previously had resulted in jerking behaviour that created substantially more stress
on all the materials. Culture lines were knotted into the grid from a small rowing
boat, a procedure that needed smooth sea and calm weather and could only be
managed during the period of slack tide (maximum 30 min). 880 m of culture line
were transferred to the grid and later harvested. Finally, the “Offshore-Ring” was
the best design. So far, individual rings of 5 m diameter showed a superior performance in comparison to the other tested carrier constructions. They remained
stable and in place during all weather conditions, provided their moorings were
tended regularly, at least after storms, which imposed some wear on them. In
addition, they allowed equipment with culture lines to be performed onshore, the
rings subsequently being towed to their mooring locations and fastened relatively
quickly during slack tide. With the ring construction the harvesting period could be
prolonged by moving complete rings onshore. Moreover, sampling of the seaweed
culture was more easily done due to the possibility of heaving up the ring construction with a ship’s crane.
To conclude, our experience is that the major key conditions for offshore culture
were fulfilled such as the pre-cultivation of healthy plants that were well attached to
the culture lines. Another key factor, i.e., reduction of mechanical abrasion, was a
major problem on the longline system, because of high turbulences. Longline
systems are hence considered unsuitable for macroalgal culture under offshore
North Sea conditions. The ladder system was more apt to damage than the
improved grid system, e.g., at the fastening points of weights, and should therefore
also be rejected in future considerations. A further problem of all carrier constructions except the rings was the necessity to fix them at permanent offshore sites.
This led to the logistic and cost problems of efficient transfer of sporelings from the
laboratory (or hatchery facility) to the grow-out location as well as appropriate
tending of the carrier system under the prevailing rough weather conditions. Labour
requirements were also enormous. Every single culture line had to be fastened to the
carrier system from a small rowing boat, and this was only suitably done during
slack tide. Work was seriously impaired by the difficulty of getting ship time and
divers, while also waiting for calm seas and all of this at the 30 min of slack tide
and during working hours. The ring construction using its present dimensions has
proven stable in offshore conditions (Helgoland Farm, Helgoland Roads, and Roter
Sand). The new ring construction, with a central steel cable and central buoy,
reduced tractive power and tension in high velocity currents and when being moved
for sampling or harvest (Fig. 11.32a–d, h–l). The two crow’s feet with the metal
cuffs greatly prevented torsion of the ring when lifted. The depth of a ring could be
adjusted by insertion of steel ropes into its cavity and the buoyancy of the central
buoy could also be adjusted by changing its size. This way the ring could be kept at
an appropriate depth to avoid exposure to stressful surface turbulence and admit
sufficient light for algal photosynthesis even with increasing weight of algae.
A major advantage of the ring system compared to the other systems was that the
ring could be equipped onshore with 80–100 m of culture line and subsequently
towed to the mooring site, where it could easily be moored by the ship’s crew. The
reverse took place at harvest time and was also most advantageous. The ring
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B.H. Buck et al.
on all the materials. Culture lines were knotted into the grid from a small rowing
boat, a procedure that needed smooth sea and calm weather and could only be
managed during the period of slack tide (maximum 30 min). 880 m of culture line
were transferred to the grid and later harvested. Finally, the “Offshore-Ring” was
the best design. So far, individual rings of 5 m diameter showed a superior performance in comparison to the other tested carrier constructions. They remained
stable and in place during all weather conditions, provided their moorings were
tended regularly, at least after storms, which imposed some wear on them. In
addition, they allowed equipment with culture lines to be performed onshore, the
rings subsequently being towed to their mooring locations and fastened relatively
quickly during slack tide. With the ring construction the harvesting period could be
prolonged by moving complete rings onshore. Moreover, sampling of the seaweed
culture was more easily done due to the possibility of heaving up the ring construction with a ship’s crane.
To conclude, our experience is that the major key conditions for offshore culture
were fulfilled such as the pre-cultivation of healthy plants that were well attached to
the culture lines. Another key factor, i.e., reduction of mechanical abrasion, was a
major problem on the longline system, because of high turbulences. Longline
systems are hence considered unsuitable for macroalgal culture under offshore
North Sea conditions. The ladder system was more apt to damage than the
improved grid system, e.g., at the fastening points of weights, and should therefore
also be rejected in future considerations. A further problem of all carrier constructions except the rings was the necessity to fix them at permanent offshore sites.
This led to the logistic and cost problems of efficient transfer of sporelings from the
laboratory (or hatchery facility) to the grow-out location as well as appropriate
tending of the carrier system under the prevailing rough weather conditions. Labour
requirements were also enormous. Every single culture line had to be fastened to the
carrier system from a small rowing boat, and this was only suitably done during
slack tide. Work was seriously impaired by the difficulty of getting ship time and
divers, while also waiting for calm seas and all of this at the 30 min of slack tide
and during working hours. The ring construction using its present dimensions has
proven stable in offshore conditions (Helgoland Farm, Helgoland Roads, and Roter
Sand). The new ring construction, with a central steel cable and central buoy,
reduced tractive power and tension in high velocity currents and when being moved
for sampling or harvest (Fig. 11.32a–d, h–l). The two crow’s feet with the metal
cuffs greatly prevented torsion of the ring when lifted. The depth of a ring could be
adjusted by insertion of steel ropes into its cavity and the buoyancy of the central
buoy could also be adjusted by changing its size. This way the ring could be kept at
an appropriate depth to avoid exposure to stressful surface turbulence and admit
sufficient light for algal photosynthesis even with increasing weight of algae.
A major advantage of the ring system compared to the other systems was that the
ring could be equipped onshore with 80–100 m of culture line and subsequently
towed to the mooring site, where it could easily be moored by the ship’s crew. The
reverse took place at harvest time and was also most advantageous. The ring
310
B.H. Buck et al.