Experimental trials on the feasibility of offshore seed production of the mussel Mytilus edulis in the German Bight: installation, technical requirements and environmental conditions

Abstract

This study summarizes the activities and Wndings during a 2 year investigation on the grow-out of blue mussels (Mytilus edulis) and the technical requirements to withstand harsh weather conditions at an oVshore location. The experimental sites were two diVerent test areas, each 5 ha in size, 12-15 m in depth, in the vicinity of the oVshore lighthouse "Roter Sand" located 15-17 nautical miles northwest of the city of Bremerhaven (Germany). Two versions of submerged longline systems were deployed: a conventional polypropylene longline in 2002 as well as a steel hawser longline in 2003, both featuring diVerent versions of buoyancy modes. The spat collectors and grow-out ropes were suspended perpendicular from the horizontal longline for several months beginning in March of each respective year. The test sites were visited and sampled on a monthly basis using research vessels. Larval abundances in the surrounding water column reached numbers of up to 1,467 individuals m ¡3 . Postlarval settlement success varied through the entire experimental period, ranging from 29 to 796 individuals of spat per meter of collector. Settled mussels reached a shell length of up to 28 mm 6 months after settlement. Based on the growth rates observed for the seed, it is projected that mussels would reach market size (50 mm) in 12-15 months post settlement, and at the observed densities, each meter of collector rope could yield 10.9 kg of harvestable mussels. The polypropylene line resisted storm conditions with wind waves of up to 6.4 m and current velocities of 1.52 m s ¡1 and was retrieved in autumn of 2002. In contrast, the steel hawser-based line did not withstand the harsh weather conditions. The steel-based line consisted of six twisted strands that were untwisted by the strong currents and turbulences and consequently the individual strands were torn. Additionally, the line was accidentally cut by a yacht in July 2003. The biological study revealed that the tested location near "Roter Sand" has the potential to become an oVshore seed production site as well as being exploitable as a grow-out site for mussel production to market size. In light of the technical results, recommendations for mussel culture strategies using a polypropylene longline system are given.

The German Case Study: Pioneer Projects of Aquaculture-Wind Farm Multi-Uses

“…Results of these studies clearly elucidated that all three bivalve species grow successfully under exposed conditions in offshore environments (e.g. Buck 2007;Brenner et al 2012;Pogoda et al 2011Pogoda et al , 2013. As the "multi-use" of offshore wind farms for aquaculture installations will facilitate the further expansion of environmentally friendly and sustainable aquaculture, most test sites of the described cultivation experiments were conducted in North Sea areas of two offshore wind farms (OWF): "Butendiek" (in operation since 2015) and "Nordergründe" 1 (currently under construction in late 2016, Fig.…”

“…Larval abundance of mussels tended to decrease with increasing distance from shore (Walter et al 2002;Buck 2017). However, several offshore sites were identified, where larval concentration was sufficient enough to facilitate adequate natural seeding (Buck 2007) (Fig. 11.7b).…”

“…Spat collectors and grow-out ropes were suspended perpendicular from the horizontal longline for several months. These multi-use experiments have shown a massive settlement at offshore cultivation lines, making thinning essential (Buck 2007) (Fig. 11.8).…”

Offshore and Multi-Use Aquaculture with Extractive Species: Seaweeds and Bivalves

“…However, at some offshore test sites grow-out has to be economically calculated, as settlement might not be sufficiently dense. Mussels (Mytilus edulis) and oysters (Ostrea edulis) were successfully cultivated in and in the vicinity of the wind farms Nordergründe (17 nautical miles off Bremerhaven) and Butendieck (ODAS, 14 nautical miles off the Island of Sylt), as well as in North-South and West-East transects crossing all potential wind farms in the German Bight (Brenner et al 2007;Buck 2007Buck , 2017Buck et al 2006b). Health conditions with regard to infestations of macro-parasites, fitness and growth performance for both species were excellent (Pogoda et al 2011(Pogoda et al , 2012(Pogoda et al , 2013Brenner et al 2007Brenner et al , 2012Brenner et al , 2014.…”

“…Health conditions with regard to infestations of macro-parasites, fitness and growth performance for both species were excellent (Pogoda et al 2011(Pogoda et al , 2012(Pogoda et al , 2013Brenner et al 2007Brenner et al , 2012Brenner et al , 2014. Various technical solutions to connect submerged infrastructures to a windmill foundation or to deploy it centrally into a wind farm area were worked out (Buck 2007;Buck et al 2006b, see also Chap. 11 "German Case Study").…”