Razor clam surveys - Firth of Clyde and Solway: report

Executive summary

This report describes surveys carried out in the Firth of Clyde in the autumn of 2023, and in Fleet Bay (Solway) in January 2024. The purpose of the surveys was to estimate the densities and sizes of razor clams, Ensis siliqua and E. magnus. These data will contribute to the evidence base for the Scottish Government’s scientific trial of electrofishing for razor clams (Scottish Government, 2017). A combination of an electrofishing rig and a towed video sled was deployed from two fishing vessels and the video recordings subsequently analysed to estimate the sizes and abundance of razor clams which had emerged onto the seabed.

Ninety-eight video tows were completed in the main areas along the Ayrshire coast where razor clams have been harvested in the scientific trial, namely Irvine, Ayr, Culzean and Turnberry Bays. For comparison three areas outside of the permitted electrofishing areas were also investigated with thirty-one video tows completed in Machrie, Carradale and Saddell Bays.

From the videos recorded along the Ayrshire coast, 2,581 individual E. siliqua were identified and measured, and an additional 1,053 razors were measured from tows conducted outside the trial fishery area. Shell length distributions across all the Firth of Clyde tows suggested the presence of three modes at around 100 – 110, 140 – 150, and 200 – 210 mm shell length. The average density within the permitted fishing area (for all size classes) was 0.47 ± 0.03 m^‑2 (mean ± SE) with a maximum of 1.4 m^‑2, whilst for tows outside the permitted fishing area the average and maximum were slightly higher at 0.72 ± 0.09 m^‑2 (mean ± SE) and 2.6 m^-2 respectively.

Comparing data from 2017 and 2023 suggests that recruitment has occurred in recent years. For the large and medium size categories there were declines in densities in Irvine, Ayr and Turnberry Bays, but an increase in Culzean Bay.

Only 209 Ensis magnus (formerly E. arcuatus) were identified from all the video tows in the Firth of Clyde and this species was generally recorded in areas of coarser sediment with the majority being found in Machrie Bay. This species is of less commercial interest because of its smaller size.

Sixteen video tows were completed in Fleet Bay (Solway). Poor weather and reduced water visibility due to freshwater run-off meant that further survey work planned for the Solway could not be completed under the current project funding. Data collected in Fleet Bay indicated four length modes, one at around 40 – 50 mm; a second around 90 – 100 mm; a third around 120 – 130 mm, and a fourth around 170 – 180 mm. The mean density considering all the size classes was 1.32 ± 0.11 m^‑2 (mean ± SE) with the median density being 1.34 m^-2. All records were assumed to be E. siliqua although the reduced video quality means that this should be confirmed in a future survey.