Seasonal variation in weight and biochemical composition of the Pacific oyster, Crassostrea gigas in relation to the gametogenic cycle and environmental conditions of the Bizert lagoon, Tunisia Salwa Dridi a,b, ⁎ , Mohamed Salah Romdhane b , M'hamed Elcafsi a a Faculté des Sciences de Tunis/Département de Biologie, Laboratoire de Physiologie et Ecophysiologie des Organismes Aquatiques, Campus Universitaire, 1060 Tunis, Tunisia b Institut National Agronomique de Tunisie/, Département des Sciences de la, Production Animale et de la Pêche, 43 Av. Charles, Nicole, 1082 Tunis, Tunisia Received 4 May 2006; received in revised form 11 October 2006; accepted 26 October 2006 Abstract Seasonal variations in biochemical composition of the bivalve Crassostrea gigas were studied from February 2002 until January 2003 in the Bizert lagoon in relation to environmental conditions and reproductive events. Separate analyses were made of gonad-visceral mass. The gametogenic cycle comprised tow phases: gametogenesis including ripeness and spawning (March– September) and sexual repose during the rest of the year. The C. gigas in the Bizert lagoon showed a clear cycle of energy storage and utilization. Total biochemical composition was about 6.50% ± 3.01 of dry weight in glycogen, 14.34% ± 3.60 of dry weight in protein and 6.67% ± 1.34 of dry weight in lipids. Lipid and glycogen concentrations in the gonad-visceral mass were inversely related: maximum concentrations of glycogen occurred between December and February and corresponded with minimum concentrations of lipids. High glycogen contents recorded in winter supported the processes of gamatogenesis and then decreased progressively to attain low values at maturity and during summer corresponded to the first emission of gametes (June) and the beginning of total spawning, however lipids are accumulated in gonads during the period of maximum ripeness (spring) and decrease in summer. During autumn, the increase of total fatty acids values appeared to be related to the available food in association with an increase in the chlorophyll a concentrations. Protein contents show many variations throughout the year. High protein levels occurred in May and in July which corresponded to oocyte maturation and then decreased in late summer (beginning of total spawning). At the expense of the available food in autumn, protein concentrations increase again to assure recovery of the tissue weight of the bivalve, which induced an increase in the condition indices values. © 2006 Elsevier B.V. All rights reserved. Keywords: Crassostrea gigas; Gametogenic cycle; Condition indices; Glycogen; Lipid; Protein; The Bizert lagoon 1. Introduction In Tunisia, the cupped oyster Crassostrea gigas was farmed for the first time in the Bizert lagoon in 1972. Juvenile oysters importation was at first from Japan and then from France, (Medhioub, 1993). Oyster production Aquaculture 263 (2007) 238 – 248 www.elsevier.com/locate/aqua-online ⁎ Corresponding author. Faculté des Sciences de Tunis/ Département de Biologie, Laboratoire de Physiologie et Ecophysiologie des Organismes Aquatiques, Campus Universitaire, 1060 Tunis, Tunisia. Tel.: + 216 22558004; fax: +216 71885480. E-mail address: salwadridi@yahoo.fr (S. Dridi). 0044-8486/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.aquaculture.2006.10.028