GROWTH AND PRODUCTION OF THE BIVALVE SCROBICULARIA-PLANA IN 2 SOUTHERN EUROPEAN ESTUARIES

Dynamics, growth and production of the bivalve Scrobicularia plana (Da Costa, 1778) were studied at Mira and Tagus estuaries on the SW and W coasts of Portugal for a period of 18 months. The pattern of recruitm ent was similar to the results obtained for southern countries of Euro pe, with two cohorts produced per year. However, mass mortality succee ded recruitment at the lower Mira estuary. At Tagus estuary, as well a s at the upper Mira estuary, recruitment was poorly successful or didn 't occur at all. These latter two populations were dominated by older age classes, suggesting a ''dominant age-class'' phenomenon, probably being the main cause for the unsuccessful recruitment. Growth paramete rs were similar to those of southern countries at lower Mira estuary. At the upper Mira estuary and Tagus estuary, those growth parameters w ere closer to northern Europe, which was partly attributed to the diff erent age structures of populations. Reductions in biomass and abundan ce occurred after recruitment and winter. Production decreased during sampling period, showing negative values for lower Mira estuary where population was very unstable. Mean values of production were 23.116 Kc al/m(2)/year for Mira estuary and 136.089 Kcal/m(2)/year for the Tagus estuary, reflecting the different age structures,habitats and relativ e importance of different cohorts.