Résumé
Populations of Abra alba (Wood) (Mollusca : Bivalvia) were studied over a fifteen-years period at three sites: the southern part of the North Sea (Gravelines area), the Bay of Seine, and the Bay of Morlaix in the western English Channel. These surveys provide an example of long-term changes in a species considered as ``opportunistic'' and as a ``biological indicator'' of disturbance. A summed cumulated function was used to compare the changes of the Abra alba populations and of mean air temperature in each area. The graph of this function shows the timing and duration of changes. Four or five main periods were identified at each site, with a succession of periods of abundance, stability, and decrease. The abundance periods were separated by ten years in the English Channel (Bays of Seine and Morlaix) and by seven years in the Gravelines region. The Abra alba populations appear to be affected by a combination of three factors: local disturbance, biological interactions (competition and predation), and climatic effects.