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Assessment of the Impacts of Hydraulic Dredging on Sedimentary Habitats in the Irish Sea

Project Outputs

The razor clam (Ensis siliqua) fishery in the north Irish Sea is distributed in a continuous band of activity from Dundalk Bay in the north to Malahide in the south at depths from 2-15m. Fishing activity has increased significantly since 2013 in response to strong market demand for product. The fishery uses hydraulic dredging to extract razor clams and disturbs sediment to a depth of 25cm. Considerable volumes of sediment are mobilised during fishing. The impacts of this type of fishing on seafloor integrity is likely to be significant on sediment structure and associated fauna. Some of the activity occurs within Dundalk Bay SAC and SPA. The activity in Dundalk has escalated since 2011 when the Fishery Natura Plan (FNP) was developed for Dundalk Bay. A condition raised during the appropriate assessment of the FNP was that the effects of the fishery would be assessed if the activity escalated.

Since mid-2015 all vessels engaged in fishing for razor clams are required to report GPS position at up to 1 minute frequency. These data are now available for up to 75 vessels fishing in the north Irish Sea and provide, for the first time, a capacity to generate fishing pressure maps which identify the hours of dredging pressure at a fine scale. These data can be used therefore to design a fishing impact study of marine benthic habitats in the area. Significant changes to marine habitats with respect to the favourable conservation status (conservation objectives) or good environmental status indicators can be reported.


The project had a 1 year duration and project work was carried out in 2016.

Project Outputs:

Report on the effects of hydraulic dredging on benthic ecosystems in the north west Irish Sea: This study was designed to investigate the effects of hydraulic dredging on seafloor habitats in shallow waters of the northwest Irish Sea. High frequency Vessel Monitoring System (VMS) data reported by the fishing fleet was mapped at 100m resolution to identify areas which had received different levels of fishing activity in the previous 6 months. Hydraulic dredge tracks are clearly visible on the seabed from images derived from multibeam acoustic data. Sediment and fauna samples were then taken by grab from these areas. All fauna were identified to species level where possible and sediment samples were analysed for particle size and organic content. A total of 240 stations were sampled.

Preliminary findings, from multivariate statistical analysis of the seafloor faunal data, indicated significant differences in faunal communities in areas which had received <5hrs of fishing compared to areas which had received >5hrs of fishing in the previous 6 months.

The preliminary findings of the study indicate that seafloor faunal communities are affected by fishing for razor clams and that this is related to the intensity of fishing effort these areas have received in previous months. The ecological significance of the changes, especially with respect to conservation objectives for sedimentary habitats and bird species using these habitats, needs to be assessed.

Expected Benefit:

Evaluation of the possible effects of towed dredges from razor clam fisheries on habitats including seasonal variation and recovery. Improved scientific advice on the impact of specific fishing activities on the conservation objectives of SACs and SPAs.

Further Information:
Contact Oliver Tully at
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