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

Results from the 2023 Firth of Clyde survey

Water column

The water column was generally well mixed although slight to moderate haloclines and/or thermoclines were present at around 4 m depth on some days (Appendix I: Figure 9). Water temperatures were between 12.0 and 14.2°C and salinities between 29.4 and 33.1.

Video tows

The Ayrshire coastline consists of a relatively narrow strip of sand interspersed with rocky outcrops and areas of coarser ground. There are also numerous obstacles, such as pipelines and cables, so the local knowledge of the skipper and crew was essential for safe operation of the survey.

A total of 98 tows were completed within the area where electrofishing is permitted and for comparison a further 31 tows were completed outside the permitted scientific trial fishing areas (Appendix II: Table 1).

The mean tow duration was 13 mins (± 2.7 std dev) with only a few tows having to be terminated early due to changing sea conditions or encountering obstacles (Appendix I: Figure 11a). Most tows were longer than 50 m (Appendix I: Figure 11b) which was the minimum distance recommended for such surveys in Fox et al. (2019). However, towing speed (Appendix I: Figure 11c) is also an important consideration as it influences the length of time an area of seabed is exposed to the electrical field. Mean towing speed was 4.0 ± 1.1 m min-1 (mean ± std dev) so that most of the estimated exposure times to the electrical field were longer than 30 s (Appendix I: Figure 11d).

The video cameras produced clear images, even on deeper tows and towards the end of the day, which allowed easy identification of organisms on the sediment (Appendix I: Figure 10). From the video observations, the sediments were mainly fine rippled sand mixed with ground shell fragments with coarser sands in some locations. Razor clam diving operations in the Clyde tend to be restricted to shallow depths, at most 20 m but typically less than 15 m. A limited number of tows were also conducted in water deeper than 15 m with the aim of seeing whether there are repositories of Ensis below the normal diving depths. However, the sediment on these deeper tows was coarse silt and mud and very few razors were observed.

Razor clam sizes

Overall, it was possible to estimate live sizes for 3,634 E. siliqua, with 71% of these measurements coming from tows conducted within the permitted electrofishing grounds. Comparing the average shell length for E. siliqua from tows in the fished and unfished areas suggests that razor clams in areas which have not been fished during the trial were slightly larger (Appendix II: Table 2). The largest average size was recorded from tows in Saddell Bay, which is outside of the trial area although the largest individual E. siliqua was recorded in Ayr Bay, a site within the permitted fishing area.

Many of the length datasets were skewed and multimodal meaning that simple comparisons of average lengths may be mis-leading and comparing the complete length distributions is a better approach. Pooling the length data from all the tows suggests there were two or three size modes present, one at around 200 – 210 mm; a second less obvious around 140 – 150 mm and a third around 100 – 110 mm (Appendix I: Figure 12a). There were also a small number of razor clams at less than 50 mm shell length, which were assumed to be E. siliqua because of the overall dominance of that species in the surveys. The size distribution of E. siliqua from tows within the scientific trial area was similar to the overall length distribution (Appendix I: Figure 12b). In comparison, the size distribution of E. siliqua from tows outside of the trial area had a slightly higher proportion of medium sized razor clams (Appendix I: Figure 12c) and Carradale and Saddell Bays, which are both in the unfished area, were dominated by large sized razors (Appendix I: Figure 13).

Ensis magnus were recorded in low numbers and the vast majority (86%) of the records came from Machrie Bay (outside of the permitted trial area). E. magnus are not normally collected by the fishery being too small to be of commercial interest.

Razor clam densities

Summary statistics for the densities of E. siliqua comparing tows undertaken within and outside of the trial area in the Firth of Clyde are given in Appendix II: Table 3. The maximum density across all three size categories was 2.60 m-2 and was found outside of the trial area, whilst within the permitted fishery area the maximum density was 1.41 m-2. Mean densities across all three size categories were 0.53 ± 0.03 (mean ± SE) for the whole survey, and 0.72 ± 0.09 m-2 (mean ± SE) and 0.47 ± 0.03 (mean ± SE) for the out-trial and in-trial areas respectively. However, the density distributions were often skewed so that the medians may be more representative, these being 0.43 m-2 for the whole survey, and 0.61 m-2 and 0.37 m-2 for the out-trial and in-trial areas respectively.

The density data can be further summarised by site. Median densities of ‘Large’ (> 150 mm shell length) E. siliqua were between 0.20 and 0.24 m-2 in Irvine, Ayr and Culzean Bays but a bit higher in Turnberry Bay (Appendix I: Figure 14; Appendix II: Table 4). Higher densities were also found in Carradale and Saddell Bays, which are outside of the trial fishery area. For ‘Medium’ (≥ 100 and < 150 mm shell length) E. siliqua, median densities were quite variable being less than 0.1 m-2 in Irvine, Carradale and Saddell Bays but up to 0.22 m-2 in Culzean Bay. Median densities of ‘Small’ (< 100 mm shell length) E. siliqua were lower and only reached marginally above 0.1 m-2 in Turnberry and Carradale Bays.

The spatial distributions of E. siliqua in the fished areas along the Ayrshire coast are illustrated in Appendix I: Figures 15 to 18 whilst data from outside the trial area are shown in Appendix I: Figures 21 and 22. Although ‘Large’ razors (≥ 150 mm shell length) were found throughout the area (Appendix I: Figures 15 and 21), the highest densities were in Turnberry Bay. The densities of ‘Medium’ sized razors (≥ 100 and < 150 mm shell length) seemed to increase moving south from Irvine Bay (Appendix I: Figures 17 and 18) while patches of ‘Small’ razors (< 100 mm shell length) were found scattered across Ayr, Culzean and Turnberry Bays (Appendix I: Figures 18 and 20). In Irvine Bay, ‘Small’ sized razors were largely confined to a patch off Ardrossan Harbour while densities of this size class in Ayr, Culzean and Turnberry Bays were slightly higher.

Considering the sites outside of the present fishery trial, ‘Large’ razors were found at densities generally higher than within the fished area at all three locations, namely Machrie, Saddell and Carradale Bays (Figure 21a) and (Appendix I: Figure 14). In contrast, ‘Medium’ sized razors seemed mainly confined to Machrie Bay (Figure 21b) where the maximum densities exceeded those within the trial fishery areas whilst ‘Small’ sized razors (< 100 mm shell length) were found distributed patchily in all three bays (Appendix I: Figure 22).

In summary, within the scientific trial fishing area, the highest densities of ‘Large’ (> 150 mm shell length) E. siliqua were observed in Turnberry Bay, although even here the maximum density was less than 1 m-2. Compared with Irvine, Ayr and Culzean Bays, densities of ‘Large’ razors in Machrie, Carradale and Saddell Bays, which are outside the permitted fishing area, tended to be higher. Whilst these differences likely reflect the recent fishing history, densities of ‘Medium’ and ‘Small’ sized razors were quite variable across all the survey sites and there was no obvious link with whether a site was within or outside of the permitted trial fishing grounds. Small sized razors were present across all surveyed sites although higher densities were confined to restricted patches, especially in Irvine and Machrie Bays.

Razor clam depth distribution

There did not appear to be any obvious relationships between the densities of E. siliqua in each size class and the water depths at the time of sampling (Figures 23a – d).

Rates of partial emergence

Partial emergence averaged 19 ± 1 % (± SE), although rates as high as 60 and 67% occurred on individual tows in Irvine Bay and Machrie Bay (Appendix I: Figure 24a). There was no obvious relationship between the percentage partial emergence and mean exposure time during the tow (Appendix I: Figure 24b). However, for water depth there did seem to be a weak relationship as partial emergence tended to be higher on deeper tows (Appendix I: Figure 24c) but there did not appear to be any relationship between partial emergence and E. siliqua densities (Appendix I: Figure 24d).

In a previous survey of the Ayrshire coast, it was noted that partial emergence rates averaged around 20% but were higher at certain sites e.g. 41% in Culzean Bay (Fox, 2018). This pattern was also seen in the present Firth of Clyde survey where partial emergence rates averaged 20% for Irvine and Ayr Bays but reached 30% in Culzean Bay. In addition to a possible effect of water depth, razor clam emergence rates may be at least partially related to sediment composition which will vary somewhat between sites.

Incidental observations

The total numbers of incidental organisms recorded on the videos are summarised in Appendix II: Table 5. Crabs (probably the common shore crab, Carcinus maenas) were the most common incidental organism overall. The next most common were sandeels (Ammodytidae), followed by hermit crabs and various starfish (mostly Asterias rubens but also occasional cushion stars). Small, stunned flatfish were also observed, although some of these were seen to recommence active swimming as the video cameras passed overhead. Two species of diving birds, the cormorant (Phalacrocorax carbo) and eider duck (Somateria mollissima) were observed adjacent to the survey vessel, and occasionally on the underwater cameras, although neither bird species was present in large numbers.