Helena Filipsson

The Baltic Sea has a generally low biodiversity, and several species are declining due to high anthropogenic pressure. One example is the Baltic Sea cod (Gadus morhua, L.), the cod has decreased since the 1980s in both biomass and abundance. One potential solution to increase the cod population is to improve settling ground and provide shelter for juveniles in artificial reefs. The
Hanö Cod Reef project aims to increase the carrying capacity of cod in the southern Baltic Sea. The man-made structures could however benefit more than cod and an increase in settling ground for example epiphytic organisms could lead to a positive bottom-up effect on the ecosystem. In this study we aimed to investigate the colonization of epiphytic organisms on Hanö Bay cod reef over
time. In February 2023 14 reefs were installed and an additional 20 reefs in 2024. The reefs were documented using an underwater ROV. The video footage was analyzed and the epiphytic organisms were categorized into groups of: blue mussel (Mytilus edulis), Hydroids (life stage of Hydrozoa spp.), Barnacles (Amphibalanus improvises), Bryozoa spp., Rhodophyta spp., Chlorophyta spp. and “No organisms”. The result showed that blue mussels are the first to colonize the reefs while macroalgae (Rhodophyta spp. and Chlorophyta spp.) had the slowest colonization rate. Blue mussel, both as migrating older individuals and as settlement by new individuals increased the competition for space. Bryozoa spp. was not detected until March 2024 emphasizing the importance of time for high biodiversity to accumulate. These findings suggest that, beyond supporting cod populations, the artificial reefs in Hanö Bay provide valuable habitat for a variety of sessile organisms, which in turn may contribute to increased biodiversity and ecosystem complexity in the area. As colonization
progresses, the reefs could attract an even wider array of species, potentially creating new niches and enhancing the resilience of local marine life.