Scottish Marine and Freshwater Science Vol 12 No 10
This report describes a survey carried out in Largo Bay, Firth of Forth in 2020. The original work plan was interrupted due to Covid-19 but a survey of the area currently being fished was completed prior to lock-down. A combination of a commercial electrofishing rig and towed video was deployed using the fishing vessel ‘Maxine’. The video recordings were subsequently analysed for the number and sizes of razor clams observed on the surface of the seabed following passage of the electrofishing equipment. These count data were converted to area densities (numbers of razor clams m-2) based on estimates of the swept area from tow start and end positions recorded from the vessel chart plotter.
The use of towed video combined with electrofishing appeared to work well in Largo Bay. The lowest number of tows completed in any one day was four but this was due to worsening weather conditions and under good conditions up to 11 tows were completed each day. The video recorded was generally of sufficient quality to allow identification of objects on the seabed with only a few tows having parts of the field of view obscured by sediment kicked up by the camera sled. Although the efficiency of the electrofishing equipment is assumed to be high, this has not been formally confirmed by comparison with other methods such as dredging (Hauton et al., 2007). The estimates of E. siliqua densities provided in this report therefore represent a minimum for the razor clam stock in Largo Bay in 2020.
Data and Resources
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2020-02-28 to 2020-03-20
UK Open Government Licence (OGL)
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Seventy-four tows were completed in and around the main Ensis harvesting area in Largo Bay between 28 February 2020 and 20 March 2020. Of these, two tows were excluded from further analysis as the video suggested that the electrical rods had not settled properly onto the seabed leaving 72 valid tows. Average towing speeds varied between 1.9 and 6.1m min 1 (mean 3.2m min-1) and estimated swept areas were between 45 to 181m2 (mean 76m2). From the video recordings, 20,538 individual E. siliqua were counted and 18,439 measured. All razor clams observed on the videos were assigned as Ensis siliqua which accords with the skipper’s observation that Ensis magnus are not found in this area.
Considering all sizes, E. siliqua densities of up to 11.0m 2 were observed (4.0 ± 2.8 mean ± std dev). For sizes above the minimum conservation reference size (MCRS) of 100mm, densities were up to 9.3m-2 with a mean of 3.0 ± 2.2m-2 (mean ± std dev). The commercial fishers tend to target Ensis above 150mm as these fetch the highest price and densities for this size group were up to 4.9m-2 with a mean of 1.4 ± 1.2m-2 (mean ± std dev). The size distribution of the razor clams suggested two length modes, one at around 90–100mm and the other around 150–160mm. These likely represent two age classes and while showing that smaller clams are present, the densities of smaller clams were generally lower than for the larger sizes.
Other organisms occasionally seen on the videos were shore crabs, hermit crabs and common starfish. Juvenile flatfish (probably dab or plaice) were only observed on two tows and sandeel (Ammodytes spp.) on one tow. Large numbers of eider ducks (Somateria mollissima) followed the survey vessel on some days and were observed on the video diving and taking emerged razor clams from the seabed. The survey vessel skipper suggested that the numbers of eider ducks in the area had increased over recent years. It may therefore be necessary to account for this additional source of mortality when considering sustainable harvest levels for Ensis in Largo Bay but estimating this predation pressure was beyond the scope of the present survey.
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