Population dynamics and secondary production of the cockle Cerastoderma edule (L.) in a backbarrier tidal flat in the Wadden Sea
Montserrat Ramón
The population biology of the bivalve Cerastoderma edule was studied by monthly sampling from April 1994 to September 1995 at Neuharlingersieler Nacken, a backbarrier tidal flat near Spiekeroog island (East Frisian Wadden Sea, North Sea). Four stations located along a tidal gradient from 56 to 80% immersion time were established in order to analyse cockle growth, mortality and production. The highest densities were recorded at the beginning of the study (1000-1250 ind. M-2) with an intense and progressive decrease through 1995. A spatial distribution pattern was observed during summer 1994, with juveniles (3-12 mm long) being found in the high intertidal whereas adults occurred along the middle and lower intertidal. Based on growth increment data from tagging-recapture experiments, the parameters of the von Bertalanffy growth curve were estimated to be: asymptotic length L??= 40 mm, and growth constant K = 0.404 y-1. A growth ring was formed in summer (July-August 1994). Annual somatic production ranged between 38.16 and 59.33 g AFDW . m-2. y-1. Total mortality ranged between Z = 0.52 year-1 and Z = 3.03 year-1. The distribution of the population was affected by passive displacements of juvenile cockles from the upper part of the intertidal toward lower zones in combination with high mortality at the station farthest from the coast, probably resulting from mussels overlying.
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