The importance of reef habitats for fish, harbor porpoise and fisheries management

This project deployed cobble reefs in Flensburg Fjord (Sønderborg Bay) to study the reef effects in relation to fish abundance, benthic flora and fauna as well as harbour porpoise abundance. Extraction of boulders from the seabed has been prohibited in Denmark since 2010. In contrast, sand, stones and small rocks are still extracted from the seabed in dedicated marine areas. Extraction of relatively small stones and rocks (cobble) has the potential to influence marine life locally, but the topic has received limited attention in Denmark. This project examined the hypothesis that deployed reefs could provide substrate for macro algae and host various fish species as well as harbour porpoise. For example, it is well known that Atlantic herring is spawning near hard surfaces, where the eggs attach to rocky substrates or macro algae growing on the rocky substrates. Other fish species may accumulate in the area to feed on both the eggs and the adult herring. The herring and the predators may further attract harbor porpoises and thereby enable trophic interactions. Moreover, this project describes methodology to restore cobble reefs in Sønderborg Bay in Denmark. Collaborating closely with local stakeholders, and the organization Als Stenrev in particular, the project provides stepwise guidelines for 1) finding evidence of previous benthic extractions, 2) identifying sites for reef deployments, 3) getting permits for the reef deployments, and 4) collaborating with the contractor eventually deploying the reef. The study found no evidence of recent cobble extraction in the Sønderborg Bay. Reefs were successfully deployed at two sites in the bay. Likewise, the project identified marine sites with natural cobble reefs and sites without any reefs for comparisons across the timeframe of the project. Underwater cameras revealed that total fish abundance increased after reef deployment. For example, the abundance of Atlantic cod appeared to respond positively to the constructed reefs. Likewise, abundance of a number of prey fish species increased after reef deployments. As expected, the abundance of flatfish decreased after reef deployment. Baited underwater recordings revealed elevated abundances of herring in the natural reef sites, but spawning herring or herring eggs were not observed. Assessments of benthic flora and fauna on natural reef sites, and comparisons with sites without reefs, indicated that reef substrates favour high abundances of many invertebrates that are important food resources for fish. Specifically, the total epifauna abundance correlated positively with the seabed coverage of rocks (mainly cobble). Previous studies have revealed high abundance of harbour porpoise in Sønderborg Bay. The present project confirmed the presence of harbour porpoise in the area, but elevated abundance of harbour porpoise near the deployed reefs was not demonstrated, probably because of limited data availability and the short time frame of the project. The project indicated that distinct biological communities are associated with reefs consisting of cobble and similar small stones and rocks. The reefs provide foraging conditions that are favourable for many fish species. These benefits may diminish locally if cobble and similar small stones and rocks are extracted. Complete colonization of the deployed reefs takes several years, suggesting that comprehensive assessments of the reef effects require long term investigations. The project was carried out independently of Horizon 2020 and EU LIFE projects.