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Summer mortality is an important economic concern for the Pacific oyster, Crassostrea gigas, industry all over the world and particularly in France. Outbreaks appear when oysters are in gonadal maturation. Bivalve defence mechanisms against pathogen invasion are assumed by circulating cells, the hemocytes. A field experiment was conducted in the Marennes-Oleron Basin (Charente-Maritime, France) in order to monitor hemocyte parameters, gametogenesis and mortality rates among C. gigas during 7 months from March to September 2002. Diploid (from natural bed and from hatchery) and triploid oysters were placed at 15 cm or 70 cm above sediment at Marennes-Oleron Basin. Triploid animals were included because they are considered as sterile animals. Mortality rates were monitored and lipid contents were analysed in order to define sexual maturation stages. Hemocyte parameters (cell mortality, enzymatic activities and phagocytosis) for the three oyster groups were monitored by flow cytometry. Results were analysed by pooling data for the whole sampling period and by separating data in three periods related to the gametogenesis process (A, B and C). Mortality outbreak occurred during the ripe gamete period (B). Results showed that triploids presented the highest values for several hemocyte parameters and the lowest mortality rates. Differences between triploids and diploids were highest during the ripe gamete period. Relationships between oyster mortality, gonadal maturation and hemocyte parameters are discussed. This is the first study monitoring hemocyte parameters of wild diploid, hatchery bred diploid and triploid oysters reared in the field.

This paper reports a comparative study of two cupped oysters, Crassostrea gigas and C. angulata, and their hybrids reared in the Marennes-Oléron Sound (France) under usual farming conditions. The relative proficiency of these species was characterized in terms of growth, survival and reproduction, three major concerns for oyster farming. Four populations of each taxon were sampled (C. gigas: Japan and France; C. angulata: Taiwan and Spain) and used as progenitors to produce six different progenies under common hatchery and nursery conditions (transfer of 5 to 6 g of seed oysters at sea and rearing in bags for 2 years). Mortality profiles differed greatly among the progenies. The C. angulata progeny of Taiwanese origin gave the poorest results, with 100% mortality during the second year. For the entire rearing period, the highest yield (+22%) was obtained with the C. gigas progeny of French origin. The sexual maturation index, lipid cycles and dry weight loss after spawning showed that spawning of C. angulata was delayed by about two weeks compared to that of C. gigas. A clear maternal effect was observed for the growth and reproductive characteristics of the hybrids. On the whole, C. gigas of French origin gave the best aquacultural results. However, severe episodes of C. gigas mortality have been observed in France in recent years, indicating that this species might not be entirely suitable for sustained oyster farming in this country.

We developed a growth model for Crassostrea gigas oyster shells based on the use of in situ temporal manganese markings to calibrate natural cathodoluminescence (CL) changes in the shell hinge sections. A 30 min to 4-h exposure period with Mn 2+ (90-120 mg l − 1) was sufficient to create a detectable mark in the shells. This makes the Mn 2+ markings the fastest mollusc shells marking technique to date. The natural CL from juvenile and adult shells cultured in four standard shellfishfarming locations along the English Channel and French Atlantic coasts, exhibited a seasonal pattern (maximum CL intensity occurring during summer periods, minimum CL intensity occurring during winter). Hydrobiological data recorded at Baie des Veys site allows us to attribute the seawater temperature as the main parameter controlling CL of shells. Chlorophyll a and seawater manganese concentration were not decisive in the luminescence intensity of the shells. A relationship between oyster hinge growth and the length of shells makes the umbo investigations a promising tool for oyster-farming and/or wild stock assessments. Shell growth varied at spatial and temporal scales (higher growth rates were observed during summer-autumn and lower during the winter period), depending on seawater temperature changes. Sub-monthly Mn 2+ markings support the fact that shell deposition can occur under temperatures below 6 °C, which has to be taken into account for both shellfish production and environmental monitoring derived from chemical compositions of the shells. Finally, our results point out the efficiency of age and shell growth rate determination by CL analysis in further shellfish ecosystem researches.

Laboratoire conchylicole de Poitou-Charentes, J/i-emer, BP 133, 17390 Ix Tremblade, France.Rgu le 4 novembre 1998 ; accept6 le 19 janvier 1999.Abstract -Summer mortality of the oyster in the Bay Marennes-01&m: Spatial variability of environment and biology using a geographical information system (GE). The summer mortality patterns was studied in 1996 on the Ronce-les-Bains oyster bank (175 ha), located on the southern part of the Bay Marennes-OlBron (Atlantic coastline). Studies on growth, sexual maturation, survival rates and environmental variables were compared at fifteen experimental sites (on-and off-bottom culture). After 7 months, cumulative mortality reached 8-19 and 23-33 % for off-bottom and on-bottom culture, respectively, which is significantly higher for the latter case. A seasonal mortality trend was noted, with a significant increase related to air temperature greater than 20 "C and with a significant pre-spawning glycogen catabolism. Based on soft tissue flesh growth results from both types of culture, several empirical models were developed to assess overall yield from a spatial point of view. A N-NW positive trend was noted for the soft tissue flesh, suggesting a decrease in food availability towards the southern part of the bay. Yields (total weight) ranged from 85 to 2 % for off-bottom cultures with 25 and 50 % emersion times, respectively. For on-bottom culture, several sites have shown a negative yield down to -10 % in the southern oyster bank sites. The highest yields were noted in areas located on the E-NE part of the Ronce oyster bank. 0 Ifremer/Cnrs/Inra/Ird/Cemagref/Elsevier, Paris Growth rates I mortality I Crassostrea gigs I Atlantic oceanR&urn& -Le bane ostr&coIe de Ronce (175 ha) au sud du bassin de Marennes-O&on (France) constitue un site atelier expCrimental pour I'Ctude des mortalit& estivales de I'huPtre creuse Crassostrea gigas. En 1997, la croissance, la maturite' sexuelle, la survie et I'environnement des e'levages B plat et sur tables sent compare's sur quinze sites expMmentaux. En 7 mois, le taux de mortalit des Clevages ;1 plat atteint 23-33 % contre 8-19 % pour les Clevages sur tables. La mortalit varie avec la &son. Elle s'intensifie avec la tempdrature, en p&iode de pr&-ponte et de catabolisme du glycog&ne. Des modbles statistiques de production de chair s&he de l'huitre sont Ctablis selon la latitude et la longitude des sites. Ces mod?.les (en poids set) sont orient& nord -nordouest et montrent que la disponibilitC alimentaire pour les cheptels d'huitres, diminue progressivement du large vers le fond de la baie. Les rendements biologiques (poids total), sont compris entre 8.5 et 2 %, respectivement pour des sites sur tables B 25 et 50 % d'exondation. Pour les tlevages ?I plat, une perte de production sup&ieure & IO % est enregistrde dans certains parts du sud du bassin. Les meilleurs sites, & 50 % de production (poids total), sont situ& dans la partie est -nord-est du bane de Ronce. 0 Ifremer/ Cnrs/lnra!Ird/Cemagref/Elsevier, Paris Croissance / mortalit I Crassostrea g...

Some of the particulate matter found in bottom sediment comes from material rejected as pseudofaeces or produces as faeces by Crassostrea gigas . The amount of pseudofaeces rejected is proportional to the amount of seston for total seston values up to 4 .6 mg 1 -' . For this values up to pseudofaeces production, the amount of faeces is mainly limitated by pseudofaeces production and reaches a constant mean value of 8 .9 mg 1 -' g DW -' when seston concentration increases .The proportion of faeces versus pseudofaeces is over 1 when seston concentrations are below 10 mg 1 -', and remains less than 1 when seston concentrations are higher than 10 mg l -' .The pseudofaeces show a lower organic content (13 ± 3 %) than the material trapped through the gills (27 ± 4%), as well as a lower percentage of proteins and lipids, suggesting a selection by the labial palps . On the contrary, an enrichment with labile carbohydrates is recorded in pseudofaeces, probably due to mucus agglutination of rejected particulate matter .The feeding of Crassostrea gigas can package and transform (agglutinate, change the organic content, change the biochemical composition) suspended matter, thus influencing both the quality and quantity of organic matter sedimentation .

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