Pilot study examining the effects of hydraulic and elevator dredge harvesting of marine bivalves such as the common cockle (Cerastoderma edule L.) on intertidal communities

The Solway Firth straddles the border between England and Scotland and forms one of the largest continuous areas of intertidal habitat in Great Britain. Each year the Solway Firth is home to ~110,000 overwintering waterfowl and it has various designations as a Ramsar Site, a Special Protection Area (SPA), a Special Area of Conservation (SAC), and a Site of Special Scientific Interest (SSSI). Cockles are important to the Solway ecosystem, and the literature on cockle dredging indicate that such activities may be carried out without major environmental effects if it is conducted on relatively well sorted sands that receive a certain amount of wave/tidal disturbance and the area is not over-fished. However, to date, there is no comparative study on the effects of suction dredge impacts compared to elevator dredge impacts. This study describes the outcomes of a pilot study examining the effects of hydraulic and elevator dredge harvesting of marine bivalves such as the common cockle (Cerastoderma edule L.) on intertidal communities. The study was conducted in Southerness Bay in the Solway Estuary and the work carried out at the Centre for Wildlife Conservation, University of Cumbria. Statistical tests indicated there were many incidences where zero counts for individual species listings caused issues with analysis. To avoid such issues, it is recommended that a minimum of 22 samples per treatment per sampling period would be required to have a 90% chance of detecting real difference at α = 0.05. Statistically significant difference in species abundance between the “Control” and Control 2” samples indicate that future studies should encompass an assessment of substrate type classification and it’s benthic diversity, prior to the initial dredge sampling being conducted. Results suggest no significant difference in adult or juvenile cockle numbers due to the different treatment regimes applied in the study. Nor were there any significant difference between adult cockle numbers found for either dredge treatment, compared to control samples post-recovery period. Baltic tellin (Macoma balthica) demonstrated a significant increase in number after the treatments, possibly due to substrate agitation and niche availability. A trend towards suction dredge treatment leading to a greater increase in Baltic tellin numbers compared the elevator dredge treatment was observed. Evaluation of other benthic species was inconclusive and would be more significantly considered in a full-scale study encompassing recommendations generated here. The indicative patterns observed and reported from this study must be interpreted within this framework and not over-extended without further supporting data across a broader spectrum of understanding gained from the full-scale study. It only provides a snapshot of a single study site with two different treatments, targeted at a single species. A full-scale would incorporate a number of procedures and actions which would not only allow for greater statistical support of the data patterns observed, but would generate a more comprehensive dataset covering a number of different aspects of the effects of suction dredge and elevator dredge impacts on intertidal communities