An assessment of the potential impact of dredging activity on the Tamar Estuary over the last century: II. Ecological changes and potential drivers

Abstract

The Tamar estuary is a Special Area of Conservation (SAC) under the EU Habitats and Species Directive and a Special Protection Area under the Wild Birds Directive (1979). The lower Tamar is also the site of the Devonport naval dockyard which requires annual maintenance dredging, as well as occasional capital dredging for new installations. The main objective of this study was to investigate whether there is any evidence of significant temporal changes in key species (intertidal macrofauna, fishes, birds) and habitats (intertidal mudflats and saltmarsh) that could be related to dredging activities in the lower Tamar. Other physical variables, such as Tamar river flow and the North Atlantic Oscillation (NAO) index, representing potential drivers of changes in the abundance of biota, were also examined. Spatial and temporal changes in the abundance of intertidal macrofauna (between 1939 and 2000) were analysed but there was insufficient comparable data to enable us to draw any conclusions about long-term changes in the Tamar estuary. Commercially and ecologically important fish species (salmon and sea trout) showed a steady decline in numbers caught in the Tamar since the 1970’s. There was a significant correlation between the number of salmon caught by rod and the Tamar river flow. The sea trout abundance was significantly negatively correlated with the NAO index, suggesting that sea trout may be adversely affected by mild winters, which have been a feature of the late 1980’s and 1990’s. There were also significant correlations between the number of salmon caught in the Tamar and other rivers of the SW of England. Ten species of wildfowl and wader birds were analysed. There were no significant correlations between over-wintering numbers and dredging activity, but there were significant declines in teal and wigeon over 30 years. These species also showed a negative correlation with the NAO index suggesting the declines were related to the milder winters; possibly reducing their need to migrate south as far as SW England. Aerial photographs of the Tamar showed that the Egypt salt marsh, creeks and mudflat maintained a remarkably consistent structure over a period of >50 years. However, there was evidence of gradual erosion (0.23 m y⁻¹) of the saltmarsh’s front cliff edge that may be related to sea level rise (~1.5 mm y⁻¹ in SW England). The study concluded that there was no evidence of ecological changes related to the dredging activity in the Tamar. However, there were significant changes in salmon and sea trout catches, and the number of over-wintering teal and wigeon, over many decades and these changes appear to be related to large scale climatic events rather than anthropogenic factors within the Tamar estuary.