Razor clam surveys - Firth of Clyde and Solway: 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. Based on counts by divers, Murray et al. (2014) reported densities of Ensis (siliqua and magnus combined) to be up to 17 m^-2 in Loch Nevis. More recently combined electrofishing towed-video surveys have been undertaken on several grounds in the Scottish scientific trial, including some areas which have been excluded from fishing for the last 5 – 6 years (Fox, 2017; 2018; 2021 and 2023). From these results, the maximum density of E. siliqua recorded to the north of Barra was around 3 m^-2 and the average was 1.7 ± 0.19 m^-2 (mean ± SE); in Largo Bay (Firth of Forth) the maximum was 11 m^‑2 and the average was 4.0 ± 0.3 m^-2 (mean ± SE); and a recent survey on Tarbert Bank (Fox, 2023) found densities of up to 6.7 m^‑2 with the average being 1.6 ± 0.11 m^-2 (mean ± SE).

The Ayrshire coast has been previously surveyed in 2017 (Fox, 2018) where the maximum density of E. siliqua was 1.6 m^-2, and the average was 0.52 ± 0.05 m^-2 (mean ± SE). In the present survey, the maximum in the permitted fishing area was 1.4 m^-2 and the mean was 0.47 ± 0.03 m^-2 (mean ± SE). For tows conducted in 2023 outside of the permitted fishing area, the densities were higher with a maximum of 2.6 m^‑2 and average 0.72 ± 0.09 m^‑2 (mean ± SE) respectively. However, compared to other areas for which data exist, the beds within the Firth of Clyde appear to have supported relatively low average densities of E. siliqua (< 1 m^-2) throughout the trial period. In contrast, densities for Fleet Bay in the Solway were up to 2.1 m^-2 with an average of 1.3 ± 0.01 m^-2 (mean ± SE) which may reflect relative fishing pressures during the trial. However, there could also be differences in productivity since the average densities in the surveyed unfished areas of the Firth of Clyde were still not particularly high.

The 2023 Ayrshire coast results showed that the densities of small E. siliqua have increased since 2017 indicating that recruitment has occurred during the scientific trial, despite the relatively low average E. siliqua densities 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, Firth of Forth (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, and the quality of the recorded videos has usually been high. However, water clarity was much poorer in the Solway because of the large amounts of riverine fine silt, along with 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.

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; Fox, 2021; Fox, 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.

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 about 2^oC warmer than during the Fleet Bay survey. Although the depletion experiment result supports 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 still 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 of razors by these birds did not appear to be as significant as in Largo Bay. 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 and active states. Immobilised fish could be vulnerable to predation 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 this survey 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 surveyed. Results showed 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 razors 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. Finally, the use of fishing vessels to conduct the survey has allowed the science team to draw on local knowledge about the grounds, and allowed skippers and crews to contribute to the design and implementation of these scientific surveys.