5.1 Salmon escape from cage farms owing to accidents of weather or operation, through poor maintenance of nets and other equipment, through inappropriate specification of containment equipment for the exposure characteristics of the site, or through damage from seals. Although there have been improvements in containment technology and husbandry practice, the absolute number of escapes may remain high as a consequence of expansions in the industry.
5.2 Escapes from farms are obviously not desirable for the farmer as stock is lost and future insurance costs may be increased. There are also detrimental effects on the environment.
5.3 The genetic and ecological effects of escapes on wild populations are complex subjects and only the most important aspects will be summarized here. The fundamental problem arises because wild salmon and their farmed cousins have very different levels of genetic variability. Wild salmon have a high level of genetic diversity both within and between populations. Between population variability is driven by selection for the particular river (or part of a river) that they originate from and these differences are maintained by their accurate homing ability as they return from the sea to breed. Thus there are many distinct populations of salmon with a relatively low rate of mixing between them. Farmed salmon arise from relatively few wild strains and thus show lower overall variability. Although some breeding programmes seek to maintain genetic variability within populations by ensuring that large numbers of broodstock are used, this is not always the case so some reared strains are lacking in variability. Large numbers of broodstock are required to ensure that relatively rare genetic components are not lost from the population. For example, several thousands of broodstock are thought to be required to maintain the evolutionary viability of a wild population.
5.4 Breeding programmes for farmed fish exert very different pressures than natural selection does in the wild. Farmed fish are selected, intentionally or otherwise, for high growth rates and for the particular environment that exists in culture situations: high stocking densities, easy access to food, reduced stress during handling and isolation from predation. Reproductive success is generally unimportant, as is the ability to find food and avoid predators. These factors are, however, under intense selection pressure in the wild. Thus farmed fish are much less fit for survival in the wild than wild salmon. However, it is likely that if farmed fish escape early in their life cycle, those fish that survive to adulthood will have at least learned to catch prey and avoid predation but they may not be any more reproductively competent.
5.5 When farmed fish escape they can breed with wild fish. It is possible that the immediate offspring of such crosses may benefit from hybrid vigor but this is not passed on to the next generation owing to the phenomenon of outbreeding depression leading to much lower fitness and productivity.
5.6 It is quite easy to see that even where escaped fish are reproductively inferior i.e. less able to participate in breeding or having poorer quality or fewer gametes (eggs and sperm), large numbers of escapes may dwarf local wild populations which may only have relatively few breeding adults in any one year. Wild fish have some protection from such events owing to their life cycle: it takes a minimum of 2 years post-hatching for salmon to go through the freshwater phase, migrate to sea as smolts and return the following year as grilse but some fish will spend more than one year at sea thus spreading the progeny of a particular year's hatch out over several future years. Thus, it is possible for a wild population to recover after some catastrophe that affects the progeny of any one year and this factor undoubtedly contributes to the success of the species (although there appears to be a general decline in the proportion of fish who have spent more than one year at sea). However, continued escapes, if maintained over several years can have very serious effects on wild populations.
5.7 To put the problem in context, if 1% of the farmed population escapes each year then, for the west coast of Scotland only, that will amount to over 200,000 fish (in 2000), which vastly exceeds the total catch of the wild population. The total wild catch for the fish farming regions - North West, West, Clyde Coast and Outer Hebrides – was 8,459 salmon by all methods in 2000. This is probably in the region of 15% of the wild population and so it is easy to calculate that a 1% loss from aquaculture exceeds not only the catch from wild fish but also probably the total adult population in this region. The actual reported loss from escapes in 2000 for the whole of Scotland was 411,433 salmon, although more than half of this came from one incident in the Northern Isles.
5.8 It has been argued that the wild populations that might be affected by escapes from fish farming are themselves already affected by often inappropriate restocking and transplanting programmes that have been practiced by fishery managers and owners for many years. Where restocking or "stock improvement" programmes are based on only a few broodstock, even where the broodstock were taken from the local population, then serious reductions in effective population size can be introduced i.e. a loss in genetic diversity. Where strains for distant rivers have been used it is likely that the phenomenon of outbreeding depression will occur with reduced fitness especially in the second and subsequent generations. Thus, it is argued that some of the negative effects of escaped farmed salmon are already present as a consequence of some fisheries management programmes.
5.9 Although this argument is valid, it does not negate the need to prevent or minimise further escapes of farmed salmon. Given that stocking with salmon has released orders of magnitude fewer fish into the wild than farm escapes, escapes of the scale currently experienced will inevitably increase the degradation of genetic diversity already present, with potential losses of genes that are important for the fitness of populations in the wild.
5.10 Various options are available to minimise loses of escaped salmon. The most drastic is complete containment and this is the only option open where losses cannot be countenanced, for example, in experimental stations where transgenes have been introduced. Complete containment, i.e. culture in tanks with multiple safety measures on the effluent water, is currently rare except for the most juvenile life stages as a consequence of economics. Another option is to ensure that escaped fish cannot breed. This is done successfully with trout by inducing a chromosomal abnormality called triploidy. The females of these fish are essentially sterile and this is desirable for trout as it prevents early sexual maturation thus ensuring that resources are not wasted in producing unwanted gonadal tissue. Triploidy can also be induced in salmon and female triploid salmon are sterile. However, these fish show reduced performance and are generally unsuitable as a culture organism.
5.11 Improvements in containment of caged fish will likely continue as net technology develops but this may be compensated for by the probable increase of sites with a greater degree of exposure. While such sites are certainly of benefit for other reasons there must be rigorous precautions taken to minimise escapes e.g. by ensuring that net strength is over-specified and that cages and moorings are adequate for extreme weather conditions. With sufficient data, it is possible to make estimates of weather extremes likely to occur within a given time period, e.g. 50 years, and this could be used to derive containment specifications. Once the correct specifications are determined it is crucial that the appropriate inspection, preventative maintenance and replacement management regimes are implemented.