Effects of trophic and environmental conditions on the growth of Crassostrea gigas in culture

In order to study the possibility of exploiting protected marine areas, comparative data on the cultivation of the oyster Cassostrea gigas in the South Tyrrenian Sea are reported. The oysters were cultured at -7 and - 13 m on long lines linked to artificial reefs. The observations, made during a 12-month period, were of the chemical-physical and trophic properties of the water column and growth rates of the oysters. Temperature ranged between 19.81 ± 4.67°C at -7 m and 18.03 ± 3.03°C at - 13 m. Salinity showed typical Mediterranean values. The area presented oligotrophic features: the chlorophyll-a (CHLa) concentration ranged between 0.05 ± 0.01 and 0.04 ± 0.02 μg 1-1 at -7 and -13 m, respectively. The labile particulate organic matter (LPOM) ranged between 344 ± 201 and 334 ± 228 μg 1-1 at -7 and - 13 m, respectively, anti the CHLa carbon/POM carbon ratio (index of the autotrophic vs heterotrophic conditions) was never above 3%. POM concentration and POM gross energy content showed significant differences (P < 0.05) at the two depths, POM bulk being greater at -13 m. The oysters, sampled monthly, had an initial average size of 11.50 ± 2.78 mm (0.0036 ± 0.01 g dry weight) and had reached 47.50 ± 12.30 mm (0.13 ± 0.04 g dry weigh) at -7 m and 41 ± 11.43 mm (0.11 ± 0.04 g dry weight) at -13 m, after 12 months. The length-weight relationship showed the best allometric coefficient for the oysters at -13 m, although the growth trends did not show significant differences. Although a correlation between food quantity and quality and somatic and valvar production in situ was not demonstrated, it is probable that the greater POM bulk at - 13 m was the cause for the better growth trajectories of these specimens.