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

Discussion

Although razor clams are widespread around the Scottish coast, densities vary greatly with commercially viable beds typically located in more sheltered locations (Marine Laboratory, 1998). In undisturbed patches, Ensis siliqua can reach densities of 200 m-2 (Fahy and Gaffney, 2001) but unfortunately most published estimates from Scotland come from beds which have been subjected to at least some level of exploitation (Murray et al., 2014). Because razor clams are also patchily distributed, average densities will be considerably lower than localised maximum densities.

Historical surveys for razor clams have either employed visual counting by divers or collected samples using mechanical or hydraulic dredges. More recently combined electrofishing towed-video surveys have been undertaken on several grounds in the Scottish scientific trial (e.g. Fox, 2021 and 2023). From these surveys the maximum average density of legal size (≥ 100 mm shell length) E. siliqua along a tow in Largo Bay (Firth of Forth) was 9.3 m‑2 and the average over all the tows was 3.0 ± 0.3 m-2 (mean ± SE). Another survey looked at Tarbert Bank which is split into an area where electrofishing fishing is permitted, and a part where fishing is not allowed (Fox, 2023). Densities of legal sized E. siliqua in the fished zone were up to 6.7 m‑2 with the average being 1.8 ± 0.2 m-2 (mean ± SE) which compared with a maximum of 2.9 m-2 and an average of 1.1 ± 0.1 m-2 (mean ± SE) in the unfished sector.

The Ayrshire coast has been previously surveyed in 2017 (Fox, 2018) where the maximum density of legal size (≥ 100 mm shell length) E. siliqua was 1.5 m-2, and the average was 0.51 ± 0.04 m-2 (mean ± SE). In the present survey, the maximum density of legal sized razors in the permitted fishing area of the Clyde was 1.1 m-2 and the mean was 0.39 ± 0.02 m-2 (mean ± SE). For tows conducted outside of the permitted fishing area, the densities were higher with a maximum of 2.2 m‑2 and average 0.64 ± 0.08 m‑2 (mean ± SE) respectively. However, when compared to areas such as Largo Bay and Tarbert Bank, the beds in the Firth of Clyde appear to have supported rather low densities of E. siliqua throughout the trial period. In contrast, densities of legal sized razors for Fleet Bay in the Solway were up to 2.05 m-2 with an average of 1.21 ± 0.11 m-2 (mean ± SE), but the spatial extent of the razor clam bed in this bay is limited. Razor clam densities in Kirkudbright and Wigtown Bays were lower averaging 0.57 ± 0.06 m‑2 (mean ± SE), giving an overall average across all three bays of 0.84 ± 0.07 m‑2 (mean ± SE). Although these levels are higher than the densities recorded in the fished bays of the Firth of Clyde, the main challenges for the Solway fishery are the difficult working conditions and strong tides which limit the amount of fishing which can be conducted each day. It is also unclear why high densities of razor clams are not found over the much larger areas of apparently suitable sand habitat in the Solway, but this could be linked to local hydrodynamics, or the large tidal range found in this area.

In terms of recruitment, both the 2023 Ayrshire coast results and those from the Solway showed the presence of small E. siliqua (< 100 mm shell length). For the Firth of Clyde this size class appears to have increased considerably since 2017 indicating that recruitment has occurred during the scientific trial, despite the relatively low average densities of E. siliqua in the area.

Combining electrofishing with towed video has worked well as a survey technique across many locations including the Outer Hebrides (Fox, 2018), Largo Bay (Fox, 2021), Tarbert Bank (Fox, 2023) and sites along the Ayrshire coast (Fox, 2018 and this report). Water clarity has not normally been a significant hindrance in these surveys, and the quality of the recorded videos has usually been high. However, water clarity was much poorer in the Solway because of the presence of large amounts of very fine silt, along with the strong tides. For sites such as the Solway, choosing survey times coincident with neap tides and avoiding periods after heavy rainfall may be additional factors which need to be considered in survey planning. In addition, since water clarity only improved an hour or so either side of high tide, and access to Kirkudbright Marina is dependent on the state of the tide, these also restrict available survey time each day.

Tow speeds in the present survey ranged from 1.5 – 8.3 m min-1 with most of the tows conducted at 2 – 6 m min-1. These speeds are similar to those when using other vessels for Ensis video survey work (Fox, 2018; 2021; 2023). The average exposure times to the electrical field were thus between 14 – 81 seconds with most of the tows having average exposure times longer than 30 seconds. In previous reports it was suggested that the proportion of partially emerged razors might provide an index for how efficiently the electrofishing gear is working. For the 2023 Firth of Clyde survey, the average was 0.20 but reached 0.30 in Culzean Bay and for Fleet Bay the mean rate of partial emergence was 0.27. In addition, a possible depth effect on partial emergence rates was identified in the Firth of Clyde data, so razor clam emergence rates may be influenced by both depth and sediment type. For the Solway results there were no apparent relationships between partial emergence and factors such as sampling depth or E. siliqua densities.

Anecdotal evidence from fishers also suggests that water temperatures and even atmospheric pressure may affect the speed with which razor clams emerge in response to electrical stimulation. In addition, Muir (2003) produced some experimental evidence that razor clam burial depths can be affected by season and wind direction which might influence their rates of emergence. The influence of environmental factors means that there is some uncertainty about the efficiency of electrofishing, although it has generally been assumed to be high (Breen et al., 2011). This aspect is important because it will affect the results from surveys based on electrofishing. If the efficiency were to vary substantially between surveys, then this would complicate comparisons across years and sites. Although this aspect has not been studied in detail, a depletion experiment was conducted on Tarbert Bank by Fox (2023). The results suggested that on average the electrofishing gear was 82% efficient based on a single 30 s exposure to the electrical field when the bottom water temperature was 8 – 9℃. This was colder than bottom temperatures during the present Firth of Clyde survey, although 1 – 2oC warmer than near bottom temperatures during parts of the Solway survey. Although the results from the depletion experiment support the assertion that electrofishing is highly efficient, the evidence would be strengthened by conducting further experiments under different water temperatures and at different sites.

One advantage of electrofishing over dredging is that the divers leave undersized razor clams on the seabed, rather than bringing them to the surface and passing them through a sorting grid which can cause shell damage and increased mortality (Fahy and Carroll, 2007). However, Muir (2003) and Murray et al. (2014) commented that stunned razors might be vulnerable to predation by benthic scavengers in the few minutes they can take to rebury. In a previous study in Largo Bay (Firth of Forth) large flocks of eider ducks were present and these birds were observed predating on stunned razors on the seabed (Fox, 2021). In the present Ayrshire coast survey, some eiders and the occasional cormorant were observed but predation by these birds did not appear to be as significant. However, the numbers of sea birds in the area could change during the year so predation rates might vary seasonally. Small sandeels and juvenile flatfish were also seen on the videos in both stunned, recovering and active states. Fish could be vulnerable to predation while immobilised although Murray et al. (2014) suggested that they would recover in a few minutes following exposure to the electrical field. There is thus some scope for electrofishing to lead to additional predation on both target and non-target organisms, but the rates at which this occurs are not particularly well understood.

The data collected in these surveys has provided further information on the razor clam populations in the Firth of Clyde, adding to results from the 2017 survey (Fox, 2018). In addition, Machrie, Carradale and Saddell Bays, which are outside of the present scientific trial area, were included showing that median densities of ’Large’ sized E. siliqua were higher at these sites compared with Irvine, Ayr and Culzean Bays, which are in the fishery trial. While it is tempting to ascribe such inter-site differences to fishing pressure alone, there may also be differences in productivity, growth and natural mortality because the density of ‘Large’ razor clams in Turnberry Bay, which is within the authorised trial area, was within the range of values seen at the unfished sites. The data from the towed video surveys has also provided useful insights into changes in the razor clam populations over time by comparing results from surveys completed a few years apart. The changes in size distributions and densities from 2017 to 2023 along the Ayrshire coast appear consistent with harvesting of larger razor clams plus recruitment of small razor clams to the beds. For the Solway, the survey provides the first estimates of razor clam sizes and densities on the fished grounds in the scientific trial. Finally, the use of fishing vessels to conduct the survey has allowed the science team to draw on local knowledge about the grounds, as well as enabling skippers and crews to contribute to the design and implementation of these scientific surveys.