Longline cultivation is worldwide being implemented, particularly within Mytilus spp. or Perna spp. cultivation (Fig. 1). One major problem concerning offshore mussel cultivation is the lack of appropriate sites for the mussel spat to attach. Longlines equipped with respective spat-collectors can provide suitable attachment structures for mussel larvae, but in a high-energy environment – such as the North Sea – they must also endure strong currents, water motions and wave actions. Compared to other structures (e.g. rafts), longlines are rather flexible and can be deployed at various depths to avoid at least some of the devastating (surface) conditions. Longlines can withstand relatively strong wind and wave action due to the flexibility of the system itself and can resist harsh winters. Furthermore, growth rates of mussels are reported to be better at longline systems. This is most probably due to the fact that longline mussels are fully exposed to the seawater and do only have a tiny spot of hart substrate on the collector. They are therefore flushed by phytoplankton-rich seawater at all times, which promotes mussel growth. Our studies on the cultivation of the blue mussel Mytilus edulis in the German Bight (15-17nm NW off the city of Bremerhaven) revealed that polypropylene-based longlines can be employed not only for spat collection but also for the grow out of juvenile mussels.
Movie Longline (Video Clip 4.6 MByte, AVI)
The general setup of a longline system is rather simple: horizontal lines are submerged, equipped with buoyant devices, a mooring system and collectors for spat collection. Particular design and dimension of the entire system primarily depend on the species cultured and specific site conditions. Our longline systems (Fig. 2) were 70 m in length and consisted of a 3-stranded polypropylene rope that was composed of 2 single 35 m ropes. The latter were linked together by shackles and heaving rings and submerged horizontally 5 m beneath the sea surface to avoid destructive water forces of the latter. Concrete blocks of 4 t each were used to anchor the longline at its ends. Each concrete block was equipped with a marker-line and a marker buoy of 300 litres. The marker lines had an additional service load of several tonnes for tautening the longline. In regular intervals, buoyancy devices and submerged spat collectors were fixed to the longline (see Fig.2). For buoyancy purposes, each 35 m segment was equipped with one 80 litres buoyancy barrel in the centre and with 115 litres barrels at the coupling to the anchor-line. Further, a few 35 litres ball-like surface marker floats were fixed to a 5 m rope, which was connected to the longline. Three additional steel barrels filled with concrete (200 kg) tightened the longline to the sea bottom at the coupling to the anchor-line and in the centre. The 3-stranded collectors (Galician Type) hold inserted horizontal pegs to prevent the attached mussels from being washed off by water motion. The collectors were hooked into the longline via shackles and submerged perpendicular to the longline by a weight of approximately 3 kg at their bottom end.
The submersed deployment of a longline as skeleton for suspended mussel cultivation eliminates the worst effects of surface waves and forces. As the polypropylene line showed to be quite robust and flexible, the system components are appropriate for application in a high-energy offshore environment and sustainable for a long-term use. Thus, by using polypropylene-based longlines instead of stiff and inflexible wire-based systems or submerged elongated fender type installations, material failures can be minimized. Furthermore, material costs can be reduced since polypropylene longlines are quite inexpensive.
A similar longline setup is currently being used in our Project "AquaLast" which is dedicated to examine the physical forces that are exerted by such a system on the grounding constructions of an offshore wind turbine.
Buck, B. H. (2007). Experimental trials on the feasibility of offshore seed production of the mussels Mytilus edulis in the German Bight: Installation, technical requirements and environmental conditions, Helgoland Marine Research 61: 87-101.