Scottish scallop stocks: results of 2023 stock assessments

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 2022. 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 Scottish commercial dredge fishery for the king scallop (Pecten maximus) began in the 1930s in the Clyde. It has since expanded around the coast of mainland Scotland and its islands to become the second most valuable shellfish fishery in Scotland. In 2023, total landings into Scotland were in excess of 7,000 tonnes with a value at first-sale of over £19 million (Scottish Sea Fisheries Statistics 2023).
• The most important areas, in terms of recent landings into Scotland, are the North East, North West and Shetland assessment areas. Along with the Irish Sea and West of Kintyre, landings from these areas made up over 80% of landings into Scotland in 2022.
• 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. A number of historically important scallop areas show a reduction in landings in recent years. Some of the historical landings fluctuations are associated with fishery closures due to the presence of amnesic or paralytic shellfish toxins, but others appear to be associated with strong/weak year classes and subsequent changes in overall stock abundance.

Stock trends

• In the East Coast assessment area, although recruitment is estimated to have been below average since 2015, spawning stock biomass (SSB) has remained fairly stable at just below the long term mean during that period. Current estimates of recruitment for 2021 and 2022 remain low. The stable stock size coupled with the decreasing landings over recent years results in a generally decreasing trend in fishing mortality (F) since 2017.
• In the North East assessment area, both SSB and recruitment have increased steadily since 2016/2017. Recruitment estimates for 2021 and 2022 are around the long term mean. F has fluctuated without trend over the last ten years.
• In the North West assessment area, recruitment steadily decreased between 2010 and 2017 but has increased since 2018 . The recent rise in recruitment has driven a similar increase in SSB although SSB remains low. Landings have steadily decreased since 2016 and coupled with the recent increase in SSB this has resulted in a sharp reduction in F.
• In Shetland, successive years of strong recruitment have driven a steady increase in SSB to the highest levels seen in the time series. Landings have remained high with a small increase in the last few years. Over a similar time period F has dropped and is now approaching the historical minimum.
• In the West of Kintyre assessment area, recruitment is estimated to have decreased between 2009 and 2019 and increased since then to around the long-term average. The SSB has followed a similar trend but is estimated to be well above the long-term average in recent years. Due to the high stock size coupled with the significant decrease in landings in recent years, the fishing mortality has declined to an extremely low level.
• There are insufficient data from the Clyde and Orkney assessment areas to perform analytical assessments or evaluate stock trends. In previous assessment reports, data were also presented for an Irish Sea assessment area. However, given the international nature of the fishery in this area, the ICES Scallop Assessment Working Group (WGScallop) is now considered to be best placed to collate data and develop stock assessments for this area.

Management considerations

• In order 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.
• With the exception of the North West assessment area, stock biomass in all areas with analytical assessments is estimated to be above the precautionary biomass reference point in 2022. Most assessed areas have seen a substantial decline in landings, resulting in declining fishing mortality to below F_MSY proxy. The exceptions to this are Shetland and North East areas where fishing mortality is estimated at just above this reference point.
• For the East Coast and West of Kintyre and assessment areas, where both F and SSB are within safe limits relative to provisional reference points, advice is for no increase in effort. However, given the current status of these stocks, a small increase in fishing effort is considered unlikely to be detrimental to the stocks.
• For the North West, while fishing mortality is considered in safe limits, SSB is currently below the precautionary reference point. The advice is for no increase in effort (such that fishing mortality remains below the F_MSY proxy) and to consider measures to safeguard the spawning stock at a level that will support maximum sustainable yield (MSY) for future generations.
• For the North East and Shetland, SSB is above the provisional reference point while the stock is fished just above F_MSY proxy. The advice is for a decrease 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, although the absolute levels of fishing mortality are not directly comparable with previous estimates as different procedures were used to derive these metrics.
• Two of the analytical assessments (North West and Shetland) show significant retrospective patterns. 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 and subsequent qualitative advice are likely to be robust to the retrospective issues.
• In some assessment areas, commercial sampling levels have fallen in recent years. 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. 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.