Razor clam surveys - Firth of Clyde and Solway: 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 these 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.

The Clyde Sea encompasses the Firth of Clyde, the Clyde Estuary, and the sea lochs to the north of the Firth and Loch Ryan to the south. The area 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 has 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. The Solway is subject to strong tides with a large maximum tidal range of 7 – 10 m in the inner firth. The sediments consist of large areas of intertidal sand and subtidal sandbanks. However, where rivers flow into the area these can bring large amounts of fine silts into the coastal waters. The Solway is also exposed to a large fetch at its western end, so sea conditions along the northern coast can be rough unless the winds are from the north to northeast.

Evidence from previous field surveys, and monitoring of the fishery, shows that even within the permitted fishing areas, the distribution of Ensis 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 area 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 survey thus made extensive use of local fishers’ knowledge, as well as fishing location data collected by Marine Directorate during the Scottish Government trial, to target those parts of the production areas which have been regularly fished during the trial.

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 Bay and Ayr Bay. 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 sand as the coastline becomes dominated by cliffs and rocky outcrops.

Nearly all recorded electrofishing during the trial in the Firth of Clyde occurred in depths shallower than the 20 m depth 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 habitat for razor clams.

The 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 for comparison 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).

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 activity during the trial in this area has occurred in depths shallower than 15 m. The deeper sediments tend towards muds and silts and are less suitable habitat for razor clams. The areas fished are therefore much more constrained than the extent of the permitted fishing 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 surveys and lacks an aft lifting derrick. This meant the video camera rig was deployed in a different manner using divers.

The initial plan was to begin survey work in Kirkudbright Bay, but extremely strong tides and large quantities of silt brought down the river led to poor visibility. The survey was shifted across to Fleet Bay where a total of 16 tows were completed. As all work had to be completed by 31^st March 2024, it was not possible to collect further survey data under this funding round due to poor weather conditions.

Post fieldwork analysis

All post fieldwork analysis of the videos followed the methods described in Fox (2023).