We use cookies to collect anonymous data to help us improve your site browsing experience.

Click 'Accept all cookies' to agree to all cookies that collect anonymous data. To only allow the cookies that make the site work, click 'Use essential cookies only.' Visit 'Set cookie preferences' to control specific cookies.

Set cookie preferences

Your cookie preferences have been saved. You can change your cookie settings at any time.

Publication - Research and analysis

Razor clam surveys - Firth of Clyde and Solway: report

Published
6 December 2024
Directorate
Marine Directorate
Topic
Marine and fisheries
ISBN
9781836018858

This report describes a survey carried out in the Firth of Clyde (2023) and Solway (2024) 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 speed (metres min‑1) and (d) average exposure time to the electrical field (seconds) for the 2023 Firth of Clyde survey.
Two histograms showing the distribution of tow durations and tow lengths for the tows conducted in the Firth of Clyde survey. The patterns are described in the results section of the main report. Two histograms showing the distribution of tow speeds and estimated exposure times to the electrical field for the tows conducted in the Firth of Clyde survey. The patterns are described in the results section of the main report.
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.
Three density histograms showing the shell length distributions as reconstructed from the videos recorded during the Firth of Clyde survey. The top plot shows all the tow data and suggests there were three modes at around 100 to 110, 140 to 150, and 200 to 210 millimetres. The middle plot shows results from tows within the trial fishery zone and is more dominated by the 100 to 110 and 200 to 210 millimetres modes. The lower plot shows data from tows outside the trial fishery zone where the three modes are more clearly distinguishable.
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.
Seven density histograms showing the shell length data broken down by sampling site. For sites within the trial zone the length distributions were more dominated by sizes in the 150 to 250 millimetres range for Irvine and Turnberry Bays. For Ayr and Culzean Bays the lengths were more evenly distributed across the larger and smaller than 150 millimetre sizes. For sites outside of the trial fishery zone, shell length distributions in Carradale and Saddell Bays were dominated by clams larger than 150 millimetres shell length, while for Machrie Bay razors around 150 millimetres were more common.
Figure 14: Boxplots of E. siliqua density estimates for the 2023 Firth of Clyde survey:
Boxplots of Ensis siliqua density estimates by site and length category. The patterns in these plots are described in the results section of the main report.

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.

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).
Bubble plots showing the spatial distribution of large sized Ensis siliqua in Irvine and Ayr Bays. 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 plots 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).
Bubble plots showing the spatial distribution of medium sized Ensis siliqua in Irvine and Ayr Bays. Razors between 100 to 150 millimetres shell length were less common in Irvine Bay but recorded 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 plots 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).
Bubble plots showing the spatial distribution of small sized Ensis siliqua in Irvine and Ayr Bays. Razors smaller than 100 millimetres shell length were found in a small patch off Ardrossan harbour but appeared more widely spread in Ayr 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 plots 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).
Bubble plots showing the spatial distribution of large and medium sized 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. 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 plots 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.
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. 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.
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. There were no obviously strong relationships although some suggestion of a positive link with water depth. 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 positive link with 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.
Two density histograms showing the shell lengths for Ensis siliqua for the Ayrshire coast, Firth of Clyde surveys conducted in 2017 and 2023. The more recent data show a shift towards a greater proportion of the reconstructed lengths being comprised of razor clams smaller than 100 mm shell length. This indicates that recruitment of young razor clams has taken place during the period of the electrofishing trial.
Figure 26: Boxplots summarising the densities of large size 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 plots show that the median densities of 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 size 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 plots show that the median densities of razor clams sized 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 size 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 plots show that the median densities of 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 Fleet Bay, Solway survey in 2024.
Water column profiles of temperature and salinity Fleet Bay, Solway. Temperature and salinity profiles were slightly cooler and fresher towards the surface on both days.
Figure 30: Example of image quality from the Fleet Bay, Solway survey. 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 Fleet Bay, Solway survey. The images are murky due to the large quantities of silt in the water, but an emerged razor clam can be seen lying on the seabed.
Figure 31: Density histogram for E. siliqua shell lengths (mm) reconstructed from video recordings in Fleet Bay, Solway in 2024. n= total number of razor clam measurements.
Density histogram showing the Ensis siliqua shell length distributions as reconstructed from the videos recorded during the Fleet Bay, Solway survey. The plot suggests the presence of four length modes, one at around 40 to 50; a second around 90 to 100; a third around 120 to 130, and a fourth around 170 to 180 millimetres.
Figure 32: Spatial density (nos m-2) distributions of E. siliqua in the 2024 Fleet Bay, 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).
Bubble plots showing the spatial distribution of large and medium size Ensis siliqua in Fleet Bay, Solway. Densities for razors larger than 150 millimetres shell length were higher in tows to the north of the bay whilst the inverse pattern was seen for razors between 100 and 150 millimetres shell length. Bubble plots showing the spatial distribution of small size Ensis siliqua in Fleet Bay, Solway. Densities for razors smaller than 100 millimetres shell length were higher in tows to the north of the bay.
Figure 33: Spatial density (nos m-2) distributions of E. siliqua in 2024 Fleet Bay, Solway survey: Small size (<100 mm shell length). (Charts created using Opensource QGIS, not to be used for navigation).
Bubble plots showing the spatial distribution of small size Ensis siliqua in Fleet Bay, Solway, with higher densities in the more central area.

Contact

Email: mss.fisheries.advice@gov.scot

Back to top