Offshore production of seafood is a promising approach to evade numerous specific problems related to aquaculture activities in coastal areas. The aim of this study was to investigate the biological performance of oysters, typical near-shore organisms and potent aquaculture candidates, in an offshore environment. Juveniles of two oyster species, Ostrea edulis and Crassostrea gigas, were transferred to an offshore cultivation site in the German North Sea and cultivated from April to October 2007. Samples were taken every six to eight weeks to determine biochemical and elemental compositions: total glycogen (TG), total protein, total lipid (TL) as well as lipid classes, fatty acids (FA) and the yields and ratios of carbon, hydrogen and nitrogen. Results show an increase in glycogen from spring to early summer for both species, which is related to high food abundance during spring phytoplankton bloom. During summer, glycogen storage revealed clear differences between O. edulis and C. gigas attributing to the reproductive activity of C. gigas but not O. edulis. TG contents ranged between 6% and 23% dry mass (DM) in O. edulis and between 5% and 16% DM in C. gigas. Total protein contents did not show significant seasonal variations and ranged between 34% and 41% DM in both species. TL contents increased during summer, although the increase was clearly more pronounced in C. gigas, followed by a decrease in both species in autumn. TL levels ranged between 7% and 14% DM in both species. Phospholipids and triacylglycerols were the main lipid classes in both oyster species, followed by sterols. FA compositions resembled those of near-shore-grown oysters. We conclude that offshore-cultivated oysters exhibit a natural biological performance, emphasizing their suitability as offshore aquaculture candidates. © 2013 Elsevier B.V.