Corbicula fluminea populations were sampled annually from 1997 to 2000 at the beginning of the autumn, i.e. at the end of the reproduction period, at 14 sites along the Saône and Rhône rivers. Recruitment in the upper reach of these rivers was generally high. Conversely, the observation of other sectors revealed generally low or no production of juveniles, missing cohorts and major year to year variations in population structure. Nevertheless, no dramatic fall in population density was observed during the surveyed four years. This suggests that the longevity of the Corbicula populations of the Saône and Rhône rivers is ensured to a great extent by the drifting of individuals from the upper course of these rivers and their tributaries. Two possible causes are put forward to explain the low recruitment observed : low production of phytoplankton that prevents adults from constituting sufficient energy reserves to ensure the full development of larvae, and/or toxic contamination, which inhibits the development of phytoplankton and it is lethal for pediveligers.
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