Spatial Variation in the Predation of Cockles by Oystercatchers at Traeth Melynog, Anglesey. II. the Pattern of Mortality

Abstract

(1) The profitability (rate of biomass intake per bird) of each site to oystercatchers Haematopus ostralegus. depended not only on cockle Cerastoderma edule density but also on cockle size and flesh content. At very low cockle densities the individuals eaten were large, containing a lot of flesh (relative to their size), but the feeding rate was low. At high cockle densities many cockles were taken but they were small, containing little flesh. At a medium density of 25-150 cockles per m2 the profitability was at its maximum. (2) Oystercatchers fed most in areas of highest profitability (i.e. 25-150 cockles per m2) and so inflicted the highest cockle mortality in these areas. Over the period 1 December to 31 March, mortality was estimated at between 28% (at 45 cockles per m2) and almost zero (at 12 and 600 cockles per m2). (3) The actual disappearance of cockles was not clearly related to their density; however the downshore movement of cockles countered the effects of predation in the most profitable sites at the bottom of the shore. The large number of old cockles present accorded with the low mortality caused by the oystercatchers and the absence of cockle fishing.