Side-scan-sonar survey of the Horse mussel - Fisheries ...

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Fig. 25 Images of camera tow 306a showing representative habitat characteristics DISCUSSION The still images analyzed verified the extent of the Modiolus bed as indicated by the side-scan-sonar images. Tows showing live Modiolus corresponded with the substratum categories 3 (some structured substratum present most likely Modiolus beds) and 4 (extensive structural substratum present most likely Modiolus beds). Overall, categories 3 and 4 covered 53% of the surveyed area. The Modiolus bed’s limits could be established for the northern, southern and southeastern parts. The northwestern limit was not found and it is possible that the reef extends further into the northwest. The biotope type of Modiolus beds off the Point of Ayre most closely resemble the habitat type describe by the EUNIS code A4.4/B-CMX.ModMx (Modiolus modiolus on circalittoral mixed sediment). Camera tow station 307 and 317 showed dense aggregations of Modiolus and most closely resembled a bed or reef structure. At the other sites individual Modiolus occurred along the video/stills transects sometimes imbedded into the sediment matrix. Due to the embedded nature, and 18
the difficulty of distinguishing live Modiolus in general, abundance estimates are likely to have underestimated true abundances at the Point of Ayre. Obvious dredge marks from scallop dredges or otter trawls, indicating fishing disturbance, were not detected within side-scan-sonar images. However, as image quality was very poor in some areas of the side-scan such trawl marks might have remained undetected. Dredge tracks can be very faint and difficult to detect in single side-scan-swath. Faint tracks might have become more visible within a full side-scan coverage of the area. Overall, the status of the reef and the potential impact form dredging activity is difficult to assess from the results of the current survey. More video tows over the area would have been needed to make a more comprehensive assessment of the status of the Modiolus bed. It is interesting to note that the area had few individuals of the target species important for the local scallop fisheries. Within the 1145 images analyzed only 5 queen scallops were identified and counted. Compared to other sites around the Isle of Man abundance of scallops seem to be extremely low in this area. Therefore it seems unlikely that these Modiolus reefs would become the target of a focused fishery, although there may be periodic exploratory attempts from vessels outside the Isle of Man that are not familiar with local grounds. Accordingly, it would be useful to understand the level of fishing activity that has occurred in this area in the recent past (Fig. 26). Figure 26: Vessel Monitoring System data for 2007/2008 showing the distribution of fishing activity, which was absent from the area surveyed in this report. Reference Holt, T.J., Rees, E.I., Hawkins, S.J. & Seed, R., 1998. Biogenic reefs (Volume IX). An overview of dynamic and sensitivity characteristics for conservation management of marine SACs. Scottish Association for Marine Science (UK Marine SACs Project), 174 pp. This comprehensive review of biogenic reefs contains numerous references to research directly relevant to horse mussels and their associated fauna. 19
Page 1 and 2: Side-scan-sonar survey of the Horse
Page 3 and 4: Due to the size of the area chosen
Page 5 and 6: RESULTS Working conditions off the
Page 7 and 8: Table 1. Summary of video tow posit
Page 9 and 10: Fig. 7 Map showing side-scan image
Page 17: Fig. 23 Map showing side-scan image

Side-scan-sonar survey of the Horse mussel - Fisheries ...

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