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

4. General Discussion

4.1 Regional Summaries

Substantial scallop fisheries have existed around the coast of Scotland for many years. In some areas, such as the Irish Sea, Shetland and Orkney there are systematic increases apparent in the landings data. However, in other areas (North West and North East), the landings are characterised by occasional and rapid increases or declines. Some of these 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).

The TSA stock assessments show that following periods of high recruitment during the mid 1990s, both of the main west coast stocks of scallops have experienced poorer levels of recruitment in recent years which has resulted in declining biomass. The continued high catches in the West of Kintyre are reflected in the recent increase in fishing mortality, although the estimates of fishing mortality 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 (as estimated in the TSA) in these areas increased during the late 1980s and in Shetland has increased again in recent years concomitant with the increases in catches.

Although no analytical assessment has been presented for the East Coast area, stock trends based on the dredge survey data suggest that SSB increased during the 2000s following a number of strong year classes. More recent recruitment appears to have been low and SSB is declining.

Historical stock trends estimated by the TSA approach show good agreement with previous scallop assessments presented in the MSS stock booklet (Scottish Government, 2010). The absolute levels of biomass, recruitment and fishing mortality presented here are not directly comparable with previous assessments as different measures have been used to define these quantities ( e.g. recruitment at age three compared with recruitment at age two previously).

The stock recruitment plots provided for the four areas assessed using TSA show little evidence of a stock recruitment relationship. One explanation for this may be that recruitment is truly independent of stock size (although others have observed density dependent effects, Vahl, 1982) and is driven more by external factors such as environmental conditions, which are not included in the model. Another is that the model estimates of SSB (individuals aged three and above) are not a good measure of spawning potential as two year old individuals are also mature but are not included in the model. The lack of a clear stock recruit relationship has implications for the type of stock projections that could be carried out for scallop stocks. Although short term projections typically make use of recent average recruitment, the simulations that would be required to investigate the medium to long term effects of changes in, for example, minimum landing size ( MLS) typically require the use of a stock recruit relationship. An alternative method for modelling future recruitment in a stock which exhibits sporadic recruitment (North Sea haddock) is presented in Needle (2008). The method generates random recruitment from a lognormal distribution with additional occasional very strong year classes. Assumptions about the distribution and frequency and size of high year classes are derived from the historical time series. Such an approach may prove useful in any scallop management strategy evaluations that are required in the future.

4.2 Reference Points

There are currently no agreed biomass or fishing mortality reference points for Scottish scallop stocks. Stock status and management considerations are therefore provided on the basis of a comparison of estimates of current fishing mortality, recruitment and biomass in relation to historical values. The lack of a stock recruitment relationship precludes the calculation of target reference points based on maximum sustainable yield. In cases where F _MSY cannot be estimated directly, proxy values based on per recruit analysis are often used ( ICES, 2010). ICES has advised on the use of F _MAX (fishing mortality at the maximum of the yield per recruit ( YPR) curve) as an appropriate proxy unless there is evidence of poor recruitment at such levels of fishing mortality. In cases where the maximum of the YPR curves is less well defined then F _0.1 (fishing mortality at which the slope of the YPR curve is 10 % of the slope at the origin) or reference points based on spawning biomass per recruit are likely to be more appropriate proxies.

The ICES advice framework also makes use of biomass reference points which are used as limits rather than targets. For many ICES fish stocks, B _lim (limit reference point for biomass) has been defined as the historical lowest observed spawning stock (B _loss) - the value below which recruitment is expected to be 'impaired' or the stock dynamics are unknown. The precautionary reference point (B _PA) is derived from this value by adjusting it to account for variability and uncertainty in the assessment.

Scallop ( Placopecten magellanicus) stocks off the north east US coast are managed in relation to a target fishing mortality of 80 % of F _MAX (used as a proxy for F _MSY). A proxy for B _MSY on the basis of the product of B _MAX (biomass per recruit at F _MAX) and the median number of recruits per tow from the survey is also used ( SAW Invertebrate Subcommittee, 2004). The threshold for being in an 'overfished condition' is defined as half of B _MAX. In New Zealand, F _0.1 is used as a target fishing mortality in the major scallop ( Pecten novaezelandiae) fisheries (New Zealand Government, 2011). However, they state that biomass reference points based on virgin biomass or B _MSY are not likely to be appropriate for stocks with highly variable recruitment and growth.

There are clearly a number of options to be explored for the calculation of reference points for Scottish scallop stocks. The calculation of fishing mortality reference points based on per recruit curves would be relatively straightforward given that the required inputs for the calculations are a direct output from the TSA assessment. In addition, there is a relatively long time series of abundance estimates (either the TSA output or from the surveys) that could potentially be used to derive biomass reference points. It is anticipated that potential reference points will investigated ahead of MSS' next round of assessments to be carried out in 2013.

4.3 Comments on the Quality of the Data and Assessment

The age-structured TSA analytical assessment method used in this report is considered to be an improvement on the methods previously used. It provides more robust estimates of stock status 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 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 length and age structured data sampled by MSS staff which are then raised to total official landings data ⁴ . The introduction of 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. When preparing data for this report we found some inconsistencies between the Shetland area landings as reported to the SSMO and those officially reported to MS. Despite a thorough investigation, the source of these discrepancies could not be traced. Underreporting to the SSMO may have been a factor in the past, but in recent years there has been 100 % compliance in terms of logsheet returns (Leslie, personal communication). Skippers may use a nominal bag weight rather than actually weighing their catch for their EU logsheets, but cross-checks with the sales notes (completed by processors) should limit biases in these data ( MS data). These inconsistencies result in estimates of population size which appear to differ by a scaling factor (fishing mortalities quite similar) and therefore conclusions regarding stock status from the NAFC stock assessments and those presented here are broadly similar.

There are insufficient age composition samples from the Clyde, East Coast, Irish Sea and Orkney to perform analytic stock assessments. The Clyde, East Coast and Orkney have historically been less important scallop fishery areas although landings from the East Coast have increased recently. The unpredictable nature of these fisheries makes the acquisition of landings samples particularly difficult. The Irish Sea is the most important of the scallop assessment areas in terms of total landings, but over half of these are landed into ports outside Scotland. Samples from Scottish ports are therefore unlikely to be representative of the fishery as a whole. A collaborative programme of work ( UK and Isle of Man) to cover sampling and stock monitoring may improve the basis for assessment and advice in this area.

In other areas for which we have conducted analytical assessments, sampling levels have typically fallen in recent years due to limited resources within MSS. Although a single year with poor sampling levels may not significantly affect the conclusions of the assessment, continued poor sampling levels are likely to result in less precise, and potentially biased, results. In an attempt to improve shellfish sampling levels, MS Compliance staff have recently been trained to take part in market sampling in a variety of locations around the Scottish coast. In addition, there are moves within MSS to redesign and/or more appropriately target sampling effort across shellfish stocks.

The survey data are an integral component of the stock assessment and provision of advice on stock status and for the East Coast are currently the only source of information. 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. The density of stations is greatest in Shetland. It is not clear whether this is required to retain a particular level of precision in the survey abundance index estimates or whether there is potential to redistribute survey effort.

In the stock assessments of the west coast, particularly for the West of Kintyre, the residuals of the model fits to data suggests that there was a change in the survey catchability at older ages during the late 1990s and early 2000s. It is not clear whether this is due to changes in actual survey catchability or whether this mismatch between the survey and catch data is a result of changes in the distribution of stock and fishery in relation to the survey. The retrospective analyses which illustrate how the final year estimates change with the inclusion of an additional year's data also show patterns which are consistent with changes in catchability (estimates of recruitment are revised downwards on an annual basis).

The outputs from the stock assessment are presented in terms of muscle weight. These are calculated from numbers at age by the use of mean muscle weights-at-age derived from a length-weight relationship and the size at age composition data collected through the market sampling programme. The source of the regional length-muscle weight relationships in FMD is not documented, but it is believed that the relationships have not been updated for a number of years. Revisiting these length-weight relationships for all assessment areas would be a major undertaking but could potentially provide more reliable estimates of spawning stock biomass. Biological data on scallops from the Shetland area are already being collected and provided by staff from NAFC Marine Centre under the Memorandum of Understanding (MoU) between NAFC Marine Centre and MSS. It is intended that in advance of the next scallop assessments, these data will have been analysed to provide updated length-weight relationships.

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 rather than on the basis of evidence to support discrete populations. Similar trends in recruitment across the West of Kintyre and North West and also in Shetland, the North East and East Coast suggest that there are linkages between some of these areas at pre-recruitment stages with similar trends in survival to age of recruitment. Adult scallops are relatively sedentary and able to swim only 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 is therefore 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. MASTS (Marine Alliance for Science and Technology for Scotland) have recently funded a project to investigate the impact of marine reserves and wind-farm developments on scallop stocks. As part of this work a model will be developed which will simulate the dispersal patterns of scallop larvae using the outputs of ocean circulation models and link these with models of growth, survival and spawning of individuals following settlement. 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.

Acknowledgements

We would like to thank Beth Leslie from NAFC Marine Centre for providing comments on an earlier version of this report.