Scottish scallop stock assessments: 2025 results

Executive summary

This report presents the results of Scottish regional scallop stock assessments carried out by the Scottish Government’s Marine Directorate (MD) based on commercial catch-at-age data and survey data up to and including 2024. Full analytical assessments are presented for the East Coast, North East, North West, Shetland and West of Kintyre scallop stocks, and available data are discussed for the Clyde and Orkney scallop stocks. The report also provides background information on Scottish fisheries for scallops, along with a description of the current management and regulatory framework.

The fisheries

• The king scallop is one of the most important shellfish fisheries in Scotland. In 2024, total landings into Scotland were approximately 7,500 tonnes with a first-sale value of over £18 million (Scottish Sea Fisheries Statistics 2024).
• The most important areas, in terms of recent landings into Scotland, are the North East, North West and Shetland assessment areas, each accounting for around 20% of the total Scottish landings.
• Some areas, such as Shetland, have shown systematic increases in reported landings over time, while in other areas the landings are characterised by substantial fluctuations.
• Since 2022, total Scottish scallop landings have increased in 2024, though still lower than peak values of 2016. The majority of the landings are dredge caught, and it is the reduction in this fishery that is responsible for the overall decline in landings since 2016. Dive caught scallops typically make up less than 10% of the total landings.

Stock trends

• In recent years in the East Coast assessment area, estimated spawning stock biomass (SSB) has increased back to the long-term average despite relatively low estimates of recruitment. Fishing mortality (F), estimated to be one of the lowest of the time series in 2022, is now estimated to be increasing.
• In the North East assessment area, recruitment and SSB have been fluctuating around an average level since estimates of strong recruitment and high SSB in the 1990s. A recent increase in catches has resulted in an increase in F, which is estimated to be well above the long-term average in 2024.
• In the North West assessment area estimates of both SSB and recruitment have risen to be above the long-term average since 2020. Catches have declined from relatively high values pre-2020, and in 2024 F is well below the long-term average due to continued low catches, coupled with an estimated continued increase in recruitment.
• In Shetland, substantially increasing recruitment estimates have resulted in increases in SSB to the highest of the time series. Landings are also the highest in the time series in 2024 but the scale of the increase in SSB has resulted in a decline in F in recent years to well below the long-term average and the lowest since pre-1990.
• In the West of Kintyre assessment area, although recent recruitment estimates remain close to the long-term average, estimated SSB continues to increase, with the highest value of the time series in 2024. This is due to the low levels of catch and resulting estimates of F which remain around the lowest levels since the beginning of the time series.
• There are insufficient data from the Clyde and Orkney assessment areas to perform analytical assessments or evaluate stock trends.

Management considerations

• To provide an overview of stock status, provisional precautionary reference points (F_0.1 and B_pa) have been defined for each stock with F_0.1 used as a proxy for F_MSY. MD advice for the assessed stocks is provided on the basis of these provisional reference points.
• In all areas, stock biomass is above biomass reference points.
• The East Coast, North West, and West of Kintyre assessment areas are estimated to be fished below the F_MSY proxy in 2024. Fishing mortality in Shetland is estimated to be at or just above the F_MSY proxy, while in the North East it is estimated to be substantially above the F_MSY proxy reference point
• For the East Coast, North West, and West of Kintyre assessment areas, both F and SSB are within safe limits relative to provisional reference points. In these areas, there could theoretically be small increases in fishing effort above the 2024 level without there being a detrimental effect on the stock (i.e. maintaining F below F_MSY proxy).
• For Shetland, where fishing mortality is estimated to be around the F_MSY proxy, advice is for no increase in effort above 2024 levels while for the North East, where fishing mortality is well above the reference point, advice is that effort should be reduced.
• For the areas without assessments (Clyde, Orkney and areas not included in the defined scallop stock areas), precautionary advice is also for no increase in effort.

Data and quality of the assessment

• In areas for which sufficient data were available, analytical assessments were conducted using SAM, a state-space stock assessment model. SAM is a widely used stock assessment method and makes use of commercial catch at-age and survey indices by age and can deal with the omission of poor quality or missing data. The estimates of abundance and fishing mortality are calculated with confidence intervals.
• Historical trends estimated by the SAM approach show good agreement with MD’s previous stock assessments.
• Of the five analytical assessments presented, Shetland shows large retrospective revisions (particularly for SSB) while three others show some revisions (East Coast, North East and North West) in either SSB or F. However, given the direction of the retrospective revisions and the biomass estimates relative to the reference points, the conclusions in terms of biomass stock status are likely to be robust. With the exception of Shetland (and North East), stocks are estimated to be fished well below F_MSY and therefore advice related to fishing effort should also be robust to retrospective uncertainty. Given the uncertainty in the Shetland assessment, results and advice should be treated with a greater degree of caution.
• Following reductions in the number of samples across all assessment areas (except Shetland) likely due to the Covid-19 pandemic, there has been some recent improvement in sampling levels since the previous assessment. Although a single year with poor sampling is unlikely to significantly affect the conclusions of the assessment, continued poor sampling levels are likely to result in less precise and potentially biased results.
• The biological population structure of Scottish scallop stocks is not well understood, though recently completed and ongoing PhDs continue to improve understanding. The assessment areas were defined in relation to the characteristics of the fisheries in the past and may not take account of any connectivity between scallop populations or be the most appropriate management units given modern day fishing patterns.