Modelling oyster Crassostrea gigas fattening with the diatom Skeletonema costatum.

The traditional fattening of the oyster Crassostrea gigas in oyster ponds o f the French Atlantic Coast is submitted to natural environmental fluctuati ons. In order to reduce the growth variability of the soft parts of the biv alve, an intensive fattening process was developed, where the conditions of temperature, particulate organic and inorganic matter (POM and PIM) are co ntrolled. This process relies on the production of the diatom Skeletonema c ostatum, which is then distributed to the oysters at an average concentrati on of 4-5 mg POM.L-1. An ecophysiological model of the oyster C. gigas, whi ch simulates the evolution of somatic and gonad-reserve compartments, was a pplied to these conditions in order to analyse the bivalve responses. Exper imentation was performed to elaborate the model and two functions were stud ied: clearance rate and pseudofecal production. At a temperature of 14 degr eesC, chosen for the fattening process, and at POM and PIM concentrations v arying respectively from 4 to 18 mg.L-1 and 15 to 55 mg.L-1, it was found t hat clearance rate was not regulated (mean of 2.09 +/- 0.11 L.h(-1).g(-1) w ith 59% of activity rate) and that ingestion rate depended on the productio n of pseudofeces. This production allows an increase of organic ingested fr action, by the mechanism of pre-ingestive selection that buffers the PIM va riability. Negative effects of PIM on growth have been studied through seve ral simulations. It appears that actual POM ration (4-5 mg.L-1) used in the fattening process, allows dry tissues growth, in spite of simulated PIM co ncentration up to 50 mg.L-1. The elaboration of the model revealed that gam etogenesis is a major determinant in bivalve energy partitioning in the ric h food conditions of this fattening process. (C) 2001 Ifremer/CNRS/Inra/IRD /Cemagref/Editions scientifiques et medicales Elsevier SAS.