The oyster population in the Maryland portion of Chesapeake Bay, USA, has declined by more than 50-fold since the early part of this century. The paper presents evidence that the mechanical destruction of habitat and stock overfishing have been important factors in the decline, even though it is commonly thought that 'water quality' and, more recently, oyster diseases are critical. Quantitative analyses show that the long-term decline of oysters largely results from habitat loss associated with intense fishing pressure early in this century, and stock overfishing from early in the century through recent times. Furthermore, the major ecological effects on Chesapeake Bay occurred well before World War 11, before industrialization and the reported prevalence of disease. To effect the recovery of the ailing Chesapeake Bay oyster stock, a 4-point management strategy is proposed.

Mussels were fed four concentrations of seston (between 0-99 and 10-3 mg total seston I 1 ), comprising three levels of organic content (71 -9,63-6 and 40-8%), made up from natural silt and two species of cultured phytoplankton. Two of the seston concentrations were below, and two above, the threshold at which pseudofaeces were produced. Measurements of physiological traits (filtration rates, pseudofaeces production, selection efficiency, absorption efficiency, absorption rates and rates of oxygen consumption) were made after 2 days and, for two of the seston concentrations, also after 12 days. When fed at a high concentration of seston of low organic content, the mussels increased their filtration rate, rejected a higher proportion of filtered material as pseudofaeces, and increased the efficiency with which filtered matter of higher organic content was selected for ingestion; this resulted in a constancy of the relationship between ingestion rate and the concentration of particulate organic matter, regardless of differences in seston organic content. Between 2 and 12 d, the mussels increased absorption rates for organics, primarily by increasing absorption efficiency, both for total organics and for the carbohydrate component of the diet. We suggest that these responses to changes in the food environment comprise physiological adjustments which result in higher net rates of absorption than would be predicted from considerations only of the organic/inorganic ratio of the suspended particles and assumptions of a non-compensating feeding behaviour.

For the first tlme for any benthic filter-feeder, this study documents each component process of nutrient acquisition through natural tidal variations of food availability. The organic content of seston available during both neap and spring tides in the bay of Marennes-Oleron, France, decreased from 28 to 8% with ~ncreasing seston concentrat~on from 10 to about 90 mg total particulate mass 1-' Throughout this tidal variation, the blue mussel Mytilus edulis L. cleared more water of particles as seston availability increased. Rejection of filtered material as pseudofaeces prior to ingestion remained a constant fraction of about 0.93 X the mass of filtered material, so that ingestion rate showed no signs of stabilising at even the highest food availabihties. We confirm that M. edulis may preferentially reject inorganic matter with pseudofaeces. More significant was the novel observation that the net selection efficiency with which filtered organics were selectively retained for ingestion increased rapidly with the rate at whlch seston was filtered, this lncrease being faster for seston of higher organic content. The result was that the organic content of Ingested matter was enriched by up to 5 times the organic content of filtered particles. Further, net absorption efficiency for ingested organics varied in strong positive relation with the organic content of ingested material. Therefore, rates of organic absorption increased with seston filtration rate, and net energy balance increased despite the decreasing organic content of particles available at higher concentrations. These collective findings demonstrate continuous interrelated changes in feeding physiology that help to maintain rates of nutrient acquisition independent of short-term fluctuations in seston composition.

The Japanese oyster (Crassostrea gigas) has been introduced in France on a large scale to replace cultivation of the Portuguese oyster (Crassostrea angulata), affected by a viral disease. The importation took place from 1971 ta 1975, with broodstock coming from British Columbia (Canada) and spat from Japan. Good growth rate and success of the natural setting on the French Atlantic coast attested to the success ofthis implantation, with a production which oeached 80000 t by 1976. The precautionary measures associated with this transfer limited the implantation of an introduced fauna, 15 years after, to: Balanus amphitrite, Aiptasia pulchella and, on the Mediterranean coast, Undaria pinnatifida and Laminaria japonica. The authors also discuss the possible role of Crassostrea gigas in spreading the viral disease. L'huître japonaise (Crassostrea gigas) a été introduite massivement en France dans le but de subvenir au remplacement de l'élevage de l'huître portugaise (Crassostrea angulata) décimée par une maladie virale. Ces importations effectuées de 1971 à 1975 concernent des géniteurs provenant de Colombie Britannique (Canada) et du naissain en provenance du Japon. De bonnes performances de croissance et le succès du captage sur la côte atlantique française traduisent la réussite de l'implantation de cette espèce dont la production a atteint, dès 1976, 80000 tonnes. Les précautîons associées à cette importation ont permis de limiter l'implantation d'une faune associée dont les principales espèces, 15 ans après, sont: Balanus amphitrite, Aiptasia pulchella et, sur la côte méditerranéenne, Undaria pinnatifida et Laminaria japonica. Enfin est discuté le rôle possible de Crassostrea gigas dans la propagation de la maladie à virus.

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