Scottish scientific electrofishery for razor clams trial - biological and ecological goals: progress report

5. Discussion

The trial has produced a large quantity of data and the evidence presented builds on previous work which explored the possibilities of an electrofishery for razor clams in Scottish waters (Breen et al., 2011; Murray et al., 2014, Fox et al., 2019). The collaboration and commitment from stakeholders and various agencies has been integral to the data collection for the trial. Most razor clam fishers and processers are supportive of this trial fishery and formed good working relationships with staff at the Marine Directorate. These working relationships were tested and strengthened as the trial progressed and challenges arose.

The Marine Directorate and public sector partners have worked closely with fishers registered with the trial. Communication and engagement with fishers was maintained effectively through the establishment and development of the Scottish Razor Clam Association (SRCA). The good working relationship between MD and SRCA has proved invaluable in ensuring a flow of information between parties to improve standards, providing administrative assistance and ensuring scientists are provided with information about fishing practices and grounds.

The electrofishing trial, especially the scientific objectives, were ambitious from the outset. Prior to the trial’s commencement there were limited research and data on the electrofishing method and local razor clam populations. The collection of specific and robust data relied on trial participants, but there was very little lead time or initial resource. This meant a lag from the launch of the trial to when data collection began (both for live and self-sampling of razor clams). The number of self-sample data sheets increased after meetings with stakeholders, and increased further after the establishment of the Scottish Razor Clam Association. Any future research or commercial fishery should consider options for mandatory data collection as part of the management plan to ensure regular stock assessments can be conducted.

Unfortunately, sampling of live razor clams was stopped from April 2020 until March 2021 due to the COVID-19 pandemic. The staff requirement, organisation, coordination, logistics of transportation of live shellfish and the laboratory requirements for the sampling of live razor clams meant sampling was not possible during the pandemic, and when sampling resumed it was on a substantially reduced basis. This meant not all trial areas could be routinely covered because of the reduced staff, resources and space available at the Marine Laboratory.

Scientists presented annual data collection and analyses updates to stakeholders, but it would have been preferable to have more frequent meetings: twice a year or quarterly to ensure all trial participants were fully informed and committed to the trial. More regular meetings (or targeted meetings for certain trial areas) may have helped improve the number of razor clam samples, especially for the earlier years of the trial.

Stock assessment

Length cohort analysis (LCA) is a commonly used method for assessing data-limited fisheries (Jones, 1984; Addison and Smith, 2003). The method assumes that the landings length frequency data adequately represent the fishery removal from a given cohort. However, because length frequency data are derived from a single year of sampling instead of a single cohort, this assumption can only be valid for a population in a steady state or equilibrium (i.e. constant recruitment and exploitation rate). The effect of these variations on the assessments was mediated by using an average of the length frequency data (2020-2022). Nevertheless, if systematic changes in recruitment or exploitation rate occurred over these three years, fishing mortality estimates may be biased.

In addition to the length frequency distribution data, equally important inputs in the length cohort analysis (LCA) are the von Bertalanffy growth parameters (L_inf and K) and natural mortality estimates (Lee, 2011). Studies have shown LCA to be very sensitive to biological input parameters. Therefore, the choice of these parameters could critically influence the assessment results (Lai & Gallucci, 1988; Jones, 1990), such as the perception of the stock status in terms of current exploitation rate relative to F_MSY. For example, the estimates of von Bertalanffy growth parameters (L_inf and K) used in the LCA can significantly affect the shape of the yield per recruit curve and estimation of fishing mortality value in relation to F_MSY. Hence, using a growth model that underestimates the von Bertalanffy growth parameters (L_inf and K) may result in underestimating the current exploitation rate (F) and vice versa if these growth parameters are overestimated. Natural mortality (M) also has a marked effect on the shape of the relative yield per recruit curve. Therefore, using a low value for M would result in an assessment overestimating the current fishing mortality which could lead to an erroneous conclusion on stock status.

Razor clam shells from the live samples were retained for age determination to derive estimates of area specific growth parameters for use in length-based stock assessments. This work is being conducted as part of the on-going PhD at SAMS and should be completed in 2024.

The Firth of Forth Largo Bay trial area benefitted from a survey in 2020 which facilitated geostatistical analyses to reveal areas of higher densities of razor clams across the ground. The razor clam abundance estimate for this trial area was 11.3 million individuals (for razor clams >100 mm, i.e., commercial size). It is notable that the length frequency distribution data for this trial area shows that landings are typically in the range of 145 to 215 mm, and that the geostatistically-derived harvest rate for the size range >150 mm was 11.83%. Harvest rates from LCA scenarios range from 1% to 21%, with a mean of 9%. These values are in reasonable agreement with the range of geostatistically-derived harvest rate values, with both methods (geostatistics and LCA) suggesting that the current rate of removals is below the F_msy proxy, under all LCA scenarios, except Scenario 3 for the Firth of Forth. The linear regression analyses of the LPUE model (Appendix 4; Figure A4.34) suggests some evidence of a declining trend although this may be influenced by changes in the fishery footprint which indicated expansion into other grounds of the trial area.

Analytical stock assessments were not completed for all of the trial areas because of the lack of length data and/or surveys. A summary of data available for each trial area is provided in Appendix 4. It is hoped that further stock assessments (likely to be length-based indicators) will be completed as part of the on-going PhD study.

Management considerations

The level of fishing activity and the removal of razor clams should be carefully aligned with the availability of the resource and an appropriate management system put in place to ensure sustainable harvesting. Management considerations for razor clams should include maintaining a healthy ecosystem (consideration of the wider flora and fauna), avoiding overfishing (recommendations for controlling access and limiting in the number of vessels) and ensuring that catches are controlled (daily catch limits and accurate reporting) in line with updated scientific advice (based on annual or regular stock assessments).

This trial was developed as cooperative research with stakeholders, scientists, compliance and other agencies involved from the conception of the original work, through to the implementation of policy (Baker et al., 2023). Clear limits were established on the number of vessels, the areas to be fished (monitored by REM), daily catch (for each vessel) and effort restrictions (maximum number of days fished). Regular updates were provided on work to ensure openness and transparency (Marine Scotland, 2019, 2020, 2021, 2022) and this has proved popular with stakeholders, other science institutes and the wider public, although this has often meant that time and resource has to be diverted to respond to the frequent requests for further information.

The DSAP project explored data pipelines and automatic reporting but also enforced the understanding that transparency is important to meet public expectations and support better fisheries governance. This trial has provided insight into how a commercial fishery could successfully be co-managed and governed, although it is acknowledged that there are further questions regarding future responsibilities, funding, expectations and conflicts of interest (Baker et al., 2023).

Stock assessment results for the Firth of Clyde and Firth of Forth trial areas suggest that razor clams in these areas are not growth overfished, with fishing mortality below F_MSY for all scenarios. The stock assessments described in this report (for the Firth of Forth and Firth of Clyde trial areas) could be used to provide catch advice and harvest rate scenarios (using a similar approach to the annual Nephrops advice) if annual surveys were completed. Fishing mortality in the Firth of Clyde and Firth of Forth appears to currently be at a sustainable level, however, results presented are preliminary and further work is required to consider alternate options for analytical assessment (for example length based indicators) and to refine model parameters. It may be possible to conduct stock assessments for the other trial areas if sufficient data are available in future. It would be sensible to develop reference points and harvest control rules alongside regular assessments, and for any future commercial fishery to include these as part of a management plan. Advice should inform a management system with a long term approach to sustainable fishing of razor clams, with the spatial resolution of stock assessments and advice aligned with the management system in place.

There are a number of options (qualitative or quantitative) which could be explored to estimate appropriate reference points which could inform how any potential future fishery is managed, and could be considered for future work. There are currently no agreed reference points for razor clam stocks in Scottish waters and these have not been explored as part of this trial (although the LCA assumed F_max as a proxy for F_msy in line with the approach taken for other shellfish stocks in Scottish waters).

The catch efficiency of electrofishing was investigated during the Tarbert Bank survey and reported to be high with an average catchability of 82% based on depletion experiments (in warmer water this could be higher: Fox, 2023). Given the high efficiency of electrofishing, the fishing gear and methodology should be kept under regular review and regulated appropriately to ensure that technological creep does not impact negatively on the razor clam stocks or wider ecosystem. This may be of particular importance for areas where the razor clam stock distribution is believed to be larger than the current fishing footprint. Surveys of some trial areas reported razors at depths deeper than the fishers currently harvest (due to constraints of SCUBA); or outwith the permitted trial area (Fox, 2021; 2023). It is likely that the close proximity of these unfished areas could provide benefits to the fishery in terms of “spill over” of both juvenile (recruitment) and adult biomass (fishers have reported that razor clams can migrate within beds) .

These unfished areas (within current trial zones) could be considered for permanent closure or no fish zones. Options for temporal or rotational closures have been successful in other fisheries and could be applicable for razor clams, but will require further understanding on growth rates, recruitment and natural mortality (the on-going PhD study should help to inform this).

Anecdotal information from the fishers confirm that undersized razors are replaced back into the substrate and can survive (Fox et al. 2019) but future work could investigate the survivability of these clams. If successful then this procedure could be formalised with a voluntary code of conduct implemented to ensure that razor clams have an increased chance of survival (and are less vulnerable to predation on the sea bed).

By mapping the spatial extent of the razor clam fishery, this trial facilitates accountability of the fleet to environmental impacts and provides information on fishery distribution for use in fisheries management and marine spatial planning. In terms of wider ecosystem management, mapping and understanding the footprint of the fishery will continue to be important for the future and valuable lessons have been learned for other fisheries about the technology and resources required to best utilise these mapping tools.

Impacts of electrofishing for razor clams

A review of the likely effects of electrofishing for razor clams on the marine environment conducted by Breen et al. (2011) reported anecdotal evidence of limited short-term direct effects of electrical fields on targeted Ensis spp. This is inferred by the fact that clams are exported live and supported by observations that most stunned individuals rebury themselves (if left) within ~10 minutes.

There are a small number of existing studies on the ecological effects of electrofishing for razor clams. The most extensive of these documents research which investigated the vulnerability of a range of vertebrate and invertebrate species to electrofishing ((Murray et al., 2014; Murray et al., 2016).

Using tank experiments, Murray et al (2014, 2016) examined short term behavioural responses to electrofishing and subsequent survival of pod razor clams (Ensis siliqua), hermit crabs (Pagura bernhardus), surf clams (Spisula solida), and starfish (Asterias rubens). Tank trials were unable to reveal a significant difference in survival between control and electrically fished individuals and impacts to animal behaviour were observed only in the short term, lasting for a maximum of eight minutes (median = 3 minutes 37 seconds) across all species tested. Similarly, observations of electrically fished animals in the natural environment, including stunned sandeels (Ammodytes marinus) also showed recovery of normal behaviour within ten minutes. Observations of impacts to the seabed were described as minimal, with tracks left behind by electrodes likened to those produced by natural debris dragging along the bottom in a strong current. Ultimately, these results led the authors to conclude that electrofishing is a low impact method of harvesting Ensis spp.

Further evidence of the low ecological impact is presented by Woolmer et al. (2011), who performed field-based surveys of marine assemblages in Wales. Macrofaunal grab samples and diver transect surveys were unable to reveal any significant difference between electrofished and control sites after 28 days. Furthermore, diver observations and video footage revealed that stunned epifauna and fish, including Ensis spp., resumed normal behaviour within ten minutes. The authors also showed how fish species commonly encountered during electrical fishing for razor clams, including dover sole (Solea solea), dab (L. limanda), and plaice (Pleuronectes platessa), frequently exhibit an escape response when disturbed by the fishing activity. As such, they reason that this escape response is likely to provide fish with effective natural protection from potential adverse effects of electrical stunning and therefore electrofishing should be considered low risk.

As part of the PhD study, detailed measurements of the induced electric field have been taken to map its distribution and intensity. Furthermore, exposure experiments are being undertaken using shore crabs (Carcinus maenas), which Fox et al. (2019) reported to be the most observed epifaunal species during video surveys of Scottish razor clam beds. After an approximate two-minute exposure period to the electric field, physiological metrics of stunned and control animals are tracked over time in order to assess the impact of stunning upon organism functioning. Results will be reported in the PhD submission, anticipated in 2024.

Conclusion

Progress has been made on all of biological and ecological goals which were:

• 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.

Information on razor clams was gathered from all around Scotland, and the trial participants have been integral to this. The fishers, vessel owners, buyers, processers, and administrative staff all played vital roles and the trial would not have continued without their engagement.

Survey work was undertaken in collaboration with SAMS, but it would have been preferential to complete surveys in all trial areas and to have experimented further uses of cameras and technologies. This is an area of work that Marine Directorate are keen to progress and we advise that the use of electronic monitoring and camera systems be considered paramount in any future commercial razor clam fishery. The electronic monitoring data analysis allowed detailed maps of the fishing footprint for each of the trial areas and also ensured compliance of the regulations.

This report has highlighted the progress made towards a sustainable razor clam fishery in Scotland. The trial is well managed under strict controls with regular communication between fishers, compliance, policy and science staff. Work will continue to attempt to complete further stock assessments for the other trial areas as part of the on-going PhD project, alongside reporting on the impacts of the electrofishing method on target and non-target species. The learning outcomes from this trial should be intrinsic to an appropriate fisheries management plan for any future research or sustainable commercial razor clam electrofishery.