Razor clam surveys - Firth of Clyde 2023 and Solway 2024-2025: report
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.
Comparison of the 2017 and 2023 Ayrshire coast surveys
Introduction
The size distributions and densities of razor clams along the Ayrshire coast were previously surveyed in 2017 using the same methodology as in 2023 (Fox, 2018; Fox et al., 2019). Although the spatial coverage was less in the earlier survey and the total number of tows smaller, this repetition allows some temporal comparisons to be made. The data from the earlier survey were re-analysed for this comparison as the site names and size class boundaries used in (Fox, 2018; Fox et al., 2019) were slightly different to those used in the present analysis.
Razor clam sizes
Summary statistics for the shell lengths suggests there has been an overall decrease in the average size of E. siliqua between 2017 and 2023. This is also seen within each site except for Irvine Bay, which had a small increase (Appendix I: Figure 25, Appendix II: Table 6). The declines in average shell lengths can be explained by the increased proportion of ‘Small’ (< 100 mm shell length) razor clams in the populations (Appendix II: Table 7). In the 2017 survey, few ‘Small’ razors were observed but by the 2023 survey they had increased by up to 22%. The changes in shell length distributions reflect a combination of recruitment, growth, natural mortality, and fishing mortality between 2017 and 2023. However, care must be taken not to over-interpret the length data on their own as changes in the razor clam densities must also be considered.
Razor clam densities
The changes in E. siliqua densities between 2017 and 2023 are shown by size class in Appendix I: Figures 26 – 28. For the ‘Large’ (≥ 150 mm shell length) and ‘Medium’ (≥ 100 mm and < 150 mm shell length) size classes there were declines in the median densities in Irvine, Ayr and Turnberry Bays, but an increase in Culzean Bay. For the ‘Small’ (< 100 mm shell length) size class, median densities increased at all four sites, influencing the noticeable drops in average sizes described above.
It should be cautioned that these results may have been influenced by the lower number of tows in the 2017 survey. This difference was particularly large for Irvine Bay where only 9 tows were completed in 2017, but 51 in 2023. Nevertheless, the reasonably consistent patterns across the sites for both length distributions and densities suggest that the overall patterns of change are probably representative.