Looking at the cage realities
HOW will marine aquaculture develop over the next few years? As the number of available sheltered sites dwindles and the need to compete with tourism and other sea users intensifies, alternatives must be studied, including landbased tank farming and offshore cages. I think we will see developments in both of these directions.
Offshores sites offer some advantages over more sheltered sites; a comparison of physical parameters is given in Table 1. Generally, offshore sites provide better conditions for the fish, although the effects of increased wave action on the stack and physical structure need to be taken into account. Also, offshore cages are more difficult to operate, requiring bigger boats and welltrained staff.
In Norway, fish farming has developed from home-made wooden cages in sheltered sites to steel or plastic cages in more exposed sites and third-generation cages used in exposed areas with waves up to 2m height. Lately, a fourth generation of cages has been produced for the open sea, which can tolerate waves of up to 4m in height.
How big should cages be? Experiments Helgeland Plast have done show that fish in 22m diameter cages give 20-30 per cent better growth than those in 16m cages, but they found no difference in growth between fish in 22m and 33m diameter cages. This suggests that 22m diameter cages are the optimal size.
Nets are the weak points in offshore cage systems. The floating structure or collar is always well-built, as much is known about the strength of the steel or other materials used in construction; a lot less is known about the design and strength of nets.
American company, N.E.T. Systems Inc’s Ocean Spar system has a vertical spar which supports the suspended net, allowing waves to pass through with little movement of the mesh.
Another new open sea structure, Arctic Farm, designed by Offshore Design A/S in Norway allows the net to be tightened at a depth of 5m, which helps prevent it from moving and hurting the fish. One such unit is in use in Norway. The cost of this system is about NOK 3.5 million (£307,000) for six cages measuring 18x18m.
Hydaq A/S have made and sold seven systems in Norway. These have been approved for use in seven metre waves and use chains and ropes to support a floating net kept suspended by the moorings.
One of the more well-known systems is the Aquasystem 104 barge which had its nets damaged in a storm last winter.
Helgeland Plast have developed and tested their offshore cages in waves with a significant wave height of up to 4m. These cages are made from three circular HDPE pipes.
Table 2 compares the cost per m3 of the various systems, assuming 15m deep nets where the depth is not determined by the design.
Blakstad (1988) investigated the economics of farming salmon in inshore and offshore cages. His results are summarised in Tables 3 and 4, and show that profits are slightly higher in offshore cages. This gives an incentive to salmon farmers who may soon have no other choice but to expand into offshore sites if they want to increase production.
Bjorn Myrseth, Marine Harvest A/S, makes an appraisal of some existing systems, their relative costs and ciencies” effi