Electrofishing for razor clams: scientific trial update - 1 February 2022 to 31 January 2023

7. MD Evidence, Data and Digital

Evidence, Data and Digital Portfolio (formerly Marine Scotland Science (MSS)) of the MD continue to take responsibility for the biological and ecological goals of the razor clam trial;

• to gather local level information on razor clam populations and stocks, including accurate data gathered by trial participants to supplement stock survey work;
• to ensure sustainable harvesting levels; and,
• to gather further information about the impacts of the electrofishing method on target and non-target species.

Data collection continued in the fifth year of the trial, although staffing resources at the Marine Laboratory were reduced and work on the trial was prioritised accordingly. Since the beginning of the trial (August 2018) samples of live razor clams were sent to the Marine Laboratory in Aberdeen on an approximately monthly basis (work was paused for a period and reduced during the COVID-19 pandemic). Razor clams were dissected and analysed to determine sex, spawning time, and length weight relationships, and a sub sample underwent further histological examination. Live sampling was stopped in December 2022, with the exception of live razor samples which were requested on an ad-hoc basis to support the PhD project (further details below).

Up to the end of the fifth trial year (January 2023) a total of 6338 live razor clams have been sampled (Table 1), with 1468 razor clams examined by histology for maturity staging and spawning activity. Not all trial zones or areas were sampled consistently, with most razor clams coming from the Firth of Clyde (total of 2053 razor clams over the time) and Firth of Forth (1746 razor clams over the time). Histological examination revealed 632 razors identified as male, 731 as female, and 2 as hermaphrodite. It was not possible to identify the sex of 103 razor clams because the individuals were in stage ‘0’ and either only protogonia cells were present or almost no gonadal tissue was available to be sampled.

To gather information on the length of landed razor clams, detailed instructions for sampling and measuring razor clams were sent to trial participants and the associated processors. As a minimum, vessels submit one sample per month when fishing. Between August 2018 and January 2023, a total of 455 razor clams sample sheets were submitted to the MD (Table 2).

All vessels participating in the trial carry REM systems which record details of vessel position, speed, and the current (amperes) output of the generator used to power the electrofishing gear at 10 second intervals. Official Scottish landings and effort data are collated by compliance staff in MD Operations and Corporate Services from fishers’ logbooks and sales notes and entered into fisheries databases (COMPASS and iFISH). These data were utilised for a number of purposes including in combination with information on vessel activity (as detected by on-board REM systems) to monitor fishing activity, in the calculation of landings per unit effort (LPUE) and in stock assessments.

For two trial areas (Firth of Forth and Firth of Clyde), for which sufficient sample data is available, length frequency data obtained from self-sampling were combined with official landings data to provide a raised annual landings-at-length distribution. These data were averaged over a four-year period (2018 – 2021) and aggregated for use in Length Cohort Analysis (LCA). Work was also undertaken using geostatistics and survey based abundance estimates. Further details of the analyses and results will be provided in the full report.

All data collected as part of the trial will be collated in a central database, which is currently under construction. Future plans also include producing publicly available data layers through the National Marine Plan platform^[2].