Increasing seaweed crop yields through organic fertilisation at the nursery stage

Extractive seaweed aquaculture is gaining attention in the western Baltic Sea and in particular the co-cultivation with other species for bioremediation or nutrient delivery. However, there are still limitations to viable seaweed production yields in a brackish habitat with a short production period for Saccharina latissima. This investigation presents the specific growth-enhancing effect of Mytilus edulis on the seaweed early nursery stages during the hatchery and during the grow out period at sea in a Baltic fjord. Gametogenesis and juvenile sporophyte development were evaluated with and without blue mussels during 9 weeks of seaweed hatchery. The presence of mussels resulted in a significantly higher abundance of large multicellular sporophytes. After the hatchery period, seedling lines were transferred into the field and installed both in the direct vicinity of and 25 m away from mussel culture ropes. The previously observed supporting effect of mussel co-culture on seaweed development during the hatchery period was still visible after 6 months at sea. Sporophytes were larger, had a higher biomass and had higher carbon content if previously combined with mussels in the hatchery. This investigation suggests that the co-cultivation of seaweed and mussels during seaweed hatchery can increase seaweed crop yields in the following grow out period at sea, with the possibility of being certified organic.