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Assessment of the Impacts of Dredging on Seagrass Habitats

Project Outputs

This project is a collaboration between the local shellfish co-op, Comharchumann Sliogéisc Chonamara and the Marine Institute. Comharchumann Sliogéisc Chonamara operates the bivalve aquaculture licence in Cill Chiaráin and Beirtreach Buí Bays in an area that also encompasses a designated Special Area of Conservation (SAC) known as the Kilkieran Bay and Islands SAC (Site Code 002111). The area is designated for the habitats mudflats and sandflats not covered by seawater at low tide, large shallow inlets and bays and reefs. The bay supports a variety of sub-tidal and intertidal sedimentary and reef habitats including habitats that are sensitive to pressures, which might arise from fishing and aquaculture, such as Maërl (corraline algae), seagrass (Zostera) and kelp reefs. The co-op dredges for oyster and scallop in the upper bay close to or on potentially sensitive habitats such as seagrass.

This study investigated the possible effect of towed dredges during winter and spring fisheries for oyster and scallop on the potentially sensitive Zostera beds and the quality of Zostera in the following summer. A before and after control impact study (BACI) was established; seagrass plots were sampled for seagrass rhizome using SCUBA in mid-winter. These plots were then either fished or not fished (controls) at different intensities and the density of seagrass growth and biodiversity of fauna associated with seagrass in the different plots was subsequently assessed in late spring and summer.


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

Project Outputs:

Report on the effects of winter-spring dredging on summer seagrass growth and biodiversity. Preliminary findings indicate that seagrass leaf and shoot density is similar in areas that were fished in the previous winter to those which were not fished and that the biodiversity supported by seagrass beds is not affected by oyster dredging during winter.

Expected Benefit:

Evaluation of the possible effects of towed dredges from oyster and scallop fisheries on sensitive Zostera beds including seasonal variation and recovery. Improved scientific advice on the impact of specific fishing activities on the conservation objectives of SACs and specific sensitive habitats within designated SACs.

Further Information:
Contact Oliver Tully at
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