Within the framework of aquaculture research carried out at the AWI, we investigate the suitability of different aquaculture candidates. For an implementation of offshore aquaculture in the German Bight only domestic North Sea species will be used and are subject of our research. The focus is on extractive organisms such as macroalgae and molluscs, which do not need to be fed and increase bioremediation by reducing nutrients in the water body. Candidates are the blue mussel (Mytilus edulis), European oyster (Ostrea edulis) and Pacific oyster (Crassostrea gigas), which filter plankton and particulate organic material from the water column and use it as food. Other extractive candidates are the macroalgae sugar kelp (Saccharina latissima, ex Laminaria saccharina) from the group of the brown seaweed and the edible red seaweed dulse (Palmaria palmata). Macroalgae culture has a long tradition in Asian countries in aquaculture and the seaweed is used as high-quality food. This market in Europe is just developing, as vegetarian and vegan nutritional movements mean considerable potential for algae products. Furthermore, seaweed contains a variety of natural substances, biochemical compounds and molecular groups which are also used for medical-pharmaceutical research and provide an additional value for aquaculture production. From the group of intensive aquaculture candidates, organisms that are fed, three species of North Sea fish are represented in our research for their biological suitability for offshore aquaculture and integrated multi-trophic aquaculture (IMTA)systems: turbot (Scophthalmus maximus, ex Psetta maxima), sea bass (Dicentrarchus labrax) and cod (Gadus morhua). In order to investigate the suitability for offshore aquaculture of candidate species from the extractive and intensive group, parameters such as growth, physiological fitness, and overall biochemical state and condition indices are analyzed. Furthermore, e.g. filtration performance or assimilation rates in order to assess integration into IMTA systems.
The turbot, Scophthalmus maximus, is one of the most valuable food fish of the Northeast Atlantic. Since the 90s, intensive production methods for all life stages of this species are known. Rearing takes place in high stocking densities, particularly in land-based recirculation systems. The culture in surface cages or submersible cages was successfully demonstrated at moderate flow rates and a relatively stable temperature. Since the growth rate of turbot decreases below 14 °C and above 20 °C, the temperatures in the German Bight do not allow year-round on-growing. Moreover, this species reacts sensitively to strong tide flow rates of up to 1.2 ms-1, which plays an important role in site selection in the German Bight.
The decline in catches of cod, Gadus morhua, led to a growing interest in the breeding of this species. For more than thirty years, the intensive production of cod has been established. The rearing requires the production of live food in different sizes and feeding protocols must be strictly followed. A key problem for cod aquaculture is their ability to escape through the mesh or other materials. Cod is one of the species with the highest rate of the escapees, which can have a negative impact on the wild population by cross-breeding. The extreme temperature tolerance range of -1 ° C to 23 ° C and an optimum temperature range from 8 ° C to 12 ° C makes this species a suitable candidate for aquaculture in the German Bight. However, it should be noted that the relatively high summer temperatures in the German Bight increase the risk of infection. The suitability of cod for offshore aquaculture has been successfully demonstrated in submersible cages off the coast of New Hampshire. Cod is a physoclist species, and there stops have to be included during the vertical movements of the cage.
The European sea bass, Dicentrarchus labrax, is a well established species in aquaculture and with a production of about 60,000 tons per year one of Europe's most cultured marine fish species. On-growing takes place mainly in surface cages in coastal areas of the Mediterranean. However, a successful rearing was demonstrated also under more exposed conditions. Since sea bass is a physoclist species, in which the swim bladder is closed and pressure compensation is only possible by slow gas exchange, the rise and the lowering of the cage needs sufficient stops to prevent barotrauma of the fish. The sea bass can tolerate temperatures in the North Sea, but the average temperature of 10 ° C in the North Sea is below its optimum temperature range for growth of 22-28 ° C. Model calculations suggest, that this lower temperature prolongs the on-growing by 4-8 months compared to the production of D. labrax in cages in the Mediterranean.
Dulse, Palmaria palmata, is an edible red algae (Rhodophyta) from the upper subtidal of the rocky coasts of the Atlantic from Greenland to Portugal and the North Sea. It is traditionally eaten dried in France, Ireland, Iceland and Canada, and is a source of minerals, vitamins, antioxidants and iodine.
The red alga propagates through spores. In aquaculture asexual reproduction is widespread, with apical tips used for the cultivation of new thalli from the branched thallus. P. palmata can be cultivated in tank culture with air to circulate. In Ireland, the red algae is grown in the sea after sowing of spores on ropes or directly attached to the ropes.
Palmaria palmata is a good candidate for integrated multi-trophic aquaculture (IMTA). In the project Offshore Site Selection (OSS), dulse showed specific growth rates (SGR) of 5-> 6 % d-1, at 16 hours of daylight and constant radiation in co-culture with turbot. The red alga also has a relatively high protein content (up to 21 % of dry matter), which can be increased by growing in integrated aquaculture. P. palmata needs a location with flow velocities of 5-10 cm s-1 for optimal nutrient and CO2 exchange on the surface of the thalli, as well as for the reduction of growth of epiphyts. In strong currents, the thalli of macroalgae become slimmer, which influences the surface:volume ratio and hence affects nutrient uptake and growth. For offshore site selection for growing dulse, wave exposure and flow rates have to be considered in order to achieve optimal growth conditions and to reduce the loss of biomass.
Another limiting factor for the growth of P. palmata in the German Bight is the relatively high water temperature in summer. Dulse does not tolerate temperatures above 19-22 ° C, depending on the sub-population. At 16.5 °C, dulse has a reduced growth rate of only 2.2 % d-1. Optimal growth of P. palmata is achieved at temperatures between 6 ° and 12 °C. Dulse is usually harvested at a length of 30-40 cm.