CONTAINMENT SPECIAL
elements in respect of containment – par- ticularly in regard of manoeuvring boats and handling cranes to avoid net damage. Another area of concern in regard to net longevity is the use of net washers – both off-site and in situ. Nets are getting larger and there is an increased interest in moving away from the use of antifoulants, making the use of net wash- ers more likely. A number of farmers identified net ‘shrinkage’ as an issue – perhaps caused by the net fibres opening slightly and thus reducing the overall dimensions of the net – which has caused operational difficulties and perhaps gives an increased likelihood of net failure. From a containment perspective, it is of particular concern if it results in the net not being effectively supported at the cage collar, which may lead to greater strain on the net and stanchions. Farmers should ensure that only appropriately dimensioned nets are used. Net handling operations are worrying, since a net which snags on equipment under water can then be easily holed. As cranes are com- monly used to lift nets it is difficult for the oper- ator to know if a net has snagged compared to when manual lifting is used. Research should be carried out into the use of limiters or other mechanisms to help prevent damage to nets which have become snagged on cage compo- nents when being lifting by crane or hauler.
Net specification and strength testing
Whilst the CoGP includes a clause that speci- fies minimum net strengths, the authors found that nets of higher specifications were often being used in practice. The authors recom- mended that the minimum net strengths should be related to site exposure. The CoGP also requires net strength testing, although there was no standard protocol to guide the tests which should be addressed. Maximum mesh sizes relative to minimum fish weights should also be prescribed.
Mooring and cages
A
quaculture cages in Scotland include plastic circular cages, square plastic cages, square steel cages and square wooden cages. In the marine environment, plastic circles and square steels are by far the most common design used today, with only the occasional use of wooden or square plastic cages. In freshwater lochs, all types may be found.
Over recent years, there has been an increas- ing use of plastic circular cages. These were considered by fish farmers to be more appro- priate for more exposed sites than hinged steel cages because they can flex with the waves – although steel cages are eminently suitable for many sites. The general move towards
www.fishfarmer-magazine.com
SARF 054
circular cages, particularly for more exposed sites, was considered by farmers and equip- ment suppliers to have significantly reduced the potential for cage and mooring failure due to storm events.
Moorings
The specification of moorings, including an- chors, chains and ropes, have increased con- siderably over the years. Although the size of cages and other floating installations has also increased considerably over this time, farmers were of the opinion that mooring specifications had nevertheless increased in real terms. A mixture of reasons were given for this increase, including that farmers and suppliers have learnt from mooring incidents over the years, that suppliers have increased the integrity of their products over time, and that farmers have invested heavily in new cages and refurbished existing ones.
The other major change in moorings over the years has been the move to the use of a grid system, particularly for plastic circular cages. The grid is an inter-connected system of anchors, chains, rope and buoys to which the cages are moored, compared to the older approach where each cage was moored individually. One advantage provided by the mooring grid approach is that different elements of the system may be able to compensate should an individual component fail. In engineering terms, this is known as ‘redundancy’. With one exception, all the fish farmers said that they now use grid systems for plastic circular cages, a move which largely happened in the mid to late 1990s. The exception is a small independ- ent company which considered that individual cage moorings provided greater flexibility. Square cages (steel and plastic) are generally moored by attaching mooring legs at individu- al points around the cage group. Mooring legs will typically consist of an anchor, chain, warp and buoy. The other form of mooring used by farmers (for any cage type) is directly into rock using a large screw called a rock pin. This is used at freshwater and sea water sites in situ-
27
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100