Razor clam surveys - Firth of Clyde and Solway: report

Comparison of the 2017 and 2023 Ayrshire coast surveys

Introduction to the Ayrshire coast 2017 and 2023 surveys

Razor clams along the Ayrshire coast were previously surveyed in 2017 using the same towed video technique (Fox, 2018; Fox et al., 2019). Although in the earlier survey the spatial coverage was less and the total number of tows completed lower, this repetition allows some temporal comparisons to be made. The data from the earlier survey have been re-analysed for this comparison as the site names and E. siliqua size class boundaries used in the original reports were slightly different.

E. siliqua shell lengths - Comparison of the 2017 and 2023 Ayrshire coast data

Summary statistics for the shell lengths suggests there has been an overall decrease in average size. This is also seen within each site except for Irvine Bay, where a small increase is apparent (Appendix I: Figure 25, Appendix II: Table 6). The declines in average shell length can be explained by the increased proportion of small-sized razors in the populations (Appendix II: Table 7). In the 2017 survey, small razors were rarely observed but in the 2023 survey they had increased by up to 22%.

These changes in the 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 these data on their own; changes in the densities of the different size categories must also be considered.

E siliqua densities - Comparison of 2017 with 2023 Ayrshire coast data

The changes in E. siliqua densities between 2017 and 2023 are visualised as box plots for the different size categories (Appendix I: Figures 26 – 28). For the large and medium size categories there were declines in median densities in Irvine, Ayr and Turnberry Bays, but an increase in Culzean Bay. For the small size group, median densities increased at all four sites, influencing the noticeable shifts in length distributions described above.

It should be noted that these results may have been influenced by the lower number of tows in the 2017 data. 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 shell length distributions and densities by size group suggest that the overall patterns are probably reliable.

In the 2017 survey, E. magnus was not recorded at all whilst in the 2023 survey it was only identified 209 times with the vast majority (86%) of these records coming from tows in Machrie Bay, an area not included in the earlier survey.