Growth and seasonal variations of fatty acid composition of the Pacific oyster Crassostrea gigas from the Bizert lagoon, Tunisia

Growth and fatty acid composition of the gonad-digestive gland of the oyster Crassostrea gigas was studied from January 2002 to January 2003. Different measurements of oyster shell length and weight were discussed in relation to the environmental conditions, especially food availability. Separation between oysters in the two phases of the gametogenic cycle demonstrated that during sexual resting, the correlation between the various growth parameters is higher than those obtained during the gonadal activity, including ripeness and spawning. The total lipids and seasonal variations in the fatty acids of the gonad-visceral mass of the cupped oyster suggests a cycle of energy storage and utilization in the Bizert lagoon. Total fatty acid content (7.97+/-1.31-4.87+/-1.60\% of dry tissue) varied significantly during the year, reaching the highest value in autumn in coincidence with the increase of nutrient availability demonstrated by the elevation in Chlorophyll a level in November. However the lowest values of total fatty acids occurred in Summer which corresponded to the intense gametogenic activity and the decrease of the correlations observed. The 16:0, 18: 1 (n-7), 18:2(n-6), 18:3(n-3), 20:5(n-3) and 22:6(n-3) fatty acids predominated and presented maximal levels during winter and autumn (sexual resting stage) and contributed to the increasing shell length, width and thickness (2.04+/-0.18cm; 0.58+/-0.08cm and 0.90+/-0.06cm from January 02 to February 02 and 1.25+/-0.33cm; 0.65+/-0.11 cm and 0.28+/-0.07cm from November 02 to January 03, respectively. Crassostrea gigas was characterized by a relatively high n-3/n-6 PUFA ratio (2.45-3.15) which was particularly high in winter. Fluctuations in the fatty acid composition of oysters between seasons are dependent upon the gonadic stages, temperature and the dietary resources available in their immediate habitat.