Razor clam surveys - Firth of Clyde and Solway: report

Results from the 2024 Solway survey

Survey results were restricted to Fleet Bay because of problems with water clarity and the weather.

Water column – Fleet Bay 2024

Temperatures during the Solway survey were between 5.9 – 7.1^oC and salinities between 28.7 – 30.5. (Appendix I: Figure 29). The water was notably cooler and less saline towards the surface on the second survey day (14^th Jan) due to freshwater runoff from the rivers into the coastal area.

Video tows – Fleet Bay 2024

Summaries of the 16 video tows completed in Fleet Bay are given in Appendix II: Table 9. The razor clam diving operations in the Solway tend to be restricted to shallow depths, at most 15 m but typically less than 10 m and this was reflected in the depth distribution of the video tows. The coastline consists of relatively small areas of sand interspersed with rocky outcrops and areas of coarser ground. Deployment of the video equipment worked well with the camera rig being lowered to the seabed and connected to the electrofishing spreader bar by the dive team using the two connecting ropes. Water clarity was extremely poor resulting in lower quality images than in other surveyed areas. However, razors were identifiable (Appendix I: Figure 30), although it was difficult to identify other non-target organisms. The images were also not clear enough to make meaningful comments on the coarseness of the sediment.

The mean tow duration was 15 mins (± 3.3 std dev) with a few tows having to be terminated early due to encountering seabed obstacles such as rocks, but the majority of tows were longer than 50 m. Towing speeds varied between 2.9 and 7.4 m min^-1 with the average being 4.0 ± 1.2 (mean ± std dev) so that most of the average exposure times to the electrical field were longer than 25 s. The average swept area of the video tows was 86 ± 16 m² (mean ± std dev), with most tows covering above 70 m².

Razor clam sizes – Fleet Bay 2024

Overall, it was possible to estimate live sizes for 1,196 E. siliqua from the video recordings. These data suggest there were three or four length modes, one less obvious mode at around 40 – 50 mm; a second around 90 – 100 mm; a third around 120 – 130 mm, and a fourth around 170 – 180 mm. By shell length category 58% were large (≥ 150 mm shell length), 30% medium (≥ 100 and < 150 mm shell length) and 12% small (< 100 mm shell length).

Razor clam densities – Fleet Bay 2024

The maximum density of E. siliqua across all sizes was 2.12 m^‑2 (Appendix II;

Table 10) and the mean was 1.32 ± 0.11 (mean ± SE). The spatial density distributions of E. siliqua for the large and medium size categories in Fleet Bay are shown in Appendix I: Figure 32 and for small size razor clams in Appendix I: Figure 33. Although large category (≥ 150 mm shell length) razors were found throughout the area, the highest densities were in the more northern patch (Appendix I: Figure 32a). Higher densities of medium sized razors (≥ 100 and < 150 mm shell length) were also found in the northern patch but had an inverse spatial distribution pattern with the densities of the larger sized razors (comparing Appendix I: Figure 32a with Appendix I: Figure 32b). Small size (< 100 mm shell length) razors were present on all the tows, but again at higher densities in the more northern patch (Appendix I: Figure 33).

Razor clam depth distribution – Fleet Bay 2024

There were no obvious relationships between E. siliqua densities and water depths at the time of sampling, so these data are not included. However, it must be noted that the range of depths sampled in Fleet Bay was relatively limited.

Rates of partial emergence – Fleet Bay 2024

The mean percentage of partially emerged E. siliqua was 27.0 ± 0.2 % (mean ± SE). There were no obvious relationships with mean exposure time to the electrical field, water depth or with E. siliqua densities.

Incidental observations – Fleet Bay 2024

Incidental observations included 14 flatfish, 12 hermit crabs, 8 crabs (probably C. maenas) and 3 sandeel. However, because of the low quality of the videos these identifications are only indicative and other incidental organisms may have been missed.