Résumé
We developed a growth model for the oyster
Crassostrea gigas cultured in Thau Lagoon. The oyster standing stock in the lagoon ranged between 10,000 and 15,000 tons a year. Two culture methods are presently in use in Thau Lagoon which are used in about the same proportions. At seeding, initial size of oysters is different among methods. The model was calibrated on (1) growth data accounted for both culture methods and (2) hydrobiological data (temperature, salinity, suspended particulate matter and chlorophyll
a), both recorded in several sites in the lagoon between March 2000 and October 2001. The lagoon is slightly eutrophic: total chlorophyll
a and total particulate matter averaged 1.2 μg l
−1 and 2.2 mg l
−1, respectively. Organic content accounted for ca. 40–50% of particulate matter. There was no seasonal trend in seston, whereas temperature and salinity were minimal in winter. Oyster growth varied among sites in response to spatial variations in seston. Growth was maximal in summer and minimal in winter because of temperature seasonality. For each location, we modelled growth as a function of particulate organic matter and temperature. Chlorophyll
a was left out of the model because of a weaker fit with growth. Growth was modelled as
G=
aPOM
b
T
c
Y
d
, where
G is the growth rate in shell length (mm day
−1) or in mass (g day
−1), POM is particulate organic matter (mg l
−1),
T is temperature (°C) and
Y is either shell length (mm) or mass (total individual mass or dry flesh mass in g). Allometry (
Y
d
) allowed us to use the same model for both culture methods. The model yielded a good fit with actual size, either as measured by shell length (
R
2=0.96) or total individual mass (
R
2=0.93).