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Publication - Advice and guidance

Razor clam surveys - Firth of Clyde 2023 and Solway 2024-2025: report

Published
25 April 2025
From
Acting Cabinet Secretary for Net Zero and Energy, +1 more … Director-General Net Zero
Directorate
Agriculture and Rural Economy Directorate, +1 more … Marine Directorate
Topic
Marine and fisheries
ISBN
9781836914624

This report describes a surveys carried out in the Firth of Clyde (2023) and Solway (2024 and 2025) to estimate the densities and sizes of razor clam, Ensis siliqua. The surveys were conducted as part of the Scottish Government’s electrofishing scientific trial.

View supporting documents Supporting documents

Appendix I: Report figures

Figure 1: Chart of the Firth of Clyde showing the study area and locations mentioned in this report. The area in which electrofishing for razor clams is permitted under the Scottish Government scientific trial is shown hatched. (Chart created using Opensource QGIS, not to be used for navigation).
Chart of the Firth of Clyde showing the location of the Scottish Government scientific trial area for electrofishing for razor clams which is located along the Ayrshire coast from just north of Ardrossan down to just south of Ballantrae.
Figure 2: Mid-points of tows conducted in Irvine Bay (purple dots) in 2023, with the tow numbers being shown next to the positions. Some intermediate tow numbers are not shown to improve readability. (Chart created using Opensource QGIS, not to be used for navigation).
Chart of Irvine Bay, Ayrshire coast showing the mid-points of the video tows conducted in the 2023 survey.
Figure 3: Mid-points of tows conducted in Ayr Bay (purple dots) in 2023, with the tow numbers being shown next to the positions. Some intermediate tow numbers are not shown to improve readability. (Chart created using Opensource QGIS, not to be used for navigation).
Chart of Ayr Bay, Ayrshire coast showing the mid-points of the video tows conducted in the 2023 survey.
Figure 4: Mid-points of tows conducted in Culzean Bay (purple dots) in 2023, with the tow numbers being shown next to the positions. Some intermediate tow numbers are not shown to improve readability. (Chart created using Opensource QGIS, not to be used for navigation).
Chart of Culzean Bay, Ayrshire coast showing the mid-points of the video tows conducted in the 2023 survey.
Figure 5: Mid-points of tows conducted in Turnberry Bay (purple dots) in 2023, with the tow numbers being shown next to the positions. Some intermediate tow numbers are not shown to improve readability. (Chart created using Opensource QGIS, not to be used for navigation).
Chart of Turnberry Bay, Ayrshire coast showing the mid-points of the video tows conducted in the 2023 survey.
Figure 6: Mid-points of tows conducted in Machrie Bay, Arran (purple dots) in 2023, with the tow numbers being shown next to the positions. Some intermediate tow numbers are not shown to improve readability. (Chart created using Opensource QGIS, not to be used for navigation).
Chart of Machrie Bay, west side of Arran, showing the mid-points of the video tows conducted in the 2023 survey.
Figure 7: Mid-points of tows conducted in Carradale and Saddell Bays (purple dots) in 2023, with the tow numbers being shown next to the positions. Some intermediate tow numbers are not shown to improve readability. (Chart created using Opensource QGIS, not to be used for navigation).
Chart of Carradale and Saddell Bays, east coast of the Mull of Kintyre, showing the mid-points of the video tows conducted in the 2023 survey.
Figure 8: Chart of the Solway showing the study area and locations mentioned in this report. The area in which electrofishing for razor clams is permitted under the Scottish Government scientific trial is shown hatched. (Chart created using Opensource QGIS, not to be used for navigation).
Chart of the Solway showing the location of the Scottish Government scientific trial area for electrofishing for razor clams which is located between Burrow Head and Kirkudbright range.
Figure 9: Water column temperature (oC) and salinity (psu) profiles from the 2023 Firth of Clyde survey.
Water column profiles of temperature and salinity for each survey day in the Firth of Clyde. Temperature and salinity profiles were either slightly cooler and fresher towards the surface on days prior to 18th October or were vertically well mixed on subsequent days.
Figure 10: Example of image quality from the 2023 Firth of Clyde survey. The three separate MacArtney Luxus (MacArtney UK Ltd., Dyce, Aberdeen) camera feeds (top row images) are combined to generate a composite equivalent to 1.5 m width on the seabed (bottom image). Video is from tow 115, Saddell Bay 27th Oct recorded in 5.7 m water depth. The trail marks left in the sand by the razor fishing electrodes are also visible.
An illustrative example of the image quality captured by the razor clam video camera sled during the Firth of Clyde survey. The images clearly show the sandy seabed with some fragments of seaweed and a fully emerged razor clam lying on the seabed. The shallow indentations caused by the passage of the electrode rods can also be clearly seen.
Figure 11: Histograms showing the distribution of (a) tow durations (mins); (b) tow lengths (metres); (c) average tow speeds (metres min‑1) and (d) average exposure times to the electrical field (seconds) for the 2023 Firth of Clyde survey.
Histograms showing the distribution of tow durations in minutes and length in meters for the tows conducted in the Firth of Clyde 2023 survey. The plot shows that the majority of tows were between 10 and 18 minutes in duration and 40 to 80 meters in length.
Figure 12: Density histograms for E. siliqua shell lengths (mm) reconstructed from the 2023 Firth of Clyde video tows: (a) all tows combined; (b) tows conducted within the scientific trial area; (c) tows conducted outside the scientific trial area. n = total number of razor clam measurements.
Density histograms showing the distribution of Ensis siliqua shell lengths as reconstructed from the recorded videos in the 2023 Firth of Clyde survey. Separate plots are shown for all the tows combined and for tows conducted in the allowed fishing area, and for tows outside of the permitted fishing area. The patterns shown in the plots in terms of their length modes are described in the Results section of the main report.
Figure 13: Density histograms for E. siliqua shell lengths (mm) reconstructed from the 2023 Firth of Clyde video tows by site. n = total number of razor clam measurements.
Density histograms showing the distribution of Ensis siliqua shell lengths as reconstructed from the recorded videos in the 2023 Firth of Clyde survey. Separate plots are shown for each of the main surveyed areas, namely Irvine, Ayr, Culzean, Turnberry, Machrie, Carradale, and Saddell Bays. The patterns shown in the plots in terms of the differences between sites are described in the Results section of the main report.
Figure 14: Boxplots of E. siliqua density estimates for the 2023 Firth of Clyde survey: IB = Irvine Bay, AB = Ayr Bay, CB = Culzean Bay, TB = Turnberry Bay, MB = Machrie Bay, CaB = Carradale Bay, SB = Saddell Bay. Vertical dashed line separates sites within the scientific trial from sites outside the permitted fishing area. Heavy horizontal lines indicate the medians, the boxes the interquartile ranges, whiskers show the quartiles +/- 1.5 times the interquartile range, circles indicate outliers: (a) All sizes of E. siliqua; (b) Large E. siliqua ≥ 150 mm shell length; (c) Medium E. siliqua ≥100 mm and < 150 mm; (d) Small E. siliqua < 100 mm shell length.
a) Boxplot showing the distribution of the mean densities for Ensis siliqua of all sizes within each tow by surveyed bays. The main patterns shown in the plot are described in the Results section of the main report. b) Boxplot showing the distribution of the mean densities for large size (larger than 150 mm shell length) Ensis siliqua within each tow by surveyed bays. The main patterns shown in the plot are described in the Results section of the main report. c) Boxplot showing the distribution of the mean densities for small size (smaller than 100 mm shell length) Ensis siliqua within each tow by surveyed bays. The main patterns shown in the plot are described in the Results section of the main report. d) Boxplot showing the distribution of the mean densities for medium size (larger than 100 mm and smaller than 150 mm shell length) Ensis siliqua within each tow by surveyed bays. The main patterns shown in the plot are described in the Results section of the main report.
Figure 15: Spatial density (nos m-2) distributions of large sized E. siliqua from the 2023 Firth of Clyde survey: (a) Irvine Bay (b) Ayr Bay (Chart created using Opensource QGIS, not to be used for navigation).
a) Bubble plot showing the spatial distribution of large sized Ensis siliqua in Irvine Bay. Razors larger than 150 millimetres shell length were found across the area with no obvious spatial pattern. b) Bubble plot showing the spatial distribution of large sized Ensis siliqua in Ayr Bay. Razors larger than 150 millimetres shell length were found across the area with no obvious spatial pattern.
Figure 16: Spatial density (nos m-2) distributions of large sized E. siliqua from the 2023 Firth of Clyde survey: Culzean and Turnberry Bays (Chart created using Opensource QGIS, not to be used for navigation).
Bubble plot showing the spatial distribution of large sized Ensis siliqua in Culzean and Turnberry Bays. Razors larger than 150 millimetres shell length were recorded on most of the tows with higher densities in Turnberry Bay compared to the other sites within the trial fishery zone.
Figure 17: Spatial density (nos m-2) distributions of medium sized E. siliqua from the 2023 Firth of Clyde survey: (a) Irvine Bay (b) Ayr Bay (Chart created using Opensource QGIS, not to be used for navigation).
a) Bubble plot showing the spatial distribution of medium sized Ensis siliqua in Irvine Bay. Razors between 100 to 150 millimetres shell length were less common in Irvine Bay but recorded on most tows in Ayr Bay. b) Bubble plot showing the spatial distribution of medium sized Ensis siliqua in Ayr Bay. Razors between 100 to 150 millimetres shell length were less common in Irvine Bay but recorded at relatively low densities on most tows in Ayr Bay.
Figure 18: Spatial density (nos m-2) distributions of medium sized E. siliqua from the 2023 Firth of Clyde survey: Culzean and Turnberry Bays (Chart created using Opensource QGIS, not to be used for navigation).
Bubble plot showing the spatial distribution of medium sized Ensis siliqua in Culzean and Turnberry Bays. Razors between 100 to 150 millimetres shell length were recorded on most of the tows and were a little more common in these sites compared to Irvine and Ayr Bays.
Figure 19: Spatial density (nos m-2) distributions of small sized E. siliqua from the 2023 Firth of Clyde survey: (a) Irvine Bay (b) Ayr Bay (Chart created using Opensource QGIS, not to be used for navigation).
a) Bubble plot showing the spatial distribution of small sized Ensis siliqua in Irvine Bay. Razors smaller than 100 millimetres shell length were found in a small patch off Ardrossan harbour but appeared more widely spread in Ayr Bay. b) Bubble plot showing the spatial distribution of small sized Ensis siliqua in Ayr Bay. Razors smaller than 100 millimetres shell length were found spread across this bay but with tows with higher densities at the southern end of the bay.
Figure 20: Spatial density (nos m-2) distributions of small sized E. siliqua from the 2023 Firth of Clyde survey: Culzean and Turnberry Bays (Chart created using Opensource QGIS, not to be used for navigation).
Bubble plot showing the spatial distribution of small sized Ensis siliqua in Culzean and Turnberry Bays. Razors smaller than 100 millimetres shell length were recorded on nearly all the tows and seemed to become more common moving south into Turnberry Bay.
Figure 21: Spatial density (nos m-2) distributions of E. siliqua from the 2023 Firth of Clyde survey, Machrie, Carradale and Saddell Bays: (a) Large sizes (> 150 mm shell length); (b) Medium sizes (>= 100 mm and < 150 mm shell length) (< 100 mm shell length). (Chart created using Opensource QGIS, not to be used for navigation).
a) Bubble plot showing the spatial distribution of large Ensis siliqua in Machrie, Carradale and Saddell Bays which are outside of the trial fishing zone. Razors larger than 150 millimetres shell length were found on nearly all the tows with little obvious spatial pattern. b) Bubble plot showing the spatial distribution of medium sized Ensis siliqua in Machrie, Carradale and Saddell Bays which are outside of the trial fishing zone. Razors between 100 to 150 millimetres shell length were more common in Machrie Bay and higher density tows were spatially clustered towards the southern end of the bay.
Figure 22: Spatial density (nos m-2) distributions of E. siliqua for Firth of Clyde in 2023, Machrie, Carradale and Saddell Bays in 2023: Small sizes (< 100 mm shell length). (Chart created using Opensource QGIS, not to be used for navigation).
Bubble plot showing the spatial distribution of small sized Ensis siliqua in Machrie, Carradale and Saddell Bays which are outside of the trial fishing zone. Razors smaller than 100 millimetres were more common in the southern end of Machrie Bay and in Carradale Bay with apparently lower densities in Saddell Bay.
Figure 23: Scatterplots of E. siliqua density estimates from the 2023 Firth of Clyde survey against water depth at the time of sampling: (a) All sizes; (b) Large size > 150 mm shell length; (c) Medium size ≥100 mm to < 150 mm shell length; (d) Small size < 100 mm shell length.
a) Two of four scatterplots showing the densities of all sizes, and the large Ensis siliqua, plotted against water depths at the time of sampling in the Firth of Clyde survey. There were no obvious patterns in these data. b) Two of four scatterplots showing the densities of the medium and small size Ensis siliqua plotted against water depths at the time of sampling in the Firth of Clyde survey. There were no obvious patterns in these data.
Figure 24: The proportion of the total E. siliqua which were partially emerged in each tow from the 2023 Firth of Clyde survey: (a) frequency distribution of partial emergence proportions and (b) plotted against the mean exposure time to the electrical field (c) plotted against the water depth at the time of the tow (d) plotted against the mean density of E. siliqua on the tow.
a) Two of four plots showing the frequency distribution of partial emergence rates for razor clams from the Clyde survey, and the partial emergence rates, plotted against exposure time to the electrical field, on each tow. The histogram shows that the majority of partial emergence rates were between 0 and 0.4. The scatterplot shows that there was no obviously strong relationship exposure time to the electrical field. b) Two of four plots showing the partial emergence rates plotted against water depth at the time of sampling, and the average density of Ensis siliqua on each tow. There were no obviously strong relationships although some suggestion of a weak positive link between partial emergence rates and water depth.
Figure 25: Density histograms for E. siliqua shell lengths (mm) estimated from the Ayrshire coast surveys in (a) 2017 and (b) 2023. Dashed vertical lines indicate the boundaries of the small, medium and large size categories; n = total number of razor clam shell lengths estimated from the video data in that year.
a) Density histogram of Ensis siliqua shell lengths from all tows in the Ayrshire coast (Firth of Clyde) survey in 2017. This plot can be compared with the data from 2023 shown in the plot below. The changes in length distributions between 2017 and 2023 are described in the Results section of the main report. b) Density histogram of Ensis siliqua shell lengths from all tows in the Ayrshire coast (Firth of Clyde) survey in 2023. This plot can be compared with the data from 2017 shown in the plot above. The changes in length distributions between 2017 and 2023 are described in the Results section of the main report.
Figure 26: Boxplots summarising the densities of large (> 150 mm shell length) E. siliqua in the 2017 (grey infill) and 2023 (open infill) Ayrshire coast surveys by site. Heavy horizontal lines indicate the medians, the boxes the interquartile ranges, whiskers show the quartiles +/- 1.5 times the interquartile range, circles indicate outliers.
Boxplots of the densities of large size Ensis siliqua by site and comparing results from the 2017 and 2023 Ayrshire coast, Firth of Clyde surveys. The plot shows that the median densities of large razor clams (larger than 150 millimetres shell length) have declined when averaged across all the surveyed sites, and within Irvine, Ayr and Turnberry Bays. However, for Culzean Bay there was a slight increase in the median density of large size razors.
Figure 27: Boxplots summarising the densities of medium (≥ 100 and < 150 mm shell length) E. siliqua in the 2017 (grey infill) and 2023 (open infill) Ayrshire coast surveys by site. Heavy horizontal lines indicate the medians, the boxes the interquartile ranges, whiskers show the quartiles +/- 1.5 times the interquartile range, circles indicate outliers.
Boxplots of the densities of medium size Ensis siliqua by site and comparing results from the 2017 and 2023 Ayrshire coast, Firth of Clyde surveys. The plot shows that the median densities of medium size razor clams (100 to 150 millimetres shell length) have declined when averaged across all the surveyed sites, and within Irvine, Ayr and Turnberry Bays. However, for Culzean Bay there was a reasonably large increase in the median density of medium size razors.
Figure 28: Boxplots summarising the densities of small (< 100 mm shell length) E. siliqua in the 2017 (grey infill) and 2023 (open infill) Ayrshire coast surveys by site. Heavy horizontal lines indicate the medians, the boxes the interquartile ranges, whiskers show the quartiles +/- 1.5 times the interquartile range, circles indicate outliers.
Boxplots of the densities of small size Ensis siliqua by site and comparing results from the 2017 and 2023 Ayrshire coast, Firth of Clyde surveys. The plot shows that the median densities of small razor clams (smaller than 100 millimetres shell length) have increased when averaged across all the surveyed sites, and within Irvine, Ayr, Culzean and Turnberry Bays.
Figure 29: Water column temperature (oC) and salinity (psu) profiles from the Solway surveys in 2024 and 2025.
Water column profiles of temperature and salinity for the Solway survey. Temperature and salinity profiles tended to be slightly cooler and fresher towards the surface except for 12th and 14th December 2024 and 21 January 2025 when the stratification was more marked. The differences between the surface and near seabed water conditions, and differences between different days, are due to the amount of freshwater runoff from the land combined with the tidal and wind conditions on the survey day.
Figure 30: Example of image quality from the survey in Fleet Bay, Solway. The three separate MacArtney Luxus (MacArtney UK Ltd., Dyce, Aberdeen) video camera feeds (top row images) are overlaid to generate a composite equivalent to 1.5 m width on the seabed (bottom image). Video for tow 130, Fleet Bay, 13th Jan 2024 in 9 m water depth. Despite the low image quality, a fully emerged razor clam is visible on the left-hand side of the composite image.
An illustrative example of the image quality captured by the razor clam video camera sled during the Solway survey. The image is from Fleet Bay on 13th January 2024 but is typically murky due to the large quantities of silt in the wate. Despite the poor image quality an emerged razor clam can be seen lying on the seabed.
Figure 31: Histograms showing the distribution of (a) tow durations (mins); (b) tow lengths (metres); (c) average tow speeds (metres min‑1) and (d) average exposure times to the electrical field (seconds) for the 2024 – 2025 Solway survey.
a) Histogram showing the distribution of tow durations in minutes for the tows conducted in the Solway in 2024 and 2025. The plot shows that the majority of tows were between 40 to 80 meters in length. b) Histogram showing the distribution of tow durations in minutes for the tows conducted in the Solway in 2024 and 2025. The plot shows that the majority of tows were between 12 and 20 minutes in duration. c) Histograms showing the distribution of estimated exposure times to the electrical field for the tows conducted in the Solway survey in 2024 and 2025. The plot shows that each part of the seabed was exposed to the electrical field for on average between 25 to 50 seconds. d) Histograms showing the distribution of tow speeds for the tows conducted in the Firth of Clyde survey. The plot shows that the majority of tow speeds averaged between 2 to 5 meters per minute.
Figure 32: Density histograms for E. siliqua shell lengths (mm) reconstructed from the 2024 – 2025 Solway video tows by site. n = total number of razor clam measurements. Vertical dashed lines indicate the length boundaries between the small, medium and large size classes.
Density histograms for E. siliqua shell lengths (mm) reconstructed from the 2024 – 2025 Solway video tows. Top plot shows all three bays in the Solway survey and the lower plots show the length distributions within Wigtown, Fleet and Kirkudbright Bays. The patterns in the length distributions are described in the Results section of the main report.
Figure 33: Boxplots of E. siliqua density estimates for Wigtown, Fleet and Kirkudbright Bays from the 2024 – 2025 Solway survey. Heavy horizontal lines indicate the medians, the boxes the interquartile ranges, whiskers show the quartiles +/- 1.5 times the interquartile range, circles indicate outliers: (a) All sizes of E. siliqua; (b) Large E. siliqua ≥ 150 mm shell length; (c) Medium E. siliqua ≥ 100 mm and < 150 mm; (d) Small E. siliqua < 100 mm shell length.
a) Boxplots of E. siliqua density estimates for Wigtown, Fleet and Kirkudbright Bays from the 2024 – 2025 Solway survey showing all sized and large razor clams (those equal or larger than 150 mm shell length). The plots show that overall and for large sized razor clams the median density was higher in Fleet Bay than Wigtown or Kirkudbright Bays. b) Boxplots of E. siliqua density estimates for Wigtown, Fleet and Kirkudbright Bays from the 2024 – 2025 Solway survey showing all medium sized razor clams (those larger or equal to 100 and less than 150 mm shell length) and small size razor clams (those less than 100 mm shell length). The plots show that for medium sized razor clams the median density was slightly higher in Fleet Bay than Wigtown or Kirkudbright Bays, and for small sized razor clams, the median density was lowest Wigtown Bay.
Figure 34: Spatial density (nos m-2) distributions of E. siliqua in the 2024 – 2025 Solway survey: (a) Large size (> 150 mm shell length) (b) Medium size (>100 and <=150 mm shell length) (Charts created using Opensource QGIS, not to be used for navigation).
a) Bubble plot showing the spatial distribution of large sized Ensis siliqua in the Solway. Razors larger than 150 millimetres shell length were recorded on nearly all the tows but densities were higher in Fleet Bay than in Wigtown or Kirkcudbright Bays. b) Bubble plot showing the spatial distribution of medium sized Ensis siliqua in the Solway. Razors equal or larger than 100, but smaller than 150 millimetres shell length were recorded on nearly all the tows. Densities were highest in Fleet Bay followed by Wigtown Bay with lower densities in Kirkcudbright Bay.
Figure 35: Spatial density (nos m-2) distributions of E. siliqua in 2024 – 2025 Solway survey: Small size (<100 mm shell length). (Charts created using Opensource QGIS, not to be used for navigation).
Bubble plot showing the spatial distribution of small sized Ensis siliqua in the Solway. Razors smaller than 100 millimetres shell length were recorded on nearly all the tows. Apart from one tow, densities were lower in Wigtown Bay but similar in Fleet Bay and Kirkcudbright Bay.
Figure 36: Scatterplots of E. siliqua density estimates from the 2024 – 2025 Solway survey against water depth at the time of sampling: (a) All sizes; (b) Large size > 150 mm shell length; (c) Medium size ≥100 mm to < 150 mm shell length; (d) Small size < 100 mm shell length.
a) Two of four scatterplots showing the densities of all sizes, and the large Ensis siliqua, plotted against water depths at the time of sampling in the Solway survey. There were no obvious patterns in these data. b) Two of four scatterplots showing the densities of medium size (equal or larger than 100 and smaller than 150 mm shell length), and small (less than 100 mm shell length) Ensis siliqua, plotted against water depths at the time of sampling in the Solway survey. There were no obvious patterns in these data.
Figure 37: The proportion of the total E. siliqua which were partially emerged in each tow from the 2024 – 2025 Solway survey: (a) frequency distribution of partial emergence proportions and (b) plotted against the mean exposure time to the electrical field (c) plotted against the water depth at the time of the tow (d) plotted against the mean density of E. siliqua on the tow.
Two of four plots showing the frequency distribution of partial emergence rates for razor clams from the Solway survey, and the partial emergence rates, plotted against exposure time to the electrical field, on each tow. The plots show that partial emergence rates were mostly in the range 0 to 0.4 and that there was no obvious relationship with exposure time. Two of four plots showing partial emergence rates for razor clams from the Solway survey plotted against water depth and Ensis siliqua density, on each tow. There were no obvious relationships of partial emergence rate and water depth or razor clam density.

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Email: mss.fisheries.advice@gov.scot

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