Farming the High Seas: Biological performance of the offshore cultivated oysters Ostrea edulis and Crassostrea gigas in the North Sea.

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 stakeholder 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 friendly and sustainable aquaculture. The aim of this study was to examine the biological adequacy of two candidate species for such an offshore ostreiculture, the European flat oyster Ostrea edulis and the Pacific oyster Crassostrea gigas. Research focused on growth, condition and survival. Oyster spat of both species was transferred to different offshore sites in the North Sea in 2004 and 2007 and cultivated for six month during growing season. Samples were taken every six to eight weeks from April to October and growth rates, condition indices and survival rates were measured. To enable a deeper insight in oyster condition, elemental (carbon, nitrogen) and biochemical compositions (protein, glycogen and lipid levels) as well as lipid class and fatty acid compositions were investigated. Furthermore, the macroparasitic infestation of oysters at offshore cultivation sites and for comparison, at nearshore wild banks, was investigated. Low infestation rates or even a complete absence of such parasites would be a major advantage for shellfish production activities and would therefore play an important role in the list of site-selection criteria for a certain region. Results showed that both oyster species grow successfully in high-energy offshore environments. Mean growth rates were similar to those measured in individuals from coastal habitats (wild banks and cultures) and the condition index revealed normal seasonal variations in both species. Survival rates of both species were extremely high. However, at one single site a high mortality rate was observed for O. edulis at the end of the experiment. Differences in the increase of shell length and dry mass were observed between sites and size classes. Taking these results into account site selection criteria for different offshore locations are presented. Biochemical results showed an increase in glycogen from spring to early summer for both species, which is related to high food abundance during spring phytoplankton bloom. However, during summer, glycogen storage revealed clear differences between O. edulis and C. gigas: glycogen levels decreased substantially in C. gigas, eventually owing to reproductive activity only of the Pacific oyster. Total protein did not show significant seasonal variations, while lipids were accumulated during growing season, more pronounced in C. gigas, and decreased in autumn. Phospholipids and triacylglycerols were the main lipid classes in both species. Fatty acids resembled the compositions of nearshore grown oysters. This indicates that oysters exhibited a natural biological performance during offshore cultivation. Additionally, no macroparasites have been found in oysters from offshore cultivation sites, which provides a high-potential advantage for aquaculture activities in the open ocean. It can be summarized that oysters are successful candidates for offshore aquaculture operations, e.g. as a multi-use of offshore wind farming areas. As oysters are bioextractive organisms, oyster cultivation as an essential component of integrated multi-trophic aquaculture (IMTA) approaches in offshore areas is recommended in particular