Scottish Marine and Freshwater Science Volume 3 Number 10: Scottish Scallop Stocks - Results of 2011 Assessments
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.
3. Results and Discussion by Area
3.1 Regional and Temporal Trends
Since the mid 1990s, total Scottish scallop landings have fluctuated between eight and 10 thousand tonnes. The majority of these are dredge caught, dive caught scallops typically making up less than 5% of the total. Temporal trends in landings vary considerably between assessment areas and are shown in Table 3.1.1 (dredge) and Table 3.1.2 (dive) for 1982 to 2010 and for a longer period in Figure 3.1.1 (total landings by all vessels into all countries). In some areas, particularly to the west of Scotland, there have been substantial fisheries over a long period of time whilst in other areas such as the East Coast, North East and Orkney, fisheries developed relatively recently. The Irish Sea, Shetland and Orkney fisheries have shown a general increase in landings over the 45 year period illustrated, whilst the landings from some of the other assessment areas have shown declines. In particular, both the North East and North West areas have, in the past, had periods of very high landings (~ 3,500 t) but have shown a marked decline in recent years.
The spatial distribution of dredge landings in 2010 is shown in Figures 3.1.2 (landings into Scotland) and 3.1.3 (landings outside Scotland). The grounds of greatest importance to the dredge fishery in 2010 were the Irish Sea (around the Isle of Man), the northeast coast of Scotland, Shetland and the statistical rectangles around the Inner Hebrides. Note that a large proportion of landings from the Irish Sea were landed into ports elsewhere in the British Isles (not Scotland). There were also small landings from the East Coast and West of Kintyre into other UK ports ( Table 3.1.3). Total landings into all countries are given in Table 3.1.4. In contrast to the dredge fisheries, the main dive fisheries in 2010, ( Figure 3.1.4 and Table 3.1.2) were located in the coastal waters of the west of Scotland and at Orkney, where the method accounts for 40% of the landings.
3.2 West of Kintyre
3.2.1 Description of the Fishery
The West of Kintyre assessment area has a long history of exploitation with periods of high and low landings ( Figure 3.1.1). The main fishing grounds are around the islands of Islay and Jura and the southern end of the Kintyre peninsula. The fishery operates year round. Landings have fluctuated between around 500 and 2,000 tonnes over the stock assessment period and in 2010 were above the long term average. The fishery is prosecuted regularly by a fleet of around 14 vessels which range from 9.9 m to approximately 20 m in length and typically land their catch into Tarbert, Tayinloan and Campbeltown. Up to six vessels from the Isle of Man may also fish this area at various times of the year.
3.2.2 Sampling Levels and Age Compositions
Sampling levels for the West of Kintyre are shown in Table 3.2.1. In 2010, the number of boats sampled varied between one and four per quarter, and the total number of scallops aged and measured was 2,124 (from 10 vessel landings sampled). Sampling levels fell substantially in 2010 when the number of sampled trips and individuals measured were both less than 50% of the 10 year average.
Catch-at-age Data
Catch-at-age data for the West of Kintyre, from 1982 to 2010 are shown in Table 3.2.2 and Figure 3.2.1. In the early part of the time series scallops eight years and older, and particularly 10+ , were well represented in the catches, but have been less evident since the 1990s. In contrast , the number of scallops at ages four and five has increased considerably since this time. There was a notable lack of three years olds in the catch in 2001 and 2006, implying poor recruitment to the fishery. There has, however, been a steady increase in three years olds since then.
3.2.3 Biological Data
The parameters of the length-weight relationships (Weight (g) = a x Length (mm) b) used for scallops in the West of Kintyre assessment area are as follows:
| a | b | Source | |
| Total (annual) | 0.001142 | 2.513 | Cook et al. (1990) |
| Muscle (annual) | 0.00006634 | 2.668 | MSS unpublished |
The mean muscle weights are shown in Figure 3.2.2 and Table 3.2.3. The mean weight at age two has shown a gradual increase over the time period, while other ages up to seven show fluctuations but no specific trends. Mean weights at older ages show noticeable temporal trends which are similar across age classes, increasing through the early 2000s and decreasing more recently.
3.2.4 Exploratory Analyses
Catch Data
Catch curves by cohort for the commercial catch-at-age data are shown in Figure 3.2.3. These show partial recruitment to the fishery up to age five. Also evident are the low 1982‑1986 year classes (recruitment in 1985-1989) and the particularly strong 1992-1994 cohorts. The gradient of the catch curves becomes increasingly steep during the mid 1990s which suggests an increase in fishing mortality.
Survey Data
Partial surveys of the west coast of Scotland began in 1988, with fuller coverage of both the West of Kintyre and North West assessment areas beginning in 1993. Details of the surveys are given in Table 3.2.4. The west coast survey has typically been carried out during early summer and, until 2000, it was very consistent in terms of timing, vessel and number of hauls. Since then, the timing has varied between August and April and a number of different vessels have been used in the survey. No comparative tows have been conducted between vessels, although catch rates are standardised by dredge width to account for differences in the number of dredges worked by each vessel. Survey data from 1993 (when full coverage began) to 2010 were used in the assessment of the West of Kintyre area.
The catch rates of scallops (all ages combined) at stations throughout the survey area between 2000 and 2010 are shown in Figure 3.2.4. Some stations, such as those off the east coast of Jura, show consistently high catch rates from year to year, whilst others show much more inter-annual variability. Table 3.2.5 shows the average catch rates by age class and year. Average catch rates of up to 15 scallops hr -1 m -1 occur for the most abundant ages. There has been a general decline in catch rates of the oldest ages in the most recent surveys. However, the index of three year olds in 2010 (10.35 scallops hr -1 m -1) is the second highest on record.
Catch curve analyses ( Figure 3.2.5) show the survey index by age on the log scale for each cohort. The steeply increasing left hand limb of these curves indicates that the youngest age classes (ages 2 and 3) have significantly lower survey catchability than older age classes. The earliest cohort curves are relatively smooth showing consistent tracking of year classes and selection. However, there appear to have been some changes in survey catchability, particularly evident in the 1998 to 2001 year classes, which do not show the typical declining pattern across ages four to seven. The increase in log catch numbers at age 10 (compared to age nine within a particular cohort) is due to the plus group effect i.e. this age class includes all individuals aged 10 and above. The magnitude of this effect is, however, highly variable from year to year and these data (the 10+ age group) are therefore excluded from the assessment.
3.2.5 Final Assessment
TSA
The exploratory catch curve analysis indicates highly variable catch rates of age two individuals in both the commercial catch and survey. In addition, the catch rates of the 10+ age group in the survey are very noisy. These data are, therefore, excluded from the final assessment. To summarise, the final TSA run uses the following data and settings:
Commercial catch-at-age data from 1982 - 2010, ages 3 - 10+
Dredge survey catch-at-age index from 1993 - 2010, ages 3 - 9
Average recruitment ( i.e. no stock-recruit relationship assumed)
Age at full selection = 6
Survey timing = 0.45 (average survey timing as proportion of year, equivalent to mid June)
Outputs from the TSA assessment are shown in Figure 3.2.6 and estimated parameter values are given in Table 3.2.6.
Standardised prediction errors from the assessment model are shown in Figures 3.2.7 (landings) and 3.2.8 (survey). TSA is a state space time series model, and these prediction errors are an analogous (but not completely equivalent) diagnostic tool to residuals or fits used to assess other stock assessment models. The catch prediction errors are well distributed and do not suggest the model is predicting landings different to that of the observed data.
The survey prediction errors, however, show a distinct dome shaped pattern at the oldest ages, with this being especially apparent at ages eight and nine. It appears that the survey 'became better' at catching these older ages as time went on - reaching a peak for most ages around 2002, and then starting to decline again, showing a more evenly distributed pattern in the most recent years. This indicates changes in survey catchability that the model does not take account of. There is currently no explanation for this trend in catchability over a time when the survey gear and spatial distribution remained relatively stable.
There is no clear relationship between stock size ( SSB) and recruitment to the fishery (at age three) for this stock, with large values for recruitment estimated at small stock sizes and small recruitments possible at both large and small stock sizes ( Figure 3.2.9).
Retrospective Analysis
The retrospective plots shown in Figure 3.2.10 indicate that the assessment tends to overestimate SSB in the final year ( i.e. that estimates are revised downwards with each additional year's data), and to underestimate mean fishing mortality, indicating there is some bias in the model, probably due to a mismatch between the signals in the catch and survey data. Recruitment is also revised as additional years are included in the stock assessment, although the estimates show neither systematic under or overestimation in the final year.
State of the Stock
The state of the stock is summarised in Figure 3.2.6. The final estimates for the stock in 2010 are:
F (average over ages 4-6) = 0.600 yr -1
SSB (total over ages 3-10+) = 566 t (muscle weight)
There are no reference points for this stock.
Estimates (and standard errors) of age structured population abundance and fishing mortality are presented in Tables 3.2.7- 3.2.10. The final estimates are smoothed across years which explains the differences between the estimates of fishing mortality at age in the first year given here and the parameter estimates in Table 3.2.6. Table 3.2.11 shows the stock summary. Mean F(4-6) showed a significant decline between 2003 and 2006, but since then is estimated to have more than doubled, although recent estimates are quite uncertain. Recruitment in 2010 is estimated as being lower than in the mid 1990s, but above the long-term average. The continued high catches combined with only moderate recruitment have, however, resulted in a declining SSB which is currently at the lowest level estimated over the 30 year time series.
Comparison with Previous Assessments
The last Scottish scallop assessment report was published in 2006 (Howell et al., 2006) and presented the results of an assessment conducted using commercial data up to and including 2003 in a quarterly VPA tuned to average fishing mortality. The assessment for West of Kintyre was updated in 2008 using data up to 2007 and this showed a substantial revision to the estimates of biomass and fishing mortality. A summary of stock status including biomass trends from the 2008 assessment can be found in the MSS Stock booklet (Scottish Government, 2010). The general trend (upwards) in estimated mean fishing mortality in this assessment is similar to that from the 2008 assessment, although the estimate is slightly lower. This may be due to the differences in the age range over which mean F is calculated in the two assessments. This assessment uses ages four to six but previous assessments used ages six to eight. The trends in SSB are also similar with an initial decline followed by an increase during the 1990s and then a further decline. The current assessment provides biomass estimates in terms of muscle weight only and absolute estimates are therefore not directly comparable with previous estimates which were of muscle plus gonad weight. Recruitment estimates from the two assessments (current and 2008) are of a comparable magnitude. Trends in the time series of recruitment at age three from the current stock assessment are similar to those previously estimated but with a time lag of one year due to the differing ages at which recruitment has been estimated (previously age two).
3.2.6 Quality of the Assessment
Landings Data
Fishers are required to provide information about quantities landed and fishing location by ICES rectangle on either EU logbooks or Shell 1 forms (under 10 m vessels). The implementation of 'the registration of buyers and sellers' legislation in the UK in 2006 requires details of the landed catch also to be recorded at the point of first sale and sales notes are cross checked against vessels landings declarations. This procedure is thought to have improved the accuracy of reported landings data in recent years.
Age Composition
The scallop market sampling levels in the West of Kintyre in 2010 were low due to lack of sampling opportunities and limited staff time. This issue is currently under review with the aim to increase sampling levels throughout the year in the West of Kintyre and other areas. Provided that previous sampling levels have been adequate, the reduction in a single year (2010) should not significantly affect the assessment of stock status.
Weights at Age
Mean muscle weights-at-age used in this assessment were derived from a length-weight relationship (unpublished MSS) and the size at age composition data collected through the market sampling programme. This allows for inter-annual variation in weights-at-age (due to changes in size‑at-age) to be accounted for in the estimate of SSB. In previous assessments, fixed values for weight-at-age were used.
Survey Data
The survey provides good coverage of the fishing grounds off the east coast of Islay and Jura and to the west of the Kintyre peninsula. However, there are relatively few stations in statistical rectangle 41E4 which covers the north of Jura and the area to the southeast of Mull, where significant landings were reported in 2010.
The survey utilises a standard commercial dredge with large belly rings and a smaller laboratory dredge with small belly rings. Younger age classes (two and three year olds) have lower survey catchability because they are smaller and are able to pass through the belly rings of the dredge. However, occasional large catches are possible because other scallops and debris block their escape through the dredge. Variability in survey vessel and timing of the survey may also result in noisy data with apparently changing catchabilities, but not a systematic trend in particular age classes.
Model Formulation
TSA is useful in that it allows both fishing mortality and survey catchability to evolve over time in a constrained manner. However, the current model formulation does not allow for survey catchability trends to differ between age classes resulting in some systematic trends in the standardised survey prediction errors in the final run.
3.2.7 Management Considerations
SSB has declined markedly in the last ten years and recent estimates of fishing mortality for the West of Kintyre assessment area are high. Under these circumstances advice is for a reduction 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 SSB should be considered. An increase in the minimum landing size has previously been proposed by MS as a possible management measure for Scottish scallop fisheries. The survival of discarded scallops is high and therefore most undersized scallops returned to the sea have the potential to grow. This measure therefore has the potential to increase the reproductive capacity of the stock, provided that there is no associated increase in fishing effort.
3.3 North West
3.3.1 Description of the Fishery
The North West assessment area covers much of the west coast of Scotland and the waters around the Hebrides. There is a long history of scallop fishing in this area ( Figure 3.1.1). The main fishing grounds are around the Inner Hebrides and South Uist. The fishery operates year round. In 2010, landings were 1,365 t, 30% of the peak of 4,500 t taken from the area in 2002. The fishery is prosecuted by a fleet of around 10 vessels which range in size from 9.9 m to approximately 18 m and land their catch into Mallaig, Kallin, Uig, Kyle, Portree, Lochinver and Ullapool. There are approximately 12 nomadic vessels which fish in this area at various times of the year, and a small but significant dive fishery which consists of approximately six dive vessels. The dive fishery operates largely in the sheltered inshore waters around Ullapool, Uig, Kyle, Uist and Barra and in 2010 accounted for just over 15% of total scallop landings in the North West area.
3.3.2 Sampling Levels and Age Compositions
Sampling levels for the North West area are shown in Table 3.3.1. In 2010, 5142 individual scallops were measured from 31 sampled vessels. This is an improvement on recent years and comparable to sampling levels from the start of the time series, although seasonal coverage varied with only one sample obtained during quarter one, compared to 15 in quarter two.
Catch-at-age Data
Catch-at-age data for the North West from 1982 to 2010 are shown in Table 3.3.2 and Figure 3.3.1. In the early part of the time series, catches were dominated by individuals in the 10+ age category, whereas more recently, the catches consist largely of four, five and six year old individuals. Since 2006, there has been a noticeable lack of three year olds in the catch suggesting poor recruitment.
3.3.3 Biological Data
The parameters of the length-weight relationships (Weight (g) = a x Length (mm) b) used for scallops in the North West assessment area are as follows:
| a | b | Source | |
|---|---|---|---|
| Total (annual) | 0.001142 | 2.513 | Cook et al. (1990) |
| Muscle (annual) | 0.00008563 | 2.668 | MSS unpublished |
The mean muscle weights are shown in Figure 3.3.2 and Table 3.3.3. The mean weights of individuals aged five to ten shows a gradual increase from the late 1980s to mid 1990s with similar inter-annual variations across age classes. Mean weights for those age categories which are less important in the catch show greater fluctuations.
3.3.4 Exploratory Analysis
Catch Data
Catch curves by cohort for the commercial catch-at-age data are shown in Figure 3.3.3. These show that age classes younger than age five are typically only partially recruited to the fishery. Clearly apparent are the very low 1982-1986 year classes, recruiting to the fishery at age three in 1985-1989, and the strong 1996-1999 cohorts, recruiting at age three in 1999-2002. These large year classes appear to have been subject to much higher fishing mortality (as evidenced by the steep slope) than those cohorts recruiting in the early 1980s. The unexpectedly low catches of the 1990 cohort at age nine, the 1991 cohort at age eight, etc. occurs across all age classes. These are due to the sudden decline in total landings in 1999, a result of the amnesic/paralytic shellfish poisoning ( ASP/ PSP) fishery closures in the assessment area. The increase in log catch numbers at age 10 (compared to age nine within a particular cohort) is an effect of the plus group i.e. this age class includes all individuals aged 10 and above.
Survey Data
Details of west coast scallop surveys which cover the North West assessment area are given in Table 3.2.4. This shows the relative consistency in terms of vessel, timing and number of hauls prior to 2000 with greater variability in all aspects since then. To account for vessel changes (working different numbers of dredges), the catch rates are standardised by total dredge width. Survey data from 1993 onwards (the first year of full spatial coverage) are included in the assessment of the North West area.
The catch rates of scallops (all ages combined) at stations throughout the survey area between 2000 and 2010 are shown in Figure 3.3.4. Total catch rates across much of the region have shown a marked decline since 2006. Table 3.3.4 shows the average catch rates by age class and year. In the most recent surveys, average catch rates have fallen across a range of age classes, most notably ages three to six. The plus group in recent years will contain the survivors of the very strong year-classes recruiting in the late 1990s and early 2000s (apparent as high catch rates at age three and four earlier in the time series) and catch rates of this age class remain relatively high.
Catch curve analyses are shown in Figure 3.3.5 and show the survey index by age (ages two to 10+) on the log scale for each cohort. Ages two and three are seen to have consistently lower survey catchability than individuals from the older age classes. The cohort curves are relatively smooth showing consistent tracking of year classes and selection. The increase in log catch numbers at age 10 (compared to age 9 within a particular cohort) is due to the plus group effect i.e. this age class includes all individuals aged 10 and above.
3.3.5 Final Assessment
TSA
The exploratory catch curve analysis shows low and highly variable catch rates of age two individuals in both the commercial catch and survey. In addition, the catch rates of the 10+ age group in the survey are very noisy. These data are therefore excluded from the final assessment. To summarise, the final TSA run uses the following data and settings:
Commercial catch-at-age data from 1982 - 2010, ages 3 - 10+
Dredge survey catch-at-age index from 1993 - 2010, ages 3 - 9
Average recruitment ( i.e. no stock-recruit relationship assumed)
Age at full selection = 6
Survey timing = 0.45 (average survey timing as proportion of year, equivalent to mid June)
Outputs from the TSA assessment are shown in Figure 3.3.6 and estimated parameters given in Table 3.3.5.
Standardised prediction errors from the assessment model are shown in Figures 3.3.7 (landings) and 3.3.8 (survey). The landings prediction errors are greatest and show some similar trends across age classes during the middle of the time series when the level of the landings fluctuates between high and low over consecutive years. In 1999, coinciding with the very low total landings due the ASP/ PSP fishery closures, all age classes show a negative landings prediction error indicating that the observed value is lower than the value predicted by the model. In subsequent years the residuals are typically positive. Fishing mortalities evolve relatively slowly in TSA which can result in systematic prediction errors when the fishery (total catches) fluctuates rapidly. Most of the prediction errors are generally of low magnitude (between plus and minus two) and not sufficient to invalidate the fit of the model to the data.
The standardised survey prediction errors suggest increasing survey catchability through the time series for ages six to eight, but decreasing catchability for ages three and four. As in the West of Kintyre, these apparent changes in survey catchability cannot be associated with any particular changes to the survey. The age nine class show a number of outlying errors with greater magnitude - one due to the abnormally high catches of age nine individuals in 2005 (significantly higher than age eight in 2004) and the other a much lower than expected catch of age nine in 2010. Prediction errors for all other age classes are generally of low magnitude.
The relationship between estimated ( SSB) and recruitment at age three is particularly poor, with high and low recruitment occurring at both small and large stock size ( Figure 3.3.9).
Retrospective Analysis
The retrospective plots are shown in Figure 3.3.10. These show that the assessment tends to overestimate SSB in the final year. The addition of data from 2009 and 2010 resulted in a significant downwards revision to the SSB estimates from around 2000 onwards, coupled with downward revision to recruitment and an upwards revision to the estimates of mean fishing mortality. Further exploration of the estimated parameters reveals that the estimated survey catchabilities show a substantial upwards revision (although the trend is still downwards) with the addition of the 2009 survey data. This results in the recruitment being revised downwards and the mean F being revised upwards. Retrospective patterns since 2009 are less apparent.
State of the Stock
The state of the stock is summarised in Figure 3.3.6. The final estimates for the stock in 2010 are:
F (average over ages 4-6) = 0.111 yr -1
SSB (total over ages 3-10+) = 2,689 t (muscle weight)
There are no reference points for this stock.
Estimates (and standard errors) of age structured population abundance and fishing mortality are presented in Tables 3.3.6- 3.3.9. The final estimates are smoothed across years which results in differences between the estimates of fishing mortality at age in the first year given here and the parameter estimates in Table 3.3.5.
Table 3.3.10 shows the stock summary. Following a period of high recruitment in the late 1990s and early 2000s, recruitment has declined and has been below the long term average since 2006. As a result SSB has also declined from the relatively very high levels of 10 years ago. Catches have been low in recent years and this is reflected by the low fishing mortality.
Comparison with Previous Assessments
The last Scottish scallop assessment report was published in 2006 (Howell et al., 2006) and presented the results of an assessment conducted using commercial data up to and including 2003 in a quarterly VPA tuned to average fishing mortality. The assessment for the North West was updated in 2008 using data up to 2007 and a summary of stock status including biomass trends can be found in the MSS Stock booklet (Scottish Government, 2010). The historical trends in SSB and recruitment as estimated in the current assessment show some agreement with those estimated previously. Biomass initially declines through the 1980s, reaching the lowest estimated values around 1990 and then increases to a maximum in 2000. (Note the current assessment provides biomass estimates in terms of muscle weight only which are not directly comparable, in terms of absolute value, to previous estimates which were of muscle plus gonad weight). The general trend in estimated mean F from the current assessment is similar to that estimated by Howell et al. (2006) although the level is slightly lower. This may be due to differences in the age range over which mean F is calculated in the two assessments. This assessment uses ages four to six, whereas previously it was for ages six to eight.
3.3.6 Quality of the Assessment
Landings Data
Fishers are required to provide information about quantities landed and fishing location by ICES rectangle on either EU logbooks or Shell 1 forms (under 10 m vessels). The implementation of 'the registration of buyers and sellers' legislation in the UK in 2006 requires details of the landed catch also to be recorded at the point of first sale and sales notes are cross checked against vessels landings declarations. This procedure is thought to have improved the accuracy of reported landings data in recent years.
Age Composition
The scallop market sampling levels (number of trips and number of scallops sampled) for the North West in 2010 were at the highest level since 2003. During the mid to late 2000s sampling levels were lower which may explain the highly variable catch curves during these years. The lack of consistency in sampling throughout the year ( i.e. some quarters not sampled) may also cause bias and/or noise in the annual catch-at-age data.
Weights at Age
Mean muscle weights-at-age are now derived from a length-weight relationship (unpublished MSS) and the size at age composition data collected through the market sampling programme. This allows for inter-annual variation in weights-at-age (due to changes in size-at-age) to be accounted for in the estimate of SSB. Previously fixed values for weight-at-age were used.
Survey Data
Survey stations are located in most of the major fishing areas of the North West assessment area. The exceptions are the area to the western tip of Skye and the inshore waters of the east coast of Lewis, Harris and North Uist where there has been some fishing activity (inferred from VMS data) in recent years.
The survey utilises a standard commercial dredge with large belly rings and a smaller laboratory dredge with small belly rings. Younger age classes (two and three year olds) have lower survey catchability because they are smaller and are able to pass through the belly rings of the dredge. However, occasional large catches are possible because other scallops and debris block their escape through the dredge. Variability in survey vessel and timing may also result in noisy data with apparently changing catchabilities, but not a systematic trend in particular age classes.
Model Formulation
TSA is useful in that it allows both fishing mortality and survey catchability to evolve over time in a constrained manner. However, the current model formulation does not allow for survey catchability trends to differ between age classes resulting in some systematic trends in the standardised survey prediction errors in the final run.
3.3.7 Management Considerations
SSB, recruitment and catch have all declined markedly in the North West assessment area over the last ten years. Measures to increase spawning stock biomass should be considered. However given the apparent temporal autocorrelation in recruitment ( i.e. successive recruitments are correlated), it is unlikely that SSB will increase quickly even at the current relatively low fishing mortality. An increase in the minimum landing size has been proposed as a possible management measure for Scottish scallop fisheries. The survival of discarded scallops is high and therefore most undersized scallops returned to the sea have the potential to grow. This measure therefore has the potential to increase the reproductive capacity of the stock, provided that there is no associated increase in fishing effort.
Management measures to control fishing mortality should be considered and 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.
3.4 Clyde
3.4.1 Description of the Fishery
The Scottish commercial scallop fishery began in the Clyde in the 1960s. Landings have fluctuated markedly over the ensuing period, declining to under 20 tonnes in 1990 and increasing since then to over 500 tonnes in 2010 ( Figure 3.1.1). The fishery is prosecuted by two Campbeltown vessels and vessels from the Isle of Man fleet (up to six vessels) which fish in the Clyde at various times of the year. The scallop vessels vary in size from 9.9 m to approximately 20 m and regularly land into Campbeltown or occasionally into Ayr and Troon.
3.4.2 Sampling Levels and Age Compositions
In 2010 over 2,500 scallops were measured in the Clyde assessment area from 11 fishing trips during quarters one to three. In previous years, sampling of Clyde landings has been rather limited in terms of seasonal coverage and number of trips sampled ( Table 3.4.1).
Catch-at-age Data
Raised catch-at-age data for the Clyde area are available in FMD from 1982 to 2010. However, given the low historical sampling levels, these data are not deemed of sufficient quality for further analysis and are not presented here.
3.4.3 Biological Data
The parameters of the length-weight relationships (Weight (g) = a x Length (mm) b) available for scallops in the Clyde assessment area are as follows:
| a | b | Source | |
|---|---|---|---|
| Total (annual) | 0.001142 | 2.513 | Cook et al. (1990) |
| Muscle (annual) | 0.00006634 | 2.668 | MSS unpublished |
3.4.4 Assessments
Due to the limited port sampling, the age composition data are insufficient for carrying out a reliable TSA assessment. No survey data are available for the Clyde assessment area.
In the absence of an assessment, advice on management or management measures specific to the Clyde assessment area have not been considered here.
3.5 Irish Sea
3.5.1 Description of the Fishery
The Irish Sea scallop assessment area covers the waters to the south west of Scotland from latitude 55˚ N to 53˚ N. The fishery began in the 1970s and landings (into Scotland) have steadily increased to a peak of 1,461 t in 2010, establishing the Irish Sea as one of the main fishing grounds in terms of landings into Scotland. At various times of the year approximately 20 large (15-20 m in length) nomadic Scottish vessels fish the Irish Sea particularly in Luce Bay, scallop grounds off Burrow Head and around the Isle of Man. These vessels normally land at Kirkudbright or the Isle of Whithorn, but depending on fishing locations, they may also land to ports on Anglesey and up the coast of England or into Peel (Isle of Man). The majority of landings from the Irish Sea assessment area are landed into ports outside Scotland with a large proportion taken by non-Scottish vessels.
A by-law, introduced by the Isle of Man Government in 2010, resulted in a number of larger vessels in the Scottish scallop fleet being excluded from fishing grounds in Isle of Man waters. Concerns about displaced fishing effort, particularly into Luce Bay (a designated SAC) in southwest Scotland resulted in a four month extension to the seasonal closure of this area in 2010-11, under The Scallops (Luce Bay) (Prohibition of Fishing) Order 2010.
3.5.2 Sampling Levels and Age Compositions
The landings from Kirkcudbright (the main Irish Sea landing port) have been sampled irregularly with a total of only 3,500 scallops measured between 2008 and 2010 ( Table 3.5.1). Over half of the landings from the Irish Sea assessment area are landed into other UK ports (out-with Scotland), which makes obtaining representative fishery data from this area particularly difficult.
Catch-at-age Data
Catch-at-age data raised to Scottish landings for the Irish Sea area are available in FMD for the mid 1980s onwards. However, given that these are based on a small number of samples taken only at Scottish ports, these data are not deemed of sufficient quality for further analysis and are not presented here.
3.5.3 Biological Data
The parameters of the length-weight relationships (Weight (g) = a x Length (mm) b) available for scallops in the Irish Sea assessment area are as follows:
| a | b | Source | |
|---|---|---|---|
| Total (annual) | 0.001142 | 2.513 | Cook et al. (1990) |
| Muscle (annual) | 0.00006634 | 2.668 | MSS unpublished |
3.5.4 Assessments
The available age composition data are insufficient for an analytical assessment, and no surveys have been undertaken in this area by MSS. Since 2007, however, Bangor University has undertaken a programme of research and monitoring of species of fisheries and conservation importance (including scallops) in the waters surrounding the Isle of Man. Their most recent assessment of stock status is based on a time series of abundance estimates from biannual dredge surveys conducted across the scallop fishing grounds around the Isle of Man (Murray et al., 2009). The index of total abundance shows an increase in recent years although this appears largely due to major increases in abundance on a limited number of scallop grounds. Applying a precautionary approach ( PA), recent advice from Bangor University (Murray et al., 2009) has been for no increase in catches, with the acknowledgement that landings figures are likely to be incomplete due to the unavailability of landings data from the other UK (non Scotland) administrations.
3.5.5 Management Considerations
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, multilateral approach to the management of stocks in the area. MSS scientists have been involved in a proposal to set up an ICES Working Group on scallops which should facilitate the scientific collaboration required to produce robust stock assessments for this 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.
East Scotland (East of 4 ⁰ W)
3.6 North East
3.6.1 Description of the Fishery
The North East scallop fishery developed in the 1980s and landings have fluctuated throughout the time series with a peak of 3,491 t in 1996 but falling to 1,267 t in 2010 ( Figure 3.1.1). Large nomadic vessels (over 15 m) fish the scallop grounds in the inner and outer Moray Firth with approximately six vessels also fishing grounds to the east coast of the northern Orkney Isles. These vessels land regularly into Wick, Buckie and Fraserburgh.
3.6.2 Sampling Levels and Age Compositions
Sampling levels for the North East area are shown in Table 3.6.1. Previously, landings from all four quarters were sampled relatively consistently, however, in recent years some quarters are un-sampled. This may reflect seasonality in fishing patterns (a lack of trips to sample), but the total number of samples obtained has also fallen (only six trips were sampled in 2010).
Catch-at-age Data
Catch-at-age data for the North East are available from 1984 to 2010. The data are shown in Table 3.6.2 and Figure 3.6.1. In the early part of the time series, catches were dominated by individuals in the 10+ age category, whereas more recently, the catches consist largely of age four to seven year olds.
3.6.3 Biological Data
The parameters of the length-weight relationships (Weight (g) = a x Length (mm) b) available for scallops in the North East assessment area are as follows:
| a | b | Source | |
|---|---|---|---|
| Total (annual) | 0.001142 | 2.513 | Cook et al. (1990) |
| Muscle (annual) | 0.00006386 | 2.668 | MSS unpublished |
The mean muscle weights are shown in Figure 3.6.2 and Table 3.6.3. The historical mean weights at age show variability, but no systematic trend until the mid 2000s when mean weights of older individuals increased up to 2008 and then declined. Inter-annual fluctuations in mean weight at age are similar across age classes.
3.6.4 Exploratory Analyses
Catch Data
Catch curves by cohort for the commercial catch-at-age data are shown in Figure 3.6.3. In general, these data appear quite variable which may be due to the rapid fluctuations which have occurred in the fishery in this area (due to effort displacement from areas closed to scallop fishing due to ASP/ PSP toxins), but which could also be due to the relatively low sampling levels during the early and latter part of the time series. The catch curves show partial recruitment to the fishery up to age five. Those from the early cohorts show an unusual increasing pattern which is due to the rapidly increasing fishery during the first 10 years (approximately) of the time series. There are above average catch curves for the 1988 to 1991 cohorts (recruitment in 1991 to 1994) which decline relatively steeply, suggesting strong year classes with higher than average mortality.
Survey Data
A partial North Sea scallop survey was conducted in 1993, with full coverage of the North East assessment area beginning in 1994. Details of the surveys are given in Table 3.6.4 and the spatial distribution of stations is shown in Figure 3.6.4 for surveys from 2000 to 2010. Since 2001, the survey has been relatively consistent in terms of timing (June/July) and vessel. However, prior to this the survey was conducted towards the end of the year, between September and December, and in one instance in the following calendar year, the 1998 survey being conducted in January/February 1999. As for the west coast survey, no comparative tows have been conducted to compare catch rates between vessels and standardisation involves dividing the catch rate by dredge width to account for differences in the number of dredges worked.
The catch rates of scallops (all ages combined) at stations across the North East area between 2000 and 2010 are shown in Figure 3.6.4. The lowest catch rates typically occur at the stations in the offshore areas of the central Moray Firth while higher catch rates are generally found in the inner Moray Firth. Table 3.6.5 shows the average catch rates by age class and year. The highest catch rates (> 10 hr -1m -1) are observed at the start of the survey time series and are of the strong 1989 and 1990 year classes, first appearing at age five and four (respectively) in the 1994 survey (and then one year older in subsequent surveys). Since then catch rates have in general been less than five individuals hr -1m -1.
Catch curve analyses are shown in Figure 3.6.5 and show the survey index by age on the log scale for each cohort. The steeply increasing left hand limb of these curves indicates that the youngest age classes (ages two and three) have significantly lower survey catchability than older age classes. The cohort curves of 1988 to 1991 are relatively smooth showing good tracking of year classes (which are particularly strong in 1989 and 1990) and consistent selection. The year class curves of the weak 1993 and 1994 cohorts are unusual in that they are relatively flat and show little decline in log numbers at older ages suggesting increasing catchability of the older ages for a period of time.
3.6.5 Final Assessment
TSA
The exploratory catch curve analysis shows low and highly variable catch rates of age two individuals in both the commercial catch and survey. In addition, the catch rates of the 10+ age group in the survey are considered too noisy to be included in the assessment. To summarise, the final TSA run for the North East area uses the following data and settings:
Commercial catch-at-age data from 1984 - 2010, ages 3 - 10+
Dredge survey catch-at-age index from 1994 - 2010, ages 3 - 9
Average recruitment ( i.e. no stock-recruit relationship assumed)
Age at full selection = 6
Survey timing = 0.58 (average survey timing as proportion of year)
Outputs from the TSA assessment are shown in Figure 3.6.6 and estimated parameter values are given in Table 3.6.6.
Standardised prediction errors from the assessment model are shown in Figures 3.6.7 (landings) and 3.6.8 (survey). The majority of the catch prediction errors are well distributed about zero. There are, however, a number of significant outliers which may be the result of noisy catch composition data due to low sampling levels, particularly during the early part of the time series.
The survey prediction errors are well distributed and do not suggest that the model is predicting survey indices different to the observed data. In contrast to the west coast, there is no evidence to indicate age-related differences in catchability trends.
There is no clear relationship between stock size ( SSB) and recruitment at age three for this stock, although examination of the data suggests that high recruitment is associated with low stock sizes ( Figure 3.6.9).
Retrospective Analysis
The results of the retrospective analysis are shown in Figure 3.6.10. There is some tendency for the assessment to overestimate SSB and underestimate fishing mortality in the final year although this appears to be less of a problem between 2009 and 2010 than 2008 and 2009. There are no systematic revisions to recruitment estimates in the final year or for any historical estimates of the stock status metrics (F and SSB).
State of the Stock
The state of the stock is summarised in Figure 3.6.6. The final estimates for the stock in 2010 are:
F(average over ages 4-6) = 0.176 yr -1
SSB (total over ages 3-10+) = 1,214 t (muscle weight)
There are no reference points for this stock.
Estimates (and standard errors) of age-structured population abundance and fishing mortality are presented in Tables 3.6.7- 3.6.10. Table 3.6.11 shows the stock summary. Fishing mortality on this stock increased rapidly during the late 1980s and early 1990s. Over the last five years it has been more stable, fluctuating at around 0.2 yr -1 (above the long term average), but with significant uncertainty surrounding the estimates throughout the time period. SSB has declined slightly in recent years after a period of relatively stable/increasing SSB since the mid 1990s. Recruitment has also declined in recent years, although it was slightly higher in 2010 than 2009.
Comparison with Previous Assessments
The last Scottish scallop assessment report was published in 2006 (Howell et al., 2006) and presented the results of an assessment conducted using commercial data up to and including 2003 in a quarterly VPA tuned to average fishing mortality). The assessment for the North East was updated in 2008 using data up to 2007 and a summary of stock status including biomass trends can be found in the MSS Stock booklet (Scottish Government, 2010). Trends in the time series of recruitment at age three from the current stock assessment are similar to those previously estimated but with a time lag of one year due to the differing ages at which recruitment has been estimated (previously age two). Trends in SSB and fishing mortality as estimated in the current assessment are broadly similar to those previously estimated. Levels of mean F and SSB are not comparable to earlier estimates as mean F was previously calculated over ages six to eight (in the current assessment it is estimated for ages four to six) and SSB was previously presented as muscle plus gonad, whereas in this assessment it is muscle only.
3.6.6 Quality of the Assessment
Landings Data
Fishers are required to provide information about quantities landed and fishing location by ICES rectangle on either EU logbooks or Shell 1 forms (under 10 m vessels). The implementation of 'the registration of buyers and sellers' legislation in the UK in 2006 requires details of the landed catch also to be recorded at the point of first sale and sales notes are cross checked against vessels landings declarations. This procedure is thought to have improved the accuracy of reported landings data in recent years.
Age Composition
The scallop market sampling levels for the North East were poor, in terms of both total number of trips and seasonal coverage of the fishery, at the start of the time series and again more recently. Catch-at-age composition data for these periods are therefore likely to be less reliable, resulting in greater uncertainty in estimated stock status.
Weights at Age
Mean muscle weights-at-age are now derived from a length-weight relationship (unpublished MSS) and the size at age composition data collected through the market sampling programme. This allows for inter-annual variation in weights-at-age (due to changes in size-at-age) to be accounted for in the estimate of SSB. Previously, fixed values for weight-at-age were used.
Survey Data
Survey stations are located in most of the major fishing areas (inferred from VMS data) of the North East assessment area. However, there appears to be significant spatial variability in the intensity of sampling (as a result of the original fixed station survey design), with some grounds such as those off the north Moray coast and to the east of Orkney much more intensely sampled than the large central offshore fishing grounds.
The survey utilises a standard commercial dredge with large belly rings and a smaller laboratory dredge with small belly rings. Younger age classes (two and three year olds) have lower survey catchability because they are smaller in length and width and are able to pass through the belly rings of the dredge. However, occasional large catches are possible because other scallops and debris block their escape through the dredge which may result in variable catchability.
Model Formulation
TSA is useful in that it allows both fishing mortality and survey catchability to evolve over time in a constrained manner. Precision estimates are provided for the parameters of interest which is extremely useful in this case as it highlights the extent of the uncertainty surrounding the estimates of mean F potentially associated with low sampling levels.
3.6.7 Management Considerations
Recruitment and SSB have been relatively stable in recent years, although below the long term average for this stock. Fishing mortality has been above the long term average. In such circumstances 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.
3.7 Shetland
3.7.1 Description of the Fishery
The Shetland scallop fishery developed in the late 1960s and landings have steadily increased to a high of 1,071t in 2010. Large scallops vessels (>15 m) generally target the grounds around the Islands of Whalsay and Fetlar in the north east of Shetland and to a lesser extent grounds in the North of Yell Sound. Around 30 dredge vessels are licensed under the Regulating Order ( RO) by the SSMO to fish at Shetland.
3.7.2 Sampling Levels and Age Compositions
Sampling levels for the Shetland area are shown in Table 3.7.1. In 2010, 6,512 individual scallops were measured from 65 sampled trips. The landings from the Shetland area have been consistently well sampled since the late 1980s.
Catch-at-age Data
Catch-at-age data for Shetland are shown in Table 3.7.2 and Figure 3.7.1 for 1985 onwards. The catches are dominated by individuals from age classes four to seven, although the 10+ category also represents a significant component of the catch. Individuals aged three and under only appear in the catch with the occurrence of a strong year class, for example age three in 2005 and 2006, and age two in 1992.
3.7.3 Biological Data
The parameters of the length-weight relationships (Weight (g) = a x Length (mm) b) available for scallops in the Shetland assessment area are as follows:
| a | b | Source | |
|---|---|---|---|
| Total (annual) | 0.001142 | 2.513 | Cook et al. (1990) |
| Muscle (annual) | 0.00007599 | 2.668 | MSS unpublished |
The mean muscle weights are shown in Figure 3.7.2 and Table 3.7.3. There is a gradually increasing trend in mean weight at age over all ages through the 2000s, which appears to have levelled off more recently. Inter-annual fluctuations in mean weight at age are similar across age classes.
3.7.4 Exploratory Analysis
Catch Data
Catch curves by cohort for the commercial catch-at-age data are shown in Figure 3.7.3. On the whole the catch curves are relatively smooth, in comparison to those from other areas, which may be due to the consistently good sampling levels in this area. The strong 1989 and 1990 year classes (recruitment in 1992 and 1993) are clearly apparent as above average catch curves, whilst the 1994 and 1995 catch curves are below average, indicating low recruitment in 1997 and 1998.
Survey Data
Details of the surveys which have been carried out at Shetland are given in Table 3.7.4. Typically, the survey has been carried out in late winter (February/March), although there have been a number of exceptions to this: the 2002 survey was carried out three months earlier than usual at the end of 2001 and the first two surveys (1995 and 1996) were carried out in May. The majority of the surveys were carried out on the FRV Clupea (1998-2008). The catch rates have been standardised by dredge width to account for changes in vessel i.e. different vessels working different numbers of dredges. Survey data from 1995 onwards are included in the assessment of the Shetland area.
The number of valid survey stations is usually between 80 and 90. However, bad weather has occasionally disrupted the survey resulting in reduced numbers of stations in 2000, 2004, 2008 and 2009. The stations which were missed on these surveys are generally those to the south west of Shetland and in other exposed locations. It is not possible to determine whether lack of data from these areas has significantly biased the survey catch‑at-age indices in these years.
The catch rates of scallops (all ages combined) at stations throughout the survey area between 2000 and 2010 are shown in Figure 3.7.4. High catch rates are relatively consistently recorded at the stations in Yell Sound on the north coast. Catch rates along the south east coast of mainland Shetland show quite significant inter-annual variability. Table 3.7.5 shows the average catch rates by age class and year. The highest catch rate (~ 9 individuals hr -1m -1) is observed at age five in 1995 and is the result of the strong 1990 year class (also observed in the commercial catch curves).
Catch curve analyses are shown in Figure 3.7.5 and show the survey index by age on the log scale for each cohort. The steeply increasing left hand limb of these curves indicates that the youngest age classes (ages two and three) have significantly lower survey catchability than older age classes. The early cohort curves in particular, are relatively smooth, showing good tracking of year classes and consistent selection. Recent year classes (2004-2006) appear to be of below average abundance.
3.7.5 Final Assessment
TSA
Catch rates of age two individuals are low and highly variable in both the commercial catch and survey (as seen from the exploratory analysis). In addition, catch rates for the 10+ age class are also quite noisy. These age classes are therefore excluded from the final assessment. To summarise, the final TSA run uses the following data and settings:
Commercial catch-at-age data from 1986-2010, ages 3-9
Dredge survey catch-at-age index from 1995-2010 (1997 missing year), ages 3-9
Average recruitment ( i.e. no stock-recruit relationship assumed)
Age at full selection = 7
Outputs from the TSA assessment are shown in Figure 3.7.6 and estimated parameter values are given in Table 3.7.6.
Standardised prediction errors from the assessment model are shown in Figures 3.7.7 (landings) and 3.7.8 (survey). Both sets of prediction errors are well distributed around zero and show no significant trends.
The stock recruitment scatter plot ( Figure 3.7.9) suggests that high values of recruitment have typically tended to occur at low values of SSB. Low recruitment has typically occurred at moderate and high levels of SSB suggesting that density dependent effects are potentially in operation ( e.g. SSB inhibiting survival of juvenile scallops to recruitment age through limitations in suitable habitat).
Retrospective Analysis
The results of the retrospective analysis are shown in Figure 3.7.10. The addition of the 2010 data results in a revision of the estimates of recent fishing mortality and SSB, although this is less apparent between 2008 and 2009. Mean F is revised upwards while SSB is revised downwards. Revisions are also made to recruitment, but there does not appear to systematic under or over estimation.
State of the Stock
The state of the stock is summarised in Figure 3.7.6. The final estimates for the stock in 2010 are:
F (average over ages 4-6) = 0.367 yr -1
SSB (total over ages 3-10+) = 622 t (muscle weight)
There are no reference points for this stock.
Estimates (and standard errors) of age-structured population abundance and fishing mortality are presented in Tables 3.7.7- 3.7.10. Table 3.7.11 shows the stock summary. Fishing mortality on this stock is estimated to have increased markedly in 2010 to the highest value in the time series (although recent estimates are quite uncertain). The historical fishing mortality is estimated to have fluctuated between around 0.1 and 0.25. Recruitment has been fairly stable in recent years at around the long term average for the time series. SSB has been stable since the mid 2000s at above average levels.
Comparison with Previous Assessments
The last Scottish scallop assessment report was published in 2006 (Howell et al., 2006) and presented the results of an assessment conducted using commercial data up to and including 2003 in a quarterly VPA tuned to average fishing mortality. This was updated in 2008 using data up to 2007 and a summary of stock status, including biomass trends can be found in the MSS Stock booklet (Scottish Government, 2010). For the current assessment, the commercial catch at age data series has been extended back in time and runs from 1986 onwards rather than from 1991 as in previous assessments. Trends in the time series of recruitment at age three from the current assessment are similar to those previously estimated but with a time lag of one year due to the differing ages at which recruitment has been estimated (previously age two). Trends in SSB and fishing mortality as estimated in the current assessment are broadly similar to those estimated previously Levels of mean F and SSB are not comparable to earlier estimates as mean F was previously calculated over ages six to eight (in the current assessment it is estimated for ages four to six) and SSB was previously presented as muscle plus gonad weight, whereas in the current assessment it is for muscle only.
NAFC Marine Centre also conduct stock assessments of the scallops around Shetland based on catch-at-age data obtained from their own commercial catch sampling. Leslie et al. (2009) use a quarterly VPA tuned using fishing effort data and including commercial data from 2000 to 2008. Estimates of total fishing mortality are of a similar magnitude to those estimated here and the trend in total numbers is also similar showing a peak in the mid 2000s. However, in terms of absolute value, numbers at age are quite different. The values estimated in this assessment are much greater than those estimated in the Leslie et al. (2009) assessment. This can be attributed to the differences in the raised commercial catch-at-age data used in the assessments. The total catch numbers used by Leslie et al. (2009) are significantly lower than MSS estimates. This appears to be due to differences in total landings as officially reported to MS (used by MSS) and those recorded by the SSMO (and used by NAFC). See discussion for further details.
3.7.6 Quality of the Assessment
Landings Data
Fishers are required to provide information about quantities landed and fishing location by ICES rectangle on either EU logbooks or Shell 1 forms (under 10 m vessels). The implementation of 'the registration of buyers and sellers' legislation in the UK in 2006 requires details of the landed catch also to be recorded at the point of first sale and sales notes are cross checked against vessels landings declarations. This procedure is thought to have improved the accuracy of reported landings data in recent years although there are unexplained anomalies with the SSMO data.
Age Composition
Scallop market sampling levels (the number of trips and number of scallops sampled) for Shetland have been consistently high since 1985 with only occasional un-sampled quarters.
Weights at Age
Mean muscle weights-at-age are now derived from a length-weight relationship (unpublished MSS) and the size at age composition data collected through the market sampling programme. This allows for inter-annual variation in weights-at-age (due to changes in size-at-age) to be accounted for in the estimate of SSB. Previously fixed values for weight-at-age were used.
Biological data on scallops from the Shetland area are 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.
Survey Data
On average, around 80 stations are sampled each year on the Shetland dredge survey, although in 2004, 2008 and 2009, poor weather substantially curtailed the survey. The Shetland assessment area is relatively small and has a significantly greater number of stations than other areas of comparable size. The survey stations are therefore at a much higher density across the fishing grounds than other areas. However, a number of important fishing areas are not surveyed. There are no survey stations located around the Outer Skerries and to the southeast of Whalsay, areas which have been fished regularly in recent years (according to VMS data).
The survey utilises a standard commercial dredge with large belly rings and a smaller laboratory dredge with small belly rings. Younger age classes (two and three year olds) have lower survey catchability because they are smaller in length and width and are able to pass through the belly rings of the dredge. However, occasional large catches are possible because other scallops and debris block their escape through the dredge which may result in variable catchability.
Model Formulation
TSA is useful in that it allows both fishing mortality and survey catchability to evolve over time in a constrained manner. It also allows for incomplete data series so that survey data with missing years (as is the case here) can be easily used.
3.7.7 Management Considerations
The VMS data for scallop vessels in the Shetland assessment area, suggests that fishing (at least by these larger vessels) occurs solely within the six mile limit of Shetland and is therefore licensed and managed under the SSMO.
SSB and recruitment currently appear relatively stable and above their long term average values. Fishing mortality has been above the long term average. In such circumstances advice is for no increase in fishing mortality.
3.8 East Coast
3.8.1 Description of the Fishery
The scallop fishery in the East Coast assessment area developed in the 1990s. There has been marked variability in the landings throughout the time period, from 299 t in 2001 to a high of over 2,500 t landed in 2007 ( Figure 3.1.1). The East Coast scallop fleet consists of approximately twenty large vessels (15 to 22 m) which are mostly nomadic and fish from Peterhead down to Eyemouth. The fishery continues year round, but has a typical seasonal pattern with peak landings occurring in the second quarter. Vessels may work an area for a period of time before moving on, often returning to the same area the following year. Intense fishing has been known to occur off Bell Rock in the Firth of Forth and also on the scallop grounds further south off the coast of Burnmouth. Vessels land into Peterhead, Aberdeen, Gourdon, Montrose or Eyemouth, depending on which grounds they have been targeting.
3.8.2 Sampling Levels and Age Compositions
Sampling of the landings has been carried out since the beginning of the fishery ( Table 3.8.1). A period of low sampling levels is apparent between 2001 and 2003 and is likely to be due to a lack of sampling opportunities given the low level of landings at this time. As landings increased in the mid 2000s, sampled numbers and trips also increased, but have declined again in 2009 and 2010. In 2010 only 6 trips and 1,287 individuals (across three quarters) were sampled.
Catch-at-age Data
Catch-at-age data for the East Coast are shown in Figure 3.8.1 for 1991 onwards. No specific age classes consistently dominate the landings and there are no apparent trends in age composition. The high landings in 1994-1995 consist mainly of young (ages 4-6) individuals from the 1989-1991 year classes, which dominate the landings in 1999 at older ages (8-10+).
3.8.3 Biological Data
The parameters of the length-weight relationships (Weight (g) = a x Length (mm) b) available for scallops in the East Coast assessment area are as follows:
| a | b | Source | |
|---|---|---|---|
| Total (annual) | 0.001142 | 2.513 | Cook et al. (1990) |
| Muscle (annual) | 0.00006386 | 2.668 | MSS unpublished |
The mean muscle weights are shown in Figure 3.8.2 and Table 3.8.3. There are no apparent systematic temporal trends although inter-annual fluctuations in mean weight at age are similar across age classes.
3.8.4 Exploratory Analysis
Catch Data
Catch curves by cohort for the commercial catch-at-age data are shown in Figure 3.8.3. On the whole, the catch curves are relatively noisy which is likely to be due to the often low levels of sampling, but also to the highly sporadic nature of the fishery. There are, however, some indications of relative year class strength, with the 1989 and 1999 year classes (recruitment in 1992 and 2002) appearing well above average and those of the mid 1990s being particularly low.
Survey Data
Details of the surveys which have been carried out in the East Coast assessment area are given in Table 3.6.4. A partial North Sea scallop survey was conducted in 1993, with full coverage of the East Coast assessment area beginning in 1994. Since 2001, the survey has been relatively consistent in terms of timing (June/July) and vessel. However, prior to this the survey was conducted towards the end of the year (September-December) and in one instance actually in the following calendar year (1998 survey conducted in January/February 1999). As in the west coast survey, no comparative tows have been conducted to compare catch rates between vessels and standardisation involves dividing the catch rate by dredge width to account for differences in the number of dredges worked.
The catch rates of scallops (all ages combined) at stations across the East Coast area between 2000 and 2010 are shown in Figure 3.8.4. During the early/mid 2000s, highest catch rates occurred in the offshore areas whilst more recently, the highest catch rates typically occur at the more inshore survey stations. Table 3.8.4 shows the average catch rates by age class and year. Catch rates by age class in this survey are generally < 5 individuals hr -1m -1 with only age five in 1994 having a catch rate much greater than this (~ 13 scallops hr -1m -1), further evidence for a strong year class in 1989. The year classes of 1998, 1999 and 2003 (recruitment in 2001, 2002 and 2006, respectively) also appear strong with above average catch rates across a range of age classes.
Catch curve analyses are shown in Figure 3.8.5 and show the survey index by age on the log scale for each cohort. The steeply increasing left hand limb of these curves indicates that the youngest age classes (ages two and three) have significantly lower survey catchability than older age classes and in fact catch rates of zero at age two are not uncommon (year classes 1996, 2001, 2004, 2006 and 2008). The cohort curves of 1998 to 2002 are relatively smooth showing good tracking of year classes (particularly the strong 1998 cohort) and consistent selection. Prior to this, the catch curves show increasing catch numbers through the cohort as it ages suggesting changes in the survey catchability of these age classes (compared to younger ages) through time.
3.8.5 Preliminary Assessment
The available commercial catch-at-age composition data are insufficient for an analytical assessment. A preliminary assessment of stock status is presented based on trends in survey data. Figure 3.8.6 shows mean standardised indices of SSB and recruitment from the survey. The SSB index is calculated as the sum of products of the catch rates at age and muscle weight at age for each year. Recruitment is given by the catch rate at age three.
A proxy for fishing mortality (exploitation rate) is calculated as the ratio of catch to survey SSB index.
The survey data show a doubling of SSB between the late 1990s and 2008 which appears to be largely due to the three strong year classes recruiting in 2001, 2002 and 2006. Evidence of a strong 2002 year class is also seen in the commercial catch data (see Section 3.8.4). Since 2007, recruitment has been low although the landings have remained at a fairly high level, resulting in a decline in SSB. The highly variable total landings are reflected in the estimates of exploitation rate (F proxy).
Comparison with Previous Assessments
Analytical assessments have not been conducted for the East Coast area. Survey data trends (in terms of a total catch rate and catch rates of individuals < 100 mm), up to 2004 were previously presented in Howell et al. (2006). The higher catch rates of small individuals (< 100 mm) in 2001-2003 and an increase in the total catch rate over this period are consistent with trends in the recruitment and SSB indices in the present assessment.
3.8.6 Quality of the Assessment
Landings Data
Fishers are required to provide information about quantities landed and fishing location by ICES rectangle on either EU logbooks or Shell 1 forms (under 10 m vessels). The implementation of 'the registration of buyers and sellers' legislation in the UK in 2006 requires details of the landed catch also to be recorded at the point of first sale and sales notes are cross checked against vessels landings declarations. This procedure is thought to have improved the accuracy of reported landings data in recent years.
Age Composition
Scallop market sampling levels (number of trips and number of scallops sampled) for the East Coast area have periodically been poor. The time series of catch composition data is therefore considered of insufficient quality for use in an analytic stock assessment. Some components of the dataset have however, been used to provide supportive qualitative information on, for example, relative year class strength.
Weights at Age
Mean muscle weights-at-age are now derived from a length-weight relationship (unpublished MSS) and the size at age composition data collected through the market sampling programme. This allows for inter-annual variation in weights-at-age (due to changes in size-at-age) to be accounted for in the estimate of SSB. Previously fixed values for weight-at-age were used.
Survey Data
Typically between 40 and 50 stations are sampled each year on the survey of the East Coast assessment area. The survey shows reasonable coverage of the scallop fishing grounds (inferred from VMS data), although station density is relatively low (compared to, for example, Shetland) particularly in the offshore areas between Fife and Montrose.
The survey utilises a standard commercial dredge with large belly rings and a smaller laboratory dredge with small belly rings. Younger age classes (two and three year olds) have lower survey catchability because they are smaller in length and width and are able to pass through the belly rings of the dredge. However, occasional large catches are possible because other scallops and debris block their escape through the dredge which may result in variable catchability.
Model Formulation
No analytical assessment model is used. The assessment is based on trends in survey catch rates.
3.8.7 Management Considerations
Although no formal analytic assessment has been carried out for the East Coast area, it is clear from the stock trends evident in the survey data analysis that the fishery in this area is sporadic and dependent on a few strong year classes. In the long term the area is therefore unlikely to be able to continue to support landings at the levels recently seen.
3.9 Orkney
3.9.1 Description of the Fishery
The Orkney scallop fishery began in the 1970s but has remained relatively small in comparison to other assessment areas. There are approximately six local dredgers (some of which also fish with nets and creels) and twelve dive boats that fish grounds in Scapa Flow, Bay of Firth and around the northern isles of Orkney and other sites locally.
3.9.2 Sampling Levels, Age Compositions
Very limited sampling of recent landings was achieved in this fishery: a total of 3,992 scallops were measured during 2007-2010 ( Table 3.9.1). There are insufficient data for assessment purposes.
Catch-at-age Data
There are some catch-at-age data raised to Scottish landings for the Orkney area available in FMD from the early 1990s. However, these are based on a very low numbers of samples and are not considered of sufficient quality for further analysis.
3.9.3 Biological Data
The parameters of the length-weight relationships (Weight (g) = a x Length (mm) b) available for scallops in the Orkney assessment area are as follows:
| a | b | Source | |
|---|---|---|---|
| Total (annual) | 0.001142 | 2.513 | Cook et al. (1990) |
| Muscle (annual) | 0.00007599 | 2.668 | MSS unpublished |
3.9.4 Assessments
The available data are insufficient for an analytical assessment and there are no surveys of this area.
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