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By moving away from coastal waters and hence reducing pressure on nearshore ecosystems, offshore aquaculture can be seen as a possible step towards the large-scale expansion of marine food production. Integrated multi-trophic aquaculture (IMTA) in nearshore water bodies has received increasing attention and could therefore play a role in the transfer of aquaculture operations to offshore areas. IMTA holds scope for multi-use of offshore areas and can bring environmental benefits from making use of waste products and transforming these into valuable co-products. Furthermore, they may act as alternative marine production systems and provide scope for alternative income options for coastal communities, e.g., by acting as nodes for farm operation and maintenance requirements. This paper summarizes the current state of knowledge on the implications of the exposed nature of offshore and open ocean sites on the biological, technological and socio-economic performance of IMTA. Of particular interest is improving knowledge about resource flows between integrated species in hydrodynamic challenging conditions that characterize offshore waters.

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Aquaculture production plays an increasingly important role to meet the global demand for aquatic products and expands continuously. Most mariculture organisms are produced in coastal areas, where space is scarce and user conflicts exist. For extensive cultures farming off the coast at offshore sites could be a solution to eliminate these problems and facilitate further expansion of environmentally sustainable aquaculture. The aim of this study was to examine the biological adequacy of two candidate species for ostreiculture, the Pacific oyster Crassostrea gigas and the European flat oyster Ostrea edulis. Growth rates, condition and mortality in offshore environments were investigated by transferring oyster spat of both species to 4 different sites in 2004 and 2007. Samples were taken every six to eight weeks from April to October and length, width, height and dry mass were measured as well as the Condition Index (CI = dry mass meat •100/dry mass shell) was calculated. Results show that both oyster species grow successfully in a high-energy environment. Mean growth rates are similar to those measured in individuals from coastal habitats (wild banks and cultures) and the CI shows seasonal variation in both species. The survival rate for both species was N 99% in 2007. However, in the previous trial in 2004 a high mortality rate was observed for O. edulis at one single site at the end of the experiment. Differences were observed in the increase of shell length and dry mass between sites and size classes. Taking these results into account site-selection criteria for different offshore locations are presented. We conclude that offshore cultivation of oysters will be successful if site-selection criteria are examined carefully when choosing a location for offshore aquaculture.

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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 offshorecultivated oysters exhibit a natural biological performance, emphasizing their suitability as offshore aquaculture candidates.