CONTAINMENT SPECIAL
net to help protect against abrasion and predator attack. More recently, the use of Dyneema®
has been
tested by the Norwegian research institute SINTEF.
There has also been interest in the use of plastic nets in recent years, with one company known to have trialled one. However, they noted difficulties in handling the nets and did not consider the use of this material further. Nevertheless, plastic nets are understood to be used to a greater extent outwith Scotland, al- though the report authors’ did not identify any specific instances of use by salmon farmers. There have been some interesting develop- ments regarding plastic nets which may make them more easy to use, including the use of in- dustrial zip fasteners to join net panels in-situ. The SARF report authors concluded that the results of research trials need to be collated and analysed and cost benefit assessments made before it could be expected that there would be widespread adoption of more inno- vative net materials.
Tensioning
Net tensioning is mainly used to help main- tain net volume in exposed sites. However, there is widespread agreement that net ten- sioning is important in deterring seal attacks and many farmers consider it their primary defence against predation, as it is harder for a seal to sufficiently deflect or grip a taut cage net to access fish. Historically, the usual method of tensioning nets has been by suspending a number of in- dividual weights from the net. However, many companies have turned to the use of sinker tubes as an alternative to individual weights on circular pens. The advantage of sinker tubes is that less weight is applied directly to the net, although some people say that cages with sinker tubes are harder to operate and require more overall floatation in the cages. Some farmers also question their effective- ness in sites with higher currents, suggesting that use in such environments puts increased strain on the net. However, others are very much of the opposite opinion and consider sinker tubes as the preferred choice in high tidal sites.
No company had any knowledge about the level of tension required to prevent preda- tor attacks or any guidance on the optimum weight to be used. Instead these issues were approached on an experimental and experi- ence basis, with some companies using much heavier weights than others, both for sinker tubes and individual weights.
The authors felt that the difference in opinion on the use of sinker tubes warranted further investigation to help farmers make informed decisions about how to optimise their net weighting systems. Similarly, the lack of infor- mation on the optimum level of tension and on the weight required to obtain that tension in different current regimes – for both individual weights and sinker tubes – may benefit from
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SARF 054
great-
er sharing of information and experience on an industry-wide basis. They recommend research into the ways that nets behave in different current regimes with differ- ent weighting systems, so that advice can be given to farmers on the optimum tension re- quired and how this may be achieved through the use of different types of equipment. While sinker tubes allow much of the cage net to be efficiently tensioned, they are limited in their ability to tension the conical base of most cage nets because the apex of the net hangs beneath the tube. As a result, farmers tension the base through the use of a single weight augmented by the weight of mortality collection equipment. Some successful preda- tor attacks on cage nets with sinker tubes were identified in the research – either suggesting that the use of sinker tubes may not always be sufficient defence against persistent predators or that they were not being used effectively. The research identified little or no use of net tensioning systems at freshwater sites and no knowledge about whether this would be a useful approach to deterring mink and otter attack.
Acoustic deterrent devices Farmers have used acoustic deterrent devices (ADDs) over many years as a means to deter seals at sea water sites, but many of the farm- ers consulted thought that seals may become used to them. However, a number of farmers were starting to experience more consistent re- sults with one product, resulting in greater up- take by Scottish fish farmers. If this brief period of successful ADD use continues, the authors considered that it may prove very beneficial to the industry, although a longer time period is required to allow full evaluation. Acoustic measures have also been used at freshwater sites – but tend to be based on devices aimed at the domestic pet market such as ultrasonic ‘cat deterrent’ devices – and there appeared to be little information about their effectiveness to help deter otter and mink activity.
PREDATORS IN SCOTLAND INCLUDE, FROM TOP: COMMON SEAL, HERON, OTTER AND MINK
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