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

Materials and methods

Introduction

This report describes results from one of a series of surveys undertaken in areas where razor clam harvesting is permitted under the Scottish Government’s scientific trial on electrofishing (Scottish Government, 2017, 2021). The aim of the present surveys was to use a combination of electrofishing with towed video to assess the quantities, densities and sizes of razor clams (Ensis spp.) in the Firth of Clyde and in the Solway.

Evidence from previous field surveys, and monitoring of the fishery, shows that even within the permitted fishing areas, the distribution of E. siliqua is patchy. Razor clams only occur at higher densities in certain locations and fishers recognise specific beds where harvesting is worthwhile. Predetermined randomised surveys covering the whole of the production areas are therefore unlikely to yield particularly useful information because much of the area is not suitable razor clam habitat due to depth of sediment type. The present surveys thus made extensive use of local fishers’ knowledge and fishing location data collected by Marine Directorate to target those parts of the production areas which have been regularly fished during the trial.

The Firth of Clyde is a semi-enclosed fjordic basin having partial exchange with the waters on the adjacent coastal shelf. The bathymetry of the Clyde basin and the distribution of the sediments have been little changed since the end of the last glacial period and mostly consist of sands and muddy sands, through to fine silts (McIntyre et al., 2012). The coastline includes numerous rocky outcrops so that the sandy habitat suitable for razor clams is only continuous within some of the larger bays.

The Solway Firth lies on the border between Scotland and England with the northern coastline in Scottish waters. This area is subject to strong tides and has a tidal range as large as 7 – 10 m in the inner firth. The sediments include areas of intertidal sand and subtidal sandbanks but rivers such as the Dee bring large amounts of fine silts into the coastal waters. The Solway is also exposed to a large fetch at its western end and sea conditions tend to be rough unless the winds are from the north to northeast. Water clarity can be particularly poor inshore and tends to improve only for an hour or so around high tide and during periods of calm weather.

Firth of Clyde field survey

The Scottish Government electrofishing trial area in the Firth of Clyde runs from just north of Ardrossan to just south of Ballantrae (Appendix I; Figure 1). However, the most extensive areas of sand suitable for razor clams are in Irvine and Ayr Bays. Further south there is suitable habitat in Culzean and Turnberry Bays, although these areas are smaller in extent. South of Girvan there are limited areas of suitable habitat as the coastline becomes dominated by cliffs and rocky outcrops.

Nearly all recorded electrofishing in the Firth of Clyde has occurred at depths within the 20 m contour. As well as depth limiting the bottom time available when using air diving equipment, the deeper sediments in the Clyde tend towards muds and silts (Pace et al., 2021), thus becoming less suitable as habitat for razor clams.

The Firth of Clyde survey was conducted using a charter fishing vessel - ‘Skye’ (PLN: CN450). Operation of the electrofishing and video equipment was as described in Fox (2023). Video tows were conducted within the permitted razor clam electrofishing area in Irvine Bay (Appendix I: Figure 2), Ayr Bay (Appendix I: Figure 3), Culzean Bay (Appendix I: Figure 4) and Turnberry Bay (Appendix I: Figure 5). In addition, tows were conducted outside of the presently permitted razor clam electrofishing area in Machrie Bay on the Arran coast (Appendix I: Figure 6), and in Carradale and Saddell Bays on the east coast of the Kintyre Peninsula (Appendix I: Figure 7) for comparison purposes.

Solway field survey

Although the permitted razor clam fishing area in the Solway extends south to deeper water (Appendix I: Figure 8), nearly all the recorded electrofishing has occurred in depths shallower than 15 m. The deeper sediments tend towards fine muds and silts and are less suitable habitat for razor clams. However, large areas of apparently suitable habitat also do not seem to support high densities of razor clams (survey vessel skipper pers. Comm.). The areas suitable for fishing are therefore much more constrained than the extent of the permitted area might suggest.

The survey was conducted using a charter fishing vessel - ‘RazorBill’ (PLN: BA57). The vessel is relatively new but smaller than the vessel used for the Firth of Clyde and lacks an aft lifting derrick. This meant the video camera rig had to be deployed in a different manner using divers, but this was successfully achieved.

The initial plan for January 2024 was to begin survey work in Kirkudbright Bay, but extremely strong tides along with large quantities of silt brought down the river led to unworkable visibility. The survey was thus shifted across to Fleet Bay where a total of 16 tows were completed. As all work had to be completed by 31st March 2024, it was not possible to collect further survey data under that funding round due to continued poor weather. Further funding was obtained to continue survey work after April 2024, but the actual survey did not take place until December 2024 and January 2025 due to contractual delays and staffing availability.

Fieldwork and post-fieldwork analysis

The equipment deployed was identical to that described in Fox (2023) with the exception that two VL20000 PB underwater video lights (Big Blue, Goleta, CA, USA) were attached to the camera sled struts for the Solway survey. These were necessary because of the inadequate natural light during the short winter days. Illumination was set at oblique angles to the cameras to reduce the amount of backscatter although this was still a problem if fine silt was present in the water column. All post fieldwork analysis of the videos followed the methods described in Fox (2023). For calculation of summary statistics, razor clam lengths were divided into three size classes: Large (> 150 mm shell length), Medium (≥ 100 and < 150 mm shell length) and Small (< 100 mm shell length). At present the Minimum Conservation Reference Size (MCRS) for E. siliqua is 100 mm so the ‘Large’ and ‘Medium’ size classes can be legally landed although fishers prefer to land the ‘Large’ size as these fetch higher prices. In this report maps of the razor clam densities (average nos m-2 along a tow) by size class are presented as bubble charts where the diameters of the bubbles are proportional to the densities. Because of the extent of the Firth of Clyde survey, these maps are presented separately for the different bays. For each size class, the scaling goes up to the maximum density recorded across the whole Firth of Clyde survey i.e. it is possible to compare the charts between sites for each size class, but care should be taken if comparing charts within a site across the different size categories.