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

Results from the 2024 to 2025 Solway survey

Due to weather, water clarity and logistical problems the Solway survey had to be split across three periods (Appendix II: Table 9). The results are presented in a combined manner but with the caveat that for Fleet Bay the merged data will not fully capture any changes in razor clam lengths and densities which occurred during 2024.

Water column

Temperatures during the Solway survey were between 5.9 – 9.1oC and salinities between 20.8 – 32.2. (Appendix I: Figure 29). The water was notably cooler and less saline towards the surface on 12th and 14th Dec, 2024. These particular CTD profiles were collected in Kirkudbright Bay, a site affected by strong freshwater runoff from the River Dee. On Jan 21st 2025 the water column profile in Wigtown Bay was also noticeably stratified but the following day was more mixed demonstrating how hydrographic conditions in the Solway can quickly change depending on the tides and wind mixing.

Video tows

Razor clam diving operations in the Solway tend to be restricted to at most 15 m water depth but typically fishing occurs in less than 10 m and this was reflected in the depth distribution of the video tows (Appendix II: Table 9). Deployment of the video equipment worked well with the camera rig being firstly lowered to the seabed and then connected to the electrofishing spreader bar by the dive team. At times water clarity was extremely poor and only cleared for about an hour either side of high tide. Along with access to Kirkudbright Marina being tidally constrained, this restricted the number of tows which could be completed each day. The high sediment loading in the water resulted in lower quality video images than for other scientific trial areas where video surveys have been conducted (Fox 2018, 2021, 2023). However, razors were identifiable (Appendix I: Figure 30), although it was difficult to reliably identify other non-target organisms.

The mean tow duration was 18.3 mins (± 3.0 std dev). Although a few tows had to be terminated early, the majority were longer than 50 m (Appendix I Figures, 31a, b). Towing speeds varied between 1.9 and 7.4 m min-1 (on a single tow with the higher speed being due to the vessel being pushed by the wind). The average towing speed was 3.3 ± 0.9 m min-1 (mean ± std dev) so that most of the exposure times to the electrical field were longer than 25 s (Appendix I Figures, 31c, d).

Razor clam sizes

Overall, it was possible to estimate live sizes for 2,823 E. siliqua from the video recordings. Mean shell lengths (± std dev) were slightly larger in Fleet Bay being 147 ± 34 mm compared with Wigtown Bay at 132 ± 34 mm and Kirkudbright Bay at 136 ± 41 mm (Appendix II: Table 1). Length frequency plots suggest there were three or four length modes in the populations, one less obvious mode at around 40 – 50 mm; a second around 70 – 80 mm; a third around 120 – 130 mm, and a fourth around 160 – 170 mm (Figure 32).

Razor clam densities

The maximum density of E. siliqua across all size classes was in Fleet Bay at 2.42 m‑2 (Appendix II: Table 11) and the mean density was 1.38 ± 0.12 (mean ± SE). Fleet Bay also had the highest median densities of ‘Large’ and ‘Medium’ sized E. siliqua across the three sites (Appendix I: Figure 33). Median densities for ‘Small’ E. siliqua (< 100 mm shell length) were below 0.02 m-2 across all three bays. Spatial distributions of E. siliqua densities in the ‘Large’ and ‘Medium’ size categories are shown in Appendix I: Figure 34, and for the ‘Small’ size razor clams in Appendix I: Figure 35. There was little apparent spatial pattern apart from the differences in overall densities between the three bays described above.

Razor clam depth distribution

There were no obvious relationships between E. siliqua densities and water depths at the time of sampling for any of the size categories (Figure 36a – d).

Rates of partial emergence

The mean (± SE) percentage of partially emerged E. siliqua was 26 ± 2 % (Figure 37a). There were no obvious relationships with mean exposure time to the electrical field, water depth or with E. siliqua densities (Figure 37b – d).

Incidental observations

Incidental observations included 71 flatfish (most being recorded from tows in Wigtown Bay), 45 hermit crabs, 19 crabs (probably C. maenas), 6 sandeel (Ammodytidae), and 3 starfish. However, because of the low quality of the videos some of these identifications were tentative and other incidental organisms may have been missed on the videos.