Scottish Marine and Freshwater Science Volume 3 Number 10: Scottish Scallop Stocks - Results of 2011 Assessments

Executive Summary

This report presents the results of Scottish regional scallop stock assessments carried out by Marine Scotland Science ( MSS) based on commercial catch-at-age and survey data up to and including 2010. Full analytical assessments are presented for the West of Kintyre, North West, North East and Shetland scallop stocks, with catch data presented for the Clyde, Irish Sea and Orkney. In addition, an exploratory analysis of the East Coast data is presented for the first time. The report also provides background information on Scottish fisheries for scallops, a description of the current management and regulatory framework and discusses potential management measures.

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 important shellfish fishery in Scotland. In 2010, total landings into Scotland were in excess of 9,000 tonnes with a value at first-sale of around £17.5 million. The landings in 2011 fell to just under 8,000 tonnes with a first-sale value of £16 million.
• The most important areas currently in terms of total landings are the Irish Sea, West of Kintyre, the North West, North East and East Coast. In 2010, over 80% of annual landings into Scotland were taken in these areas.
• Some areas, such as the Irish Sea, have shown systematic increases in reported landings, while in other areas the landings are characterised by occasional and rapid increases or declines. Some of these are associated with fishery closures due to the presence of amnesic or paralytic shellfish toxins, but others appear to be associated with strong year classes.

Stock Status

• The Time Series Analysis ( TSA) stock assessments (performed for the first time) show that following periods of high recruitment during the mid 1990s, the main west coast stocks experienced poorer levels of recruitment in recent years which has resulted in declining biomass since then. The continued high catches in the West of Kintyre area are reflected in the recent increase in fishing mortality, although the estimates of fishing mortality (as estimated in the TSA) are relatively uncertain. In contrast, the lower catches from the North West have resulted in a lower estimate of fishing mortality in this area.
• To the north east of Scotland (North East and Shetland assessment areas), recruitment is estimated to have been strong during the early 1990s, coinciding with increased catches, and more moderate in recent years. The fishing mortality in these areas increased during the late 1980s and in Shetland has increased again in recent years with the increases in catches.
• Stock trends based on the dredge survey data for the East Coast area suggest that spawning stock biomass ( SSB) increased during the 2000s following a number of strong year classes. More recent recruitment appears to have been low and SSB is declining.
• There are insufficient data from the Clyde, the Irish Sea and Orkney assessment areas to perform analytical assessments or evaluate stock trends.

Management Considerations

• There are no agreed biomass or fishing mortality reference points for Scottish scallop stocks. MSS management advice is therefore currently provided on the basis of estimates of recent fishing mortality, recruitment and biomass in relation to historical values.
• For the West of Kintyre, where the stock is at a very reduced level, advice is for a reduction in fishing mortality (F). In the North West assessment area where fishing mortality has reduced but stock levels remain low, advice is for no increase in fishing mortality.
• In the North East and Shetland assessment areas, advice is for no increase in fishing mortality.
• Management measures to control fishing mortality could include: effort restrictions (through limits on kWdays or fleet size), spatial and temporal closures or limits on the quantity landed, either alone or in combination.
• Measures to increase the SSB should be considered for both the West of Kintyre and the North West assessment areas. An increase in minimum landing size was previously advised by MSS. This has the potential to increase the reproductive capacity of the stocks, providing there is no associated increase in fishing effort.
• Several administrations have interests and responsibilities for scallop fisheries in the Irish Sea. This highlights the need to bring together data from different sources to develop a more consistent, inclusive approach to the assessment and management of stocks in the area. The area has sustained particularly high landings over a long period of time with no apparent detriment to abundance. In such circumstances, advice is for no increase in effort.
• The lack of any clear stock recruitment relationship for scallops precludes the calculation of targets or reference points based on maximum sustainable yield. Other options based on per recruit analysis are discussed in the report and it is hoped to investigate these ahead of the next assessment scheduled for 2013.

Data and Quality of the Assessment

• In areas for which sufficient data were available, an age-structured Time Series Analysis ( TSA) analytical assessment method was used. TSA provides more robust estimates of stock status than the method previously used as it makes use of multiple data sources (commercial catch-at-age and survey indices by age) and can cope with the omission of poor quality or missing data. In addition, the estimates of abundance and fishing mortality are calculated with confidence intervals.
• The estimates from TSA are smoothed through time reflecting the fact that fisheries and stocks are likely to show gradual year to year changes. As a result, the estimates are slow to respond, for example, when the data do suggest that there has been a sudden change in the fishery. This can potentially result in under or over estimation of recent fishing mortality.
• Historical trends estimated by the TSA approach show good agreement with MSS' previous stock assessments. The absolute levels of biomass, recruitment and fishing mortality estimated are not directly comparable with previous estimates as different procedures were used to derive these metrics.
• Commercial sampling levels have typically fallen in recent years due to limited staff and other higher work priorities. 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 surveys are an essential component of the assessment in that they provide fishery independent indices of abundance. They show reasonably good coverage of the fished areas according to scallop dredge VMS data (over the recent period for which these data are available) with the exception of the West of Kintyre which could potentially result in biased abundance indices. Additional (or a redistribution of) survey stations in this area may provide a more representative index.
• TSA estimates that there has been an increase in survey catchability particularly in West of Kintyre scallops. It is not known whether this is related to changes in actual survey catchability (for example related to gear or design) or to a change in the distribution of stock and fishery in relation to the survey. The latter could potentially imply that the actual decrease in biomass is smaller than that estimated by the assessment which, therefore, provides a conservative estimate of biomass in this area.
• The population structure of Scottish scallop stocks is not well understood, and the assessment areas were defined to reflect the characteristics of the fisheries in the past. A recently funded MASTS (Marine Alliance for Science and Technology for Scotland) project aims to develop a spatial population model for scallops by combining ocean circulation models to simulate the dispersal patterns of larvae with models of the growth, survival and spawning of settled individuals. This should provide further information about the magnitude and scale of connectivity between scallop populations around Scotland which may have implications for the areas on which the assessments are based.