Scottish Scallop Stocks: Results of 2025 Stock Assessments

4. General discussion

4.1. Regional summaries

Substantial scallop fisheries have existed around the coast of Scotland for many years. While landings in the Shetland area have been systemically increasing over time (and landings at Orkney show a general increase), other areas show significant periodic fluctuations. Historically there have been occasional rapid increases/decreases in landings from some areas (East Coast, North East and North West), some of which are associated with fishery closures due to ASP/PSP toxins, but others appear to be due to the appearance of strong year classes (increases in recruitment) and an increase in stock size. In recent years (since around 2015), there have been major declines in the landings from some of the historically most important scallop areas (East Coast, North West, West of Kintyre) and a decline in total landings from the Scottish assessment areas, though there is evidence of an improved fishery since the last assessment in 2025 with an increase in landings in some areas and overall. Anecdotal information from the Scottish fishing industry suggests that the recent variations are due to some of the large nomadic dredgers moving between fishing grounds and spending a significant amount of time fishing elsewhere around the coast of the UK in some years.

The SAM stock assessments show that following periods of lower recruitment during the late 1990s, the stocks around Shetland and East Coast of Scotland experienced higher recruitment during the 2000s and 2010s resulting in increased SSB.

In recent years, estimates of recruitment at the East Coast are relatively low, but following a decrease in SSB from around 2013 to 2018, and a subsequent decrease in catches, estimated SSB has increased to around the long-term average in the last year of the assessment. F, estimated to be one of the lowest of the time series in 2022, is now estimated to be increasing.

In contrast, the stock assessment for Shetland shows a recent rapid increase in recruitment (although this plateaued from 2023-2024) and SSB estimates continue to increase higher than any other point in the time series. This means that although landings in Shetland are at a historical high, F continues to decline due to the steep increase in estimated biomass.

In the North East, after estimates of strong recruitment and high SSB in the 1990s, recruitment and SSB have been fluctuating around an average level since then; following a period of decline (2013-2017), SSB has subsequently increased to above the long-term average in 2024. The increase in catches in 2023 and 2024 has resulted in an increase in F which is estimated to be well above the long-term average in 2024.

In the West of Kintyre, after low recruitment until the early 1990s, good recruitment in the 2000s and 2010s resulted in increases in SSB. Recent recruitment has been much lower (close to or just below the long-term average). However, as a result of the very low landings (and resulting estimates of F which are around the lowest of the time series), estimated SSB continues to increase.

In the North West estimates of recruitment and SSB peaked around the early 2000s and subsequently declined towards the late 2010s. Since then both recruitment and SSB have increased, and the recent very low catches have resulted in a decline in fishing mortality to near the lowest in the time series.

The historical stock trends estimated in this assessment show good agreement with the previous scallop assessments carried out in 2023 (Dobby et al., 2025) which also employed the SAM approach. The 2023 assessments also showed general agreement with those from Dobby et al., 2017, despite the change in approach from TSA to SAM.

As in the previous assessment, the stock recruitment plots provided for the five areas assessed using SAM show little evidence of a stock recruitment relationship and for this reason, recruitment continues to be modelled as a random walk. One explanation for this lack of relationship is that the model estimates of SSB (biomass of individuals aged three and above) may not be a good measure of spawning potential, either because a proportion of two year old individuals are also likely to be mature but are not included in the model, or because SSB does not sufficiently account for the greater reproductive output of larger individuals. Another reason could be that recruitment is largely independent of stock size (although others have observed density dependent effects; e.g. Vahl, 1982) and is driven more by external factors such as environmental conditions, which are not explicitly included in the models.

4.2. Stock status and management considerations

An evaluation of stock status is provided in relation to the precautionary reference points (F_0.1, used as a proxy for F_MSY, and B_pa) previously developed from the 2023 stock assessment outputs. Stock status in relation to precautionary reference points is summarised below. *denotes above F_MSY proxy or below B_pa:

The East Coast, North West, and West of Kintyre assessment areas are being fished below the F_MSY proxy in the most recent year of the assessment, although that has not always been the case historically. Fishing mortality in Shetland is estimated to be around the F_MSY proxy (just above) while in the North East, it is estimated to be substantially above the reference point.

In all areas, stock biomass is above the biomass reference point, and Shetland and West of Kintyre are currently ‘well above’ biomass reference points.

Scallops around Scotland are currently managed through technical conservation measures (MLS regulations) and through measures to restrict effort (zonal dredge limits; bar length and dredge bag restrictions (The Regulation of Scallop Dredges (Scotland) Order 2005)). Management advice is provided on the basis of qualitative precautionary considerations and the assumption of a relationship between fishing effort and fishing mortality.

Recruitment is clearly important to the fishery. In most of the assessment areas, good recruitment appears to drive upturns in SSB, and these peaks in recruitment are often associated with subsequent high landings. Even within longer term upward or downward trends there are fluctuations, and successive recruitments appear to be correlated, with high and lows evolving over four to eight years. During periods of low recruitment there may be a need to reduce fishing mortality to enable a stock to rebuild to (or be maintained at) a level that allows for MSY in future.

In the East Coast, North West and West of Kintyre assessment areas, both F and SSB are estimated to be within safe limits relative to provisional reference points. In these areas, where fishing mortality is below the reference point, 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, the advice is for no increase in effort^[2] (above 2024 levels) while for the North East, where fishing mortality is well above the reference point, 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.

4.3. Comments on the quality of the data and assessment

The accuracy and precision of the estimates of stock status depend on the quality of both the total commercial catch-at-age data and the survey indices-at-age. The catch-at-age data are derived from landings length- and age-structured data sampled by MD staff which are then raised to total official landings data. The introduction of UK Buyers and Sellers’ legislation in 2006 is thought to have improved the accuracy of reported landings, although given that Scottish scallop fisheries are not regulated through TACs there is actually no incentive for fishers to underreport or misreport scallops.

There are insufficient data from the Clyde and Orkney areas to perform analytic stock assessments. Clyde and Orkney have historically been less important scallop fishery areas, and the unpredictable nature of these fisheries can make the acquisition of landings samples particularly difficult. The number of samples from the Clyde has increased in recent years and the resulting data in combination with the developing survey time series (first conducted in 2019) are being explored with the aim of providing an assessment for this area in the near future. The roll out of REM across the scallop dredge fleet may also provide data which could in future be utilised in the assessment process for both data limited and data rich areas (e.g. high resolution effort data to develop landings-per-unit effort indicators).

In the scallop areas for which MD have conducted analytical assessments, sampling levels have historically been relatively good for the west of Scotland areas (and Shetland) and poorer for the North Sea assessment areas. In recent years there have been reductions in the number of samples across all assessment areas (except Shetland) likely due to the Covid-19 pandemic, although sampling effort seems to be slowly recovering in some areas with a slight improvement in levels over the past two years since the last assessment.

A single year with poor sampling levels may not significantly affect the conclusions of the assessment but continued poor sampling levels are likely to lead to less robust results. In order to account for the variation in sampling levels over time, the number of catch samples per year is used to weight the data such that the catch data from those years with a greater number of samples is given a greater weight in the assessment, hence reducing the impact of particularly noisy data from years with poor sampling. In years where sampling data are entirely missing (East Coast in 2001 and 2023), observations of total landings (biomass) have been included in the assessment model in the absence of catch numbers at age. Whilst this means the assessment has some information on catch in these years, estimates of numbers at age by the model will be still be less accurate than if landings sampling data were available. Work is ongoing within MD to explore ways of improving future sampling levels including, working with local scallop processors.

The current analytical assessments rely on age-structured input data. In some areas (particularly the West of Kintyre) these data appear noisier which could be due to age reading errors although equally it could reflect heterogeneity of the stock or fishery in the area. Within MD, age-reading training is conducted on a regular basis. However, ICES have identified the need for internationally agreed ageing protocols along with additional quality control and assurance to validate age and growth estimates and improve consistency among readers. A recently established ICES workshop has been tasked with progressing some of these issues (Workshop on Scallop Ageing; ICES, 2023b).

The survey data are an integral component of the stock assessments. The surveys show reasonably good coverage of the fished areas according to scallop dredge VMS data with the exception of the West of Kintyre where there are a number of areas with apparently high fishing effort which are not surveyed (Dobby et al. 2017). The density of stations is greatest in Shetland although in some years no survey (or only a partial survey) has been completed due to poor weather; in 2024 the Shetland survey was cut short due to storms. It is not clear whether such a high density of stations is required to retain a particular level of precision in the survey abundance index estimates. In recent years a number of stations to the west of Scotland have not been surveyed due to the presence of newly designated marine protected areas (MPAs) and in the North Sea due to renewable energy developments. MD is exploring a number of options to replace any survey stations lost (or likely to be lost) due to these developments. Lower coverage may lead to increased uncertainty in the recent recruitment and biomass estimates.

Previously an analysis of the historical survey data suggested that the survey index was relatively insensitive to the inclusion/exclusion of a small number of survey stations. Many fish stock assessments now make use of modelled survey indices (e.g. Northern Shelf haddock and cod; ICES 2024a) and these can provide a more reliable index with variable survey coverage and also provide a statistically sound approach to the derivation of survey confidence intervals which can be included as assessment input data. Work is ongoing within MD to develop a modelled survey index for scallops, though it is not yet ready for inclusion in these assessments.

The analytic stock assessments presented here utilize SAM which is a state-space stock assessment model (Nielsen and Berg, 2014; Berg and Nielsen, 2016) similar to Time Series Analysis (TSA) which was used for the previous stock assessments (Dobby et al, 2017). The new approach has substantial flexibility in terms of model configuration and input data, allowing for both process and observation error and providing model outputs with estimates of precision. SAM has the advantage (over TSA) of being a widely used stock assessment method with a substantial network of user support, and in addition utilizes an online platform on which to run assessments and make data and results publicly available (stockassessment.org).

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 Fbar_(4-8). In all cases, SSB is revised downwards and Fbar_(4-8) upwards with the inclusion of additional data and Mohn’s rho estimates of average bias fall outwith the ICES guidelines (ICES, 2020) for either SSB, Fbar_(4-8) or both. In such situations ICES may consider either scaling the assessment results to account for the bias before using them as the basis of numerical catch advice, or potentially making some additional precautionary considerations. For these scallop stocks, advice is qualitative (rather than numerical) and the conclusions in terms of biomass stock status are likely to be robust to the retrospective bias in this case (given the direction of bias and the biomass estimates relative to the reference point). 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 a degree of retrospective uncertainty. Given the uncertainty in the Shetland assessment, results and advice should be treated with a greater degree of caution.

The last two Shetland stock assessments, conducted using SAM, have estimated a steep increase in SSB since around 2017. Although there are large confidence intervals around the estimates of recruitment and SSB in recent years, the lower confidence bound is well above the biomass reference point and thus the uncertainty associated with the estimate is unlikely to affect the conclusions on stock status in this case. Current work in development with UHI-Shetland combines the data presented in this assessment with UHI’s commercial landings per unit effort (LPUE) series and survey data collected by UHI-Shetland between 2015 and 2019. Preliminary results show tighter confidence intervals compared to assessments using UHI or MD data alone, and so in future this combined approach could provide a more robust assessment of the Shetland stock.

The current stock assessments provide an indication of stock status (and dynamics) in the assessment areas as currently defined. These areas were, however, based on the characteristics of fisheries in the past rather than on the basis of evidence of discrete populations. The population structure of Scottish scallop stocks is not well understood. Scallops are sedentary in nature and only able to swim limited distances. Larvae, however, inhabit the water column for three weeks or more, during which time they may drift a substantial distance (dependent on water circulation, tides and wind driven currents) from the parent population before settling to the sea bed. There appear to be some similarities in interannual fluctuations in recruitment between assessment areas (e.g. North East and East Coast) which may indicate that there are linkages between some of these areas at pre-recruitment stages with similar trends in survival to age of recruitment. There is potential for population linkage across substantial distances. Habitats suitable for scallops are patchily distributed and some patches of adult population may provide a source of larvae for others.

A recently completed PhD project found evidence of high genetic connectivity between Scottish scallop populations sampled in the different assessment areas but concluded that they are not fully demographically connected (ICES, 2026). Genetic transfer is instead more likely to be via ‘stepping stone dispersal’ through populations between those sampled, rather than through direct larval dispersal between areas. There is therefore no current evidence to suggest that a change to the current separate assessment units of the Scottish scallop population is required. Another PhD project on larval dispersal modelling around Scotland is currently on-going (ICES, 2026) which will continue to provide an improved understanding of the nature and extent of connectivity between scallop populations around Scotland and further afield.