Razor clam trial: annual report 2023-2024

8. Other Work Streams

8.1 Report on the Survey the razor clam grounds in the Firth of Clyde and in the Solway in 2023/24

Razor clam surveys - Firth of Clyde and Solway: report - gov.scot (www.gov.scot)

The Executive Summary from the report is as follows: -

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 Ensis. magnus. This 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.

8.1.1 Firth of Clyde: Method

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.

8.1.2 Firth of Clyde: Findings

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 E. 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.

8.1.3 Solway Firth: Method

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.

8.1.4 Solway Firth: Findings

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.

Analysis of growth rates on samples of E. siliqua from 5 sites within the trial area has been completed. As well as the preliminary results being included in a policy brief provided to Marine Directorate, the data has been written up as a paper. Following receipt of final edits from the co-authors the plan is to submit the manuscript to the Fisheries Research journal for peer review.

The abstract from the manuscript is as follows: -

Global landings of razor clams have increased by around 15 times since the turn of the century. Harvesting techniques range from hand-picking and use of brine solutions to force the clams to emerge to more intensive mechanical and hydraulic dredging. In Scotland, use of electrofishing has become popular for harvesting the pod razor (Ensis siliqua) and this fishery is presently the focus of a government scientific trial to explore its long-term sustainability. Growth estimates are an important component of fisheries management contributing to yield-per-recruit and natural mortality estimation which can be especially useful for data-poor stocks. However, growth rate estimates should be periodically re-evaluated, particularly when harvesting practices have changed. Interpretation of external shell growth increments alongside validation by oxygen isotope analyses was used to generate length-at-age data for E. siliqua collected from five sites within the scientific trial. Von Bertalanffy growth curves were then fitted to these data using a hierarchical Bayesian approach. Across all five sites the mean of the asymptotic length (L∞) was estimated to be 191 mm and the mean of the Brody coefficient (k) estimated at 0.27 y-1. The 95% credible intervals for L∞ were from 182 to 206 mm, and for k were between 0.17 and 0.33 y-1. These new estimates are consistent with previous published results from Scottish sites suggesting that recent changes to harvesting practices have not led to substantial changes in E. siliqua growth rates.

8.2 Further Solway Survey

An application was submitted to the Scottish Marine Fund to support a repeat of the survey in the Solway which was undertaken in early 2023. The 2023 survey was affected by poor water clarity and as a result only covered part of the grounds being fished within the trial. It was hoped to repeat the work under more favourable conditions during the summer months. Although the grant was awarded, formal notification of contract along with a requirement to tender for a survey vessel again delayed the start of the actual work well beyond the desirable start-date. At present we are waiting for a spell of calm weather and are in regular discussions with the skipper of the vessel in the Solway. As the survey must be completed before the end of January 2025 we are in a similar position to last year and results will be dependent on environmental conditions. As previously communicated, the timing and restrictions around the Scottish Marine Fund continue to cause logistical problems for surveys of the razor clam grounds which would ideally be conducted during the summer months.

8.3 PhD on effects of electrofishing on non-target organisms

The student undertaking this PhD decided to take a suspension of studies until Jan 2025 due to their beginning a job with JNCC. This delays their planned thesis submission date until later in 2025. However, the data from the experiments they undertook into the recovery of non-target organisms from electrical exposure have already been included in the policy brief written for Marine Directorate, while the E. siliqua growth rate work has been written up as a manuscript for which is nearly ready for submission (see manuscript abstract above).