DYNAMICS AND GROWTH OF THE COCKLE, CERASTODERMA-EDULE, ON AN INTERTIDAL MUD-FLAT IN THE DANISH WADDEN SEA - EFFECTS OF SUBMERSION TIME AND DENSITY

A descriptive study was conducted on a dense intertidal population of the infaunal bivalve Cerastoderma edule in the Danish Wadden Sea to investigate effects of submersion time and intraspecific density on recruitment and shell growth. Following a severe winter (1981/82), a successful settlement occurred in June-July 1982. Spat numbers varied along the tidal gradient. At the end of July they reached a maximum density of about 60 000 ind.m-2 in the lower intertidal zone. Their abundance declined rapidly during the following two months. After that the mortality rate was low and in the lower intertidal zone 2500 ind.m-2 survived to the age of 2. Recruitment was insignificant both in 1983 and 1984, probably due to the high standing stock of cockles born in 1982. The distribution pattern of cockles along the tidal transect established during settlement in July 1982 existed until the population vanished in the severe winter of 1984/85. The spat grew from settlement until mid-September and resumed growth during the following spring and early summer. The mean length attained by the cockles after 3 growing seasons at the station with the largest cockles was only 18.4 mm, which is far below lengths recorded elsewhere. Annual growth rates of individual cockles generally showed an increase with increasing submersion time, in particular during their second growing season. In their third year, however, growth rate was extremely low at the station with the highest density. Intraspecific competition among cockles is suspected to be a major cause of such growth rates exhibited by cockles at high densities (>2000.m-2) both during the present study and an earlier (1932-34) study in this area by Thamdrup. Cockles in nearby low-density populations showed higher growth rates, reaching mean lenghts of more than 26 mm after 2 or 3 growing seasons.