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.
