RELATION BETWEEN WATER QUALITY AND DORSAL THICKNESS OF MUSSEL (MYTILUS EDULIS) AND ITS ECOLOGICAL IMPLICATIONS FOR WINTERING OYSTERCATCHERS (HAEMATOPUS OSTRALEGUS)

legus) as a model species is successful in predicting the impacts of environmental changes in aquatic ecosystems. At its heart is a food depletion model which assumes that consumers will tend to starve in highly depleted areas and seasons. Since dorsal hammering Oystercatchers selectively deplete dorsally thin-shelled mussels (Mytilus edulis), we measured the dorsal thickness of the shells of small (34–46 mm), medium (44–46 mm) and large (54–56 mm) length classes and assessed the water quality across the seasons (September to March) in the Exe estuary, Southwest England. Water temperature decreased across the season from autumn whereas salinity increased until late October and then decreased during rest of the season. In parallel, mussel shell thickness decreased until mid-winter and then started to increase. This increase could be associated with the Oystercatchers ’ selective predation on thin-shelled mus-sels. Multiple regressions confirmed that the changes in the salinity across the season were as-sociated with the overall thickness variations of the shells. We conclude that salinity had a sig-nificant influence on the thickness variations of medium and large classes of mussels, and water temperature significantly influenced the dorsal thickness of large mussels. Changes in water quality did not significantly influence small mussels. Our result suggests that, despite the selective removal of significant numbers of thinner-shelled mussels by Oystercatchers, the density of thin-shelled prey available to them may increase because of changes in the salinity and water temperature. Key words: mussel thickness, Oystercatchers, salinity, water temperature, wintering seaso