Shetland 2021 drop-down video (DDV) survey exploring the occurrence of flapper skate eggs and horse mussels (Modiolus modiolus): preliminary results

Richard L. Shelmerdine, Rachel Shucksmith

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This work was commissioned and funded by NatureScot under the ‘Framework Agreement for Marine Survey, Analysis and Interpretation to Support Nature Conservation’. The aim was to carry out drop-down video (DDV) drifts at 60 specified sampling stations to the east of Shetland to provide more comprehensive information on skate egg occurrence south of Lunna and the extent and abundance of horse mussels southwest of Whalsay and south of Samphrey.Flapper skate, Dipturus intermedius (Parnell, 1837), are a Priority Marine Feature (PMF) that form part of the common skate complex, which also includes the common blue skate, Dipturus batis (Linnaeus, 1758). Although D. intermedius usually inhabit deeper muddy habitats, it is thought that these demersal elasmobranchs prefer to lay their eggs in shallower water on gravel or among rocks. Existing skate egg records included three occurrences from a directed DDV survey in 2019 and from a citizen science diver record from 2017 of more than 40 eggs reported to the Shark Trust.Horse mussel beds, Modiolus modiolus (Linnaeus, 1758), are a PMF and are listed as OSPAR habitats and Annex I habitats of the EU Habitats directive. Several horse mussel beds have already been identified in Shetland waters. Existing M. modiolus records were identified southwest of Whalsay in 2017 and south of Samphrey in 2010 but both surveys had limited information.Fifty-one sampling stations were surveyed in July 2021. Skate eggs, M. modiolus, and maerl were recorded. Skate eggs were confirmed from three different drifts, with an additional three drifts showing possible sightings. A total of 23 eggs were recorded from Drift 9 over a 43 second period and 29+ eggs recorded from Drift 44 over a 29 second period. However, some of these may have also been recorded from Drift 9, as the drifts overlapped, so should not be treated as total numbers for that location. A further 8-10 skate eggs were recorded at four other drifts. Diver surveys would be recommended to confirm whether the eggs were hatched with many having an aged appearance. This would allow for a better understanding of whether the area is an active egg site.Modiolus modiolus were recorded from three drifts at the site southwest of Whalsay. Abundances were high enough to constitute a bed along two of the drifts (Drifts 48 and 51) with cover ranging from 10-49%, compared with <10% for the third drift. Due to tidal conditions, it was not possible to sample the site south of Samphrey.A small patch of maerl with a live cover of 20-30% was recorded from Drift 4 south of Lunna. The bathymetry shows the patch was located within a large rock area on a sandy bottom alongside a rock face. However, it was not possible to determine the size of the area covered by the live maerl. It seems unlikely, from this evidence, that the maerl would cover a sufficient area to constitute a bed.The work confirmed the presence of skate eggs and a small patch of maerl to the south of Lunna, and a M. modiolus bed southwest of Whalsay. The survey provided greater spatial accuracy to previous public (citizen science diver) reporting of skate eggs and has provided additional skate egg locations in the wider area. Skate egg records were located on, and in among, boulders. However, the findings also highlight the potential need for further diver surveys to confirm the age of the eggs and whether they have already hatched.
Original languageEnglish
PublisherNAFC Marine Centre
Commissioning bodyNatureScot
Number of pages41
Publication statusPublished - Aug 2021


  • Flapper skate
  • Skate egg
  • Dipturus intermedius
  • Shetland
  • Horse mussel
  • Modiolus modiolus
  • Maerl
  • DDV
  • marine survey


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