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REVIEW AND SYNTHESIS OF THE ENVIRONMENTAL IMPACTS OF AQUACULTURE
CHAPTER FOUR DISEASE IMPACTS ON WILD AND FARMED STOCKS
4.1 Cage farms may cause ecological effects stemming from the release of parasites and pathogens. It is, however, difficult to find diseased animals in the marine environment - such animals quickly succumb to predation. It is, therefore, not easy to be confident about the frequency or significance of transfer of pathogens to wild stocks.
4.2 One example involved a monogenean parasite of salmon Gyrodactylus salaris, which was transferred from resistant Baltic salmon populations to Norwegian populations lacking resistance as a result of movements of farmed fish stocks in the mid-1970s. This resulted in the extinction of many wild populations. Restrictions in the movement of live material between countries are enforced but this parasite still presents a significant potential threat to wild stocks in Scotland. Although aquaculture represents a possible method of transmission it is thought that inadvertent transfer by anglers represents a more significant risk.
4.3 Sea lice infestations are endemic in most salmonid culture areas and, in recent years, declines in wild salmonid populations have led to the widespread belief that there is a link between farming and this decline. In Scotland, the main focus has been on the marked population declines of wild sea trout Salmo trutta, particularly in the north-west where salmon culture is concentrated. On their first visit to sea in the spring of the year following hatching, sea trout may be confronted with very high concentrations of infective sea lice larval stages and quickly become infested with lice. Although these fish may choose to return to fresh water to avoid the parasite it is likely that many are severely compromised. A burden of only 10 adult lice is thought to be sufficient to cause mortality, especially in immature fish already under stress.
4.4 The position is less clear with wild Atlantic salmon Salmo salar, also in general decline. Smolts of this species migrate directly to the ocean without remaining in the coastal or estuarine zone, as is the case with sea trout. It was previously thought that wild salmon would not be exposed to the same degree of infestation owing to the limited period of contact. However, it is now suggested that, particularly in long sea loch systems with several fish farms, salmon may receive sufficient infestation to compromise their survival. This hypothesis is not easy to test, as it is difficult to catch salmon smolts in coastal waters, particularly in such a way as to protect the fish from skin/scale damage that may remove any early lice stages present. However, researchers in Norway have recently made significant progress in this area using a fishing net with an aquarium in the cod-end designed to minimise damage to the fish. The results from a co-operative research project between the Institute of Marine Research, Bergen, Norway and the University of Bergen indicate that more than 86% of the wild postsmolts of Atlantic salmon migrating out of the Sognefjord, and between 48.5% and 81.5% of the postsmolts from the Nordfjord were killed as a direct consequence of sea lice infections during the spring of 1999. The surviving fish were probably weakened because of the infection. Only two fjords were investigated at that time, but it seems probable that postsmolts from other fjords also experience the same problem and there is every likelihood that a similar situation may exist in some of the longer sea lochs in Scotland.
4.5 Although the relationship between sea lice infection and the decline of wild populations is striking, and is additional to the widespread decline of migratory salmonids in areas without fish farms, there is as yet no absolute proof of a causal link. In spite of this, and owing to the increasing body of supporting (although as yet inconclusive) evidence, the burden of opinion has recently begun to swing in favour of accepting the likelihood that lice from farms constitute a direct threat to wild salmonids.
4.6 Lice infestation has always been a significant economic and health problem for the industry and has tended to be tackled site by site, company by company. Most large salmon producing countries now recognise the value of an integrated approach to lice management. The main features of strategies to reduce lice numbers include:
- Regular monitoring of lice numbers
- Co-ordinated chemical treatments between farms sharing the same water body
- Single generation sites
- Fallowing of management areas to break lice cycles
- Treatment of lice in the spring when lice numbers are low
4.7 These features were adopted in Scotland in 1999 as part of the industry's National Sea Lice Strategy.
4.8 In the past, fish farmers had access to only a few treatment agents. A consequence of this limited group of medicines was reduced efficacy, caused by resistance. There are, however, several new lice treatment agents on the market that are proving more effective in reducing lice numbers on farmed fish.
4.9 Even with greater access to effective sea lice treatment agents it is uncertain that total lice numbers can be brought down to low enough levels to fully protect wild salmonids. This is a consequence of the continuously increasing numbers of fish entering culture: the numbers of farmed fish far exceeds the collective size of wild populations. Any decrease in lice numbers occurring through a lowering of acceptable lice levels on farmed fish is likely to be compensated for through future increases in production. Given that there will always be economic and environmental constraints on the frequency of therapeutic application, it would appear that if lice from salmon farming are a major contributor to declines in wild populations, we will have to await a much more radical solution e.g. a totally effective vaccine.
4.10 In Scotland, the farmer controls lice burdens and the data collected on lice burdens remains commercially sensitive and not generally available, except confidentially through Area Management Agreements (AMA). AMAs are aimed primarily at tackling sea lice and bring fish farming, wild fisheries and regulatory interests together. There are currently 7 such AMAs in Scotland.
4.11 It is likely that the burdens of lice acceptable to the farmer are higher than the levels probably required to minimise effects on wild fish. The situation in Norway is different in that lice levels are monitored by state veterinarians on a regular basis and, when lice levels rise, treatment is compulsory.
4.12 The use of non-chemical methods of lice control ( e.g. cleaner wrasse) remains widespread in Norway but is little used in Scotland. Recent commercial scale trials of wrasse use in Scotland as part of an integrated lice management programme have shown positive economic benefits with a concomitant decrease in the use of chemical treatments.
4.13 With the exception of sea lice, there appears to be little significant transfer of parasites between farmed Atlantic salmon and wild populations - in fact reverse transfer is more apparent. Little research has been reported on the parasitic interactions of other cultured species and wild populations.
4.14 The potential for bacterial and viral diseases to be transmitted from farmed fish to wild is real. Furunculosis (caused by the bacteria Aeromonas salmonicida) was believed to have been re-introduced to Norway via cultured-fish imports from Scotland in 1985 causing severe damage to both farmed and wild populations. Furunculosis is no longer a problem in fish farming owing to effective vaccination programmes.
4.15 During and since the major outbreak of infectious salmonid anaemia (ISA) in several Scottish fish farms in 1998-1999 there have been several claims of a threat to wild populations. The presence of ISA in wild populations was confirmed in Scotland (Scottish Executive Press Release, 04/11/99) but it is not clear whether this was a consequence of the outbreak in farmed stocks nor is it clear what impact the disease had on wild populations. Fisheries biologists have also expressed concerns about the possibility of Infectious Pancreatic Necrosis virus (IPN) transfer between farmed and wild stocks. IPN is widespread in some farming areas and it appears that it can be passed to wild stocks. However, very few samples have been analysed from wild populations and further monitoring is required to determine the degree to which transfer is occurring and whether it has significance for wild populations.
Summary
4.16 Wild salmon and sea trout are at risk from infective larval sea lice that may be associated with marine salmon farms. Salmon are most at risk in long fjordic systems where they have to pass several farms during their migration to sea. The transfer of other parasites from farmed to wild fish is not thought to be a major problem at present. The introduction of the parasite Gyrodactylus salaris from Scandinavia would probably devastate the Scottish wild salmonid population although it is not thought that transfers relating to farming represent the only or greatest risk of introduction. The potential exists for transfer of infectious diseases such as Infectious Salmonid Anaemia (ISA) and Infectious Pancreatic Necrosis (IPN) from farmed to wild stocks but the real level of risk is not quantifiable given present knowledge.
Research Gaps
4.17 Research to quantify the factors responsible for the transmission of lice between farms and wild fish. Improvements in understanding the mode and rate of transmission are essential in providing information on the relationships between infection of wild populations, lice burden on farms and separation distances between migratory fish routes and fish farms. This type of research would also bring greater understanding of the mechanisms by which farmed fish become infected with sea lice from wild populations and from other farms. This would help to determine the reasons why some sites have much fewer lice problems than others do and, therefore, assist in the selection of better sites for salmon culture.
4.18 Further work is required to determine the factors affecting the risk of transmission of a variety of fish diseases between farmed and wild populations.
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