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

Executive summary

This report describes surveys carried out in the Firth of Clyde in the autumn of 2023, and in the Solway in January and December 2024, and January 2025 to estimate the densities and sizes of razor clams and contribute to the evidence base for the Scottish Government’s scientific trial of electrofishing (Scottish Government, 2017). A combination of an electrofishing rig and a towed video sled was deployed from two fishing vessels and the sizes and abundance of Ensis siliqua and E. magnus estimated from the video recordings.

Ninety-eight video tows were completed in the main areas along the Ayrshire coast where razor clams have been harvested during the scientific trial (Irvine, Ayr, Culzean and Turnberry Bays). For comparison, Machrie, Carradale and Saddell Bays, which are outside of the permitted electrofishing grounds, were also investigated.

The lengths of 2,581 individual E. siliqua were recorded from the fished areas with an additional 1,053 measurements from tows conducted outside of the permitted fishing area. Shell length distributions suggested the presence of three length modes at around 100 – 110, 140 – 150, and 200 – 210 mm shell length. The average density of E. siliqua within the permitted fishing area (for all size categories) was 0.47 ± 0.03 m‑2 (mean ± SE) whilst for tows outside the permitted fishing area the average was slightly higher at 0.72 ± 0.09 m‑2 (mean ± SE).

Only 209 Ensis magnus (formerly E. arcuatus) were identified with the majority being found in areas of coarser sediment in Machrie Bay.

Most of the Ayr coast sites had also been surveyed in 2017 using a similar video-based approach which allowed a comparison with the more recent data. This showed that recruitment of small (< 100 mm shell length) E. siliqua has occurred in recent years. For the large (≥ 150 mm shell length) and medium (≥ 100 and < 150 mm shell length) size classes there had been declines in densities in Irvine, Ayr and Turnberry Bays, but an increase in Culzean Bay.

In the Solway forty-nine video tows were completed. However, poor weather, strong tides and reduced water visibility meant that the survey was split across several time periods, and the quality of the recorded video footage was generally poorer than for the Firth of Clyde. Estimated razor clam densities for the Solway thus represent a minimum estimate as some animals may have been obscured and not counted. The length frequency plots indicated the presence of three length modes with one at around 70 – 80 mm, a second around 120 – 130 mm, and a third around 160 – 170 mm shell length. The mean density considering all the size categories was 0.99 ± 0.08 m‑2 (mean ± SE). There were some differences between the three bays in the survey with densities of legal-size E. siliqua (≥ 100 mm shell length based on the current Minimum Conservation Reference Size) being noticeably lower in Wigtown and Kirkudbright Bays compared with Fleet Bay.