The interactive roles of predation and tidal elevation in structuring populations of the edible cockle, Cerastoderma edule

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The size and age structure of the Cerastoderma edule population at Traeth Melynog, North Wales, varies dramatically with tidal level. In areas low on the shore up to 96% of cockle spat fail to survive their first summer, but mortality rate subsequently declines and remains at a low level. By contrast cockles high on the shore suffer moderate mortality during their first year (47%), but increasing rates thereafter. High-shore populations consequently consist mainly of smaller (younger) individuals and low-shore ones of a transient spatfall, plus a few larger and older individuals. The potential role of shore crabs, Carcinus maenas, and oystercatchers, Haematopus ostralegus in the determination of these patterns is assessed. Shore crabs move up into the intertidal to feed with each flood tide from about April to December. They selectively consume cockles < 15 mm in length, taking an estimated 236 × 10 3 cockles, or 2432 g dry flesh year −1 per linear meter of shoreline, mostly from lower shore levels. Oystercatchers are present only during winter and preferentially select large cockles of at least 20 mm length. They are estimated to remove 9 × 10 3 cockles, or 1204 g dry flesh year −1 per linear meter of shoreline, most of this from mid- and high-shore levels. These results indicate that shore crabs are far more important predators than previously suspected, taking 25 × the numbers and 2 × the biomass cosumed by oystercatchers. Predation also appears to be the key factor controlling the structure of the C. edule population. Crabs consume almost all the cockles settling low on the shore during their first summer, but avoid older individuals, which subsequently survive and grow well under low levels of oystercatcher predation. On the high shore, crabs are unimportant and the cockles survive well as they slowly grow into the size range attractive to oystercatchers. Thereafter they suffer increasingly severe winter mortality and are soon eliminated.