Calcimass and carbonate production by molluscs on the tidal flats in the Dutch Wadden Sea: II the edible cockle, cerastoderma edule

Abstract

Production of shell lime by the edible cockle Cerastoderma (Cardium) edule living on the tidal flats of the Dutch Wadden Sea was assessed from estimates gathered twice-annually during the period 1968 to 1982 on abundance, age composition and growth rates at 15 sampling places on Balgzand, a 50 km 2 tidal flat area. Large-scale surveys showed Balgzand to be a representative for the 1300 km 2 tidal flats of the Dutch Wadden Sea. Calcimass of the shells of the standing stock of living cockles increased during the period of observation from about 50 to about 200 g · m −2. The long-term average amounted to 104 g · m −2. Mean annual values for production and elimination were estimated at 118 and 114 g · m −2 · a −1, respectively. Turn-over rates thus amounted to about 1.1 a −1. As a consequence of the high between-years variability of the rates of both recruitment and mortality, all population parameters varied strongly from year to year. Mean annual carbonate production by cockles on the tidal flats of the Wadden Sea is estimated at 234 thousand m 3 (156 million kg) but this estimate is far from precise (95% confidence limits being about 100 to 400 thousand m 3). About 3 4 of this quantity will become available for shell fishery. The present level of exploitation of the stock of cockle shells slightly exceeds the lower limit of the long-term production estimate.