4.0 INTERACTIONS BETWEEN AQUACULTURE AND WILD SALMONIDS
4.1 The key issue identified by the SEA relates to the interactions between salmonid finfish aquaculture and wild salmonid populations. The purpose of this section is to provide background information on wild salmonids (Atlantic salmon and sea trout); their protected status; population trends and abundance; and their interactions with finfish aquaculture, focusing on sea-lice and escapes.
4.2 The two species of primary interest in relation to interactions with finfish aquaculture are Atlantic salmon ( Salmo salar) and sea trout ( Salmo trutta).
Atlantic Salmon 
4.3 Atlantic salmon is an anadromous species, i.e. fish migrate from the sea to spawn in fresh water. Atlantic salmon spawn in freshwater rivers and streams throughout Scotland, and it is in these rivers and streams that the young fish, first known as fry and later as parr, live and grow. After one or more years - commonly two or three years in Scottish rivers - the young fish undergo a range of morphological and physiological changes (at this stage they become known as smolts). It is as smolts that they move down rivers, generally in April and May, to reach the sea and begin their migration to the northern oceans where they feed for a period of one, two or sometimes three years. At the end of this period the salmon return to the Scottish coast and enter the rivers where they previously lived. Relatively little is known about the migration pathways of post-smolts or returning adults. Fish spending one year at sea and then returning are called "grilse". Those that remain at sea for two or three years before returning are called "multi-sea-winter salmon".
4.4 Salmon return to Scottish rivers throughout the year; the greatest number come back during summer or early summer, with good numbers coming back during the late autumn and spring. Fish returning to Scottish rivers between January and June are called "spring salmon". In late autumn or early winter they spawn, many of them doing so near to the places where they lived when they were parr. Most salmon (90-95%) die after spawning. Some survive to return to sea.
4.5 The eggs hatch in late spring, with the newly hatched fish (alevins) remaining in the gravels of the stream and riverbeds near the nest for several weeks, before leaving into the stream proper in May or June as fry. This behaviour of Atlantic salmon (returning to spawn in the river where they were born) has resulted in genetically distinct stock between rivers  .
Sea Trout 
4.6 Sea trout are the sea-running form of the brown trout ( Salmo trutta). Some sea trout may migrate to feed in salt water, but all spawn in fresh water in the late autumn. When they return from the sea seems to depend on their place of birth and how far up a river system they hatched. In east coast rivers, for example, sea trout heading for the upper reaches come in from the sea from April to June. They may spend most of the summer in the main river stems, waiting for high water conditions in autumn before moving upstream to spawn. Sea trout tend to live longer than salmon, and may make several annual spawning visits. This tendency is greater in highland than lowland rivers.
4.7 The Convention for the Conservation of Salmon in the North Atlantic Ocean (1982) seeks to promote the conservation, restoration, enhancement and rational management of salmon stocks. Atlantic salmon are listed in Appendix III of the Bern Convention (in fresh water only). The Habitats Directive  implements the Bern Convention; Atlantic salmon are listed in Annexes II (in fresh water only)  and V  of the Directive.
4.8 The UK population of Atlantic salmon is important in a European context. There are 17 Special Areas of Conservation ( SACs) for Atlantic salmon in Scotland ( Figure 2) and 33 in the UK as a whole. These SACs are in fresh water only; estuarine and marine sites are excluded.
4.9 Scottish Planning Policy (2010) includes a presumption against the development of marine finfish farms on the north and east coasts of Scotland to safeguard migratory fish species.
4.10 Both Atlantic salmon and sea trout are identified as priority species in the UK Biodiversity Action Plan, following the review of priority species and habitats in 2007.
Population Trends and Abundance 
4.11 The NASCO Salmon Rivers Database (2011)  shows that there are 398 rivers supporting salmon populations within Scotland. Of these:
- 341 are "not threatened with loss";
- fifteen river systems are "threatened with loss" (on Arran and in Western Scotland);
- twelve river systems are being restored (on the East Coast, Clyde Coast, north-west Scotland and Solway Firth);
- one river system is "maintained" (on the Clyde Coast); and
- nine river systems are "lost" (on Arran, Clyde Coast and north-west Scotland).
The status of a further 20 systems (on the Clyde Coast, West and north-west Scotland, Outer Hebrides and Moray Firth) is classified as "unknown".
4.12 Atlantic salmon population levels in Scotland are measured using fisheries effort (catch) data from rod and line fisheries and net fisheries. Data collection began in 1952. This collected data shows that annual catches have declined since the late 1960s ( Figure 3).
4.13 In terms of abundance, it appears that juvenile salmon production remains healthy for the majority of salmon rivers in Scotland, but that the survival of salmon at sea has declined since records began. In consequence, fewer salmon return to the Scottish coast. This decline in abundance appears to be more marked in early-running salmon ("spring salmon").
Figure 3. Annual reported catch of salmon (caught and retained) in Scotland, 1952 -2008, by method (source: Crawley 2010  )
4.14 Marine Scotland catch figures indicate a similar decline in sea trout population numbers since 1952 ( Figure 4). Taking the time series as a whole, analysis of the catch data suggests contrasting overall trends on the east and west coasts. In the absence of other evidence to the contrary, the lack of clear trend in the east coast rod catches may be taken to indicate no clear long term trend in the numbers of fish both entering fresh water and escaping to spawn. In contrast, sea trout catches in west coast fisheries have declined markedly over the same period and are currently among the lowest recorded in the time series.
4.15 The reasons for these declines in population levels are not clear. Potential reasons include poor marine survival, lower numbers of returning adults, changes to land use, water pollution, predation, non-native species, and the influence of aquaculture.  Salmon farming is only one of a range of factors which may impact on wild salmon and sea trout. At present it is not possible to determine either the absolute or relative influence of these different factors.
Figure 4. Annual reported catch of sea trout (caught and retained) in Scotland, 1952 -2008, by method (source: Crawley 2010  )
Wild Salmonid Interactions with Finfish Aquaculture
4.16 Salmonid finfish aquaculture is located on the west coast of Scotland, the Western Isles, Orkney and Shetland ( Figure 2). Concerns about the influence of salmonid finfish aquaculture on wild salmonid populations centre on the potential effects of sea-lice and escapes of farmed fish.
4.17 The Sea-Lice Working Group Report  prepared by the World Wildlife Fund ( WWF) refers collectively to sea-lice as numerous species of copepod crustaceans that are naturally occurring external parasites attaching to and feeding in the skin of marine and anadromous fish. Lepeophtheirus salmonis ( L. salmonis) is a specialist parasite of salmonids and is found on wild salmonids around the UK marine waters throughout the year, and at all stages of the parasite lifecycle  . This species is of concern in Scottish waters, where it is known to affect both wild and farmed salmonid populations.
4.18 Sea-lice affect salmon by attaching to and penetrating the epidermal tissues of the host fish. With time, and if in sufficient numbers, they can cause severe erosion of the fins and scales of the host. As skin damage and lesions develop, the underlying tissues are exposed and this can result in bacterial infections and disease.
4.19 Sea-lice occur naturally on wild salmonid species. Adult Atlantic salmon caught returning to the Scottish coast from the North Atlantic have been recorded with sea-lice burdens as high as 150 in number, while still reporting good physiological condition  . Adult wild salmon are able to counter infestation with sea-lice through actions such as jumping out of the water, or returning to freshwater environments (where sea-lice do not survive). These mechanisms are not available to farmed salmon.
4.20 Salmon farms have been shown to be a more important contributor than wild fish to the total lice in the marine environment , and there is a strong correlation between sea-lice levels on fish farms and in the local environment  , particularly at certain stages of the salmon-rearing cycle , .
4.21 Sea-lice are capable of travelling large distances. Studies suggest that the extent of lice larvae dispersal depends largely on prevailing winds, currents and local topography  . There is some evidence to suggest that larvae can disperse as far as 27 km from source under favourable conditions  .
4.22 The extent to which wild salmonid populations may be affected by sea-lice is not clear. The WWF report concluded that "the evidence is largely indirect or circumstantial that sea lice emanating from salmon farms can and do exert detrimental effects on wild salmonids … it is not plausible to draw a single over-riding conclusion regarding the potential negative impacts of sea lice on all wild fish stocks world-wide. Nevertheless, we believe that the weight of evidence is that sea lice of farm origin can present, in some locations and for some host species populations, a significant threat. Hence, a concerted precautionary approach both to sea lice control throughout the aquaculture industry and to the management of farm interactions with wild salmonids is expedient"  .
4.23 The Code of Good Practice recognises the potential for sea-lice from finfish aquaculture to affect wild salmonids and recommends measures to manage the potential for cross-infection of farmed fish populations, re-infection of farmed fish from lice populations in the vicinity of the cages, and infection of wild salmonids from dispersing sea-lice populations. Studies have also suggested that escaped farm salmonids may act as a conduit for lice transmission between fish farms and for wild salmonid populations  .
4.24 Escaped salmonids have the potential to interact with wild salmonid populations through inter-breeding, competition for resources and/or displacement, and transfer of parasites or diseases  .
4.25 Farmed salmon are genetically different from wild salmon, with farm juveniles typically growing faster and being more aggressive than wild fish, which can provide a competitive advantage during certain life stages. As a result, large numbers of escaped farmed salmon have the potential to impact significantly on wild salmon populations, through resource competition and competitive displacement at the juvenile stages at the local population level. Studies in coastal waters in Norway, for example, have shown that the invasion of escaped farmed salmon, particularly spawning females and juveniles, can result in significant reductions in smolt production through competition and competitive displacement  .
4.26 There is also potential for inter-breeding between escaped and wild fish, resulting in changes to the genetic integrity of wild salmon populations. This can pose a significant risk to the genetically distinct wild populations found in Scotland's rivers, through effects such as reduced local adaptation of wild stocks  .
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