CHANGES IN ABRA-ALBA (MOLLUSCA, BIVALVIA) POPULATIONS IN THE ENGLISH-CHANNEL AND NORTH-SEA, 1977-1991 - RELATION WITH CLIMATIC FACTORS

Populations of Abra alba (Wood) (Mollusca : Bivalvia) were studied ove r a fifteen-years period at three sites: the southern part of the Nort h Sea (Gravelines area), the Bay of Seine, and the Bay of Morlaix in t he western English Channel. These surveys provide an example of long-t erm changes in a species considered as ''opportunistic'' and as a ''bi ological indicator'' of disturbance. A summed cumulated function was u sed to compare the changes of the Abra alba populations and of mean ai r temperature in each area. The graph of this function shows the timin g and duration of changes. Four or five main periods were identified a t each site, with a succession of periods of abundance, stability, and decrease. The abundance periods were separated by ten years in the En glish Channel (Bays of Seine and Morlaix) and by seven years in the Gr avelines region. The Abra alba populations appear to be affected by a combination of three factors : local disturbance, biological interacti ons (competition and predation), and climatic effects.