catch limit has been set at 1000t. Catch records do not indicate a decline in CPUE, and the IFCA has therefore decided that the fishery is sustainable at these levels. As recent catches are below long-term averages, and CPUE is stable, we conclude that there is no concern for fishing pressure. In this area, seed mussel beds are close to the Studland to Portland Special Area of Conservation (SAC). Because of the proximity to this marine protected area, a Test of Likely Significant Effect (TLSE) must be carried out to assess the impact of dredging. The most recent TLSE was carried out in 2022. It recommended that fishing could occur if catch limits and restrictions were adhered to. However, this is not an assessment of stock size or fishing pressure and therefore does not affect the stock score.  After seed mussels have been relain for on-growing, they are able to keep producing eggs until the final harvest. This means that they can contribute to the wider mussel population. The areas selected for relaying are sheltered, and therefore relain mussels are more likely than those on wild beds to reach maturity and breed. Therefore, seed harvesting is not expected to compromise overall stock levels.  Until recently, it was assumed that mussel beds do not survive winter months and are entirely removed each year by natural processes. Fishing of the seed beds, which also removes most of the bed, was considered to replicate natural patterns. However, recent data has indicated that some mussels do survive the winter. Further evidence is therefore needed to confirm the impacts of harvesting on wild mussel beds and how this affects stock levels, habitats and reproductive capacity. If stability and productivity of seed beds is shown to be impacted by fishing, this could be cause for increased concern for stocks.

There are several management measures in place for wild mussel harvesting in the Southern IFCA district. However, it is not possible to fully assess the appropriateness or effectiveness of management measures due to the challenges in monitoring wild mussels.The majority of dredged mussels in the UK are from the coast around Northern Ireland. MMO data from 2016-2020 indicates Northern Irish landings were 1397 tonnes, with smaller fisheries around the coasts of Dorset (70 t), Cornwall (25 t) and South Wales (17 t).In Dorset, within the Southern IFCA district, the fishery dredges wild seed, which is then transplanted to licenced on-growing sites. Once at market size the mussels are then harvested, predominantly by dredge. The management of both seed and on-growing (aka transplanted or relay) sites is considered in this criterion.There is no harvest strategy for blue mussels in this area, and there appear to have been no recent surveys of wild mussel beds. Transplanted mussel beds are often private, meaning that publicly available information about these beds is limited. There are, however, a number of management measures to control fishing activity in mussel seed and relay sites.For seed mussels:Applications must be made to remove seed mussels. Southern IFCA set a catch limit of 1,000 tonnes in 2021 and 2022, and catches appear to have been within this limit at 380t in 2021 and 430t in 2022.There are area restrictions limiting where mussel seed fishing can take place.Mussels smaller than 50mm cannot be removed unless they are going to be relain.For relaying:All mussels must be relayed for a minimum of 6 months on a Several Fishery lay within Poole Harbour.Transplanted shellfish beds must be established before being fished, and permits are required to dredge areas where mussels have been relain.Other measures include:Fishers must notify SIFCA before they start fishing for mussel, and they must have satellite tracking (Vessel Monitoring System, VMS) switched on during fishing.There are gear and usage restrictions for dredges to ensure only shellfish are caught.Fishing is prohibited in some areas to protect vulnerable habitats like seagrass, although exceptions for relaying may be allowed.Habitat Regulation Assessments set conditions about how and where fishing can take place.Much of the management of mussel harvesting has focussed on the effects of fisheries on marine protected areas (MPAs). This has been considered an appropriate approach, as seedbeds have been considered ephemeral (short lived) and unlikely to survive winter storms or predation. It was therefore thought that complete removals of beds by fishing would replicate natural conditions and would not increase pressure on wild stocks. However, some seed beds have been shown to survive between years, indicating that the beds may not all be ephemeral. Additional research in the Wadden Sea indicates that fished beds are less stable over time than those not fished. As the sustainability of the fishery is predicated on the understanding that seed beds are ephemeral, further evidence is needed to support this management approach.Alternative seed sources are being explored, such as using seed settled on ropes, but no information regarding the adoption of this is available.Further evidence is needed to confirm that the current management approaches are appropriate for this fishery.

Blue mussels are predominantly harvested using dredges. Fishing using dredge is likely to cause habitat impacts as it disturbs sediments and benthic species. Some management measures are in place to protect vulnerable habitats and to reduce bycatch.Mussel cultivation involves collecting juvenile, or ‘seed’ mussels from natural beds and relaying them in another area. There, they are left on the seabed until they are large enough to be removed and sold to the market. Usually, dredges are used for both seed collection and final harvesting, meaning that the seabed is dredged in two different places during the process. Between 2016 and 2020, most wild UK mussels were cultivated using this process, accounting for 92% of all mussel landings.Dredges used to harvest mussels are often a different design from those used in other fisheries and have been shown to have a lower impact. Due to the approach for harvesting seed and mature mussels, we understand that this lower impact dredge gear is used in the Poole harbour fishery.Mussel dredges do not use teeth, which is the main source of habitat damage in scallop dredging. However, studies indicate that mussel dredging results in 2-5cm furrows in the seabed. Medium- and long-term impacts appear to vary. In Denmark, species diversity on the seabed was reduced where mussel dredging occurred less than 4 months prior. However, this has not been shown to affect the sediment texture or organic content. In areas not dredged for over 4 years, results varied, with one area showing long term impacts on the epibenthic community whilst another did not. A more recent study showed changes in species and reduced biomass. In all dredged areas included in one study, there was less shell debris and gravel, which may impact settlement of bivalves and other species that attach to shells.There is also evidence that disturbance of mussel bed structure reduces growth and accumulation of biomass in the mussels directly. In high densities, mussels can bind to some areas of seabed and provide a habitat for many other species. Through seabed disturbance, mussel collection has the potential to impact habitats including wild mussel beds, sediments and wider associated species.More broadly, studies on dredging have indicated a wide range of habitat impacts, although they are not specific to mussel dredges. The impacts vary, depending on the ecosystem, including type of seabed, the species that live there, and the level of natural disturbance from waves and storms. In more disturbed areas, the habitats and species may be faster growing and more easily able to recover. Sheltered and inshore areas are more likely to have slow-growing, delicate species, like maerl, seagrass, horse mussels and seafans. Sandy areas are generally less sensitive and more easily able to recover. Gravelly seabed communities are more susceptible because they are quite stable and therefore more likely to have larger, longer-lived species.There are some management measures in place to protect sensitive habitats from seed fishing. In some areas, seed fishing is prohibited, including in marine protected areas (MPAs). Where mussel dredging takes place near MPAs, Tests of Likely Significant Effect (TLSE) must be undertaken to understand the impact on protected features. Within MPAs, Habitats Regulation Assessments (HRA) (England) are used to assess impacts of the fishery (alone or in combination with other activities) on the protected features. Relay sites and fisheries can only be opened if the assessments confirm that the fishery will not cause adverse impacts on habitats.In areas where mussels have been relain, the low frequency of dredging activity at relay sites is identified as having a low risk of affecting the benthic community. There are some indications of positive impacts from relaying. There are records of an increase in certain species, i.e. small polychaete worms, after more intensive dredging activity, which may benefit protected species. If relay locations restore lost bivalve beds, some changes could be positive as these filter feeders recover ecosystem services and habitats. Mussel cultivation has been shown to significantly change the occurrence of some species of the infaunal community, which in some areas has been shown to increase in the number of epibenthic species.Reports suggest that farming of mussels may provide a direct food source for species such as Goldeneye ducks, which are in unfavourable condition, and curlew and redshank. There is no information about approaches fishers may use to control this predation. Relay sites for mussels may also attract predators, such as crabs and starfish. They may be removed using pots, dredges or mop like tools to entangle starfish.Deposits from shellfish beds, known as mussel mud or pseudofaeces, will change the local environment. Most mussel farms occupy shallow coastal waters with soft sediment, dominated by detrital organisms. They are not expected to be negatively impacted by the deposits.Through translocating mussel seed to cultivation sites, there is some risk of introduction and spread of pathogens, parasites, and/or non-native invasive species. Available information indicates that the risks of this are currently relatively low for mussel relay.Non-target and endangered, threatened and protected (ETP) species are rarely reported in mussel dredge fisheries. Monitoring in the Irish Sea showed negligible impact on non-target species. In the Southern IFCA district, specific gear restrictions limit the catch to predominantly shellfish.Where extensive mussel fisheries occur, the depletion of wild seed stocks may have an effect on trophic interaction (food chain links) within the system. More information is needed to assess this.