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5.12 Management of predators is also important as if seals attack farmed stock in cages there is a high risk of damage to the net. Sites seem to vary as to the degree that seals present a problem and farmers have three basic strategies: 1) acoustic deterrents are transducers placed in the water that are programmed to emit high powered sounds of a frequency that is unpleasant to the seals thus excluding them from the immediate area; 2) the use of a second net designed to keep seals from gaining access to the fish net (not regularly used for large cages); and 3) maintaining the fish net at high tension thus preventing seals from being able to bite through to the fish. Each of these measures is often supplemented with occasional shooting of "rogue" seals. Shooting is, however, relatively inefficient as it is often difficult for farmers to identify the particular rogue seal.
5.13 Although the above measures probably have no ecological impact on seal populations, which are thriving nationally, the use of acoustic deterrents has been questioned because of the potential problems caused for cetaceans. Cetaceans – dolphins, porpoises and whales – are much more sensitive to acoustic noise and a high pitched sound that might inconvenience a seal might cause pain to a cetacean. Thus it is likely that powerful acoustic deterrents exclude cetaceans from a large area. A Canadian study indicated that killer whales were excluded from a 10 km radius of such a device. This has obvious implications for exposed sites where sound transmission distances might be considerably greater than in enclosed sea lochs. Thus although effect seal management is crucial in maintaining the containment integrity of fish cages, acoustic deterrents have other environmental impacts diminishing their usefulness. At present there is insufficient information on coastal marine noise from other sources to easily quantify the degree of extra hazard to cetaceans.
Summary
5.14 Escapees from fish farms may interbreed with wild population resulting in losses of genetic variability, including loss of naturally selected adaptations, thus leading to reduced fitness and performance. Non-local genes have been introduced into wild salmonid populations for over a century, as a consequence of restocking programmes intended to increase population sizes. However, the effect of these programmes is probably insignificant compared with that caused by farm escapes simply owing to the large scale of escapes in comparison with the wild populations. Escapes from salmon farms, therefore, constitute a major threat to wild populations. Current methods to reduce fish farm escapes by reducing net damage from predators include the use of acoustic deterrents to exclude seals from the farm area. While these probably have no great consequence for seal populations they may exclude whales, dolphins and porpoises from a much larger area owing to their greater sensitivity to underwater acoustic noise.
Research Gaps
5.15 Continued surveillance of the presence of escaped fish in wild populations and quantification of the effects in terms of population fitness.
5.16 Improvements in marking or tagging fish to enable easy identification of escapees.
5.17 New methods for reducing fertility of farmed fish.
5.18 Improved containment technologies, including technologies for reducing the costs of operation of fully contained systems.
5.19 Assessment of the effects of seal scarers on cetaceans
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