Scottish Government Policy (as stated in the draft Marine Plan) is to support, with due regard to the marine environment and carrying capacity, the industry’s target to grow marine finfish (including farmed Atlantic salmon) production sustainably to 210,000 tonnes sustainably by 2020.
There remain a number of significant concerns with regard to the sustainability of the salmon aquaculture industry in Scotland, including the release of chemicals used as sea lice treatments, benthic impacts due to organic enrichment from waste food and faeces and the sustainability of food sources. However, the most significant concerns for wild fish interests are the potential negative effects of sea lice and escapes.
- Sea lice: A number of reviews have considered the effects of interactions between salmon farms and wild salmonids (e.g. Revie et al 2009). In summarizing the information relating to impacts of sea lice from fish farms on Scottish sea trout and salmon, Marine Scotland Science (April 2013) stated: ‘Scientific evidence from Norway and Ireland indicates a detrimental effect of sea lice on sea trout and salmon populations. There is increasing scientific evidence that this is also the case for sea trout in Scotland although scientific studies investigating the case for Scottish salmon are currently lacking. Salmon aquaculture results in elevated numbers of sea lice in open water and hence is likely to have an adverse impact on populations of wild salmonids in some circumstances. The magnitude of any such impact in relation to overall mortality levels is not known. However, concerns that there may be a significant impact of aquaculture have been raised due to declines in catches of both salmon and sea trout on the Scottish West Coast’.
- Escapes: Farm escapees (from both cages in freshwater and marine cages) have the potential to interbreed with wild fish. Scottish salmon have unique genetic lineage specific to each river system (and sometimes unique to individual tributaries). Cross-bred offspring are less fit for survival at sea thereby putting the wild population at risk. In addition, escaped juvenile fish in freshwater may out-compete wild fish prior to migration and can develop to sexual maturity as parr, interbreeding with adult returning fish
The Norwegian Institute for Nature Research (NINA) has recently released a short documentary on Salmon lice on sea trout and Atlantic salmon which can be viewed below:
ASFB provides guidance to Boards/Trusts with regard to the aquaculture planning process, including the current Audit and Review Process set up to transfer aquaculture operations from Crown Estate development consents to planning consent under the Town and Country Planning Act. In addition, ASFB plays a central role in working with Government, Parliament, Local Authority Planners, Marine Scotland Science, SEPA and SNH in an attempt to ensure that the industry operates in a manner compatible with the needs of wild salmonids.
ASFB provides guidance to Boards/Trusts with regard to the aquaculture planning process. In addition, ASFB plays a central role in working with Government, Parliament, Local Authority Planners, Marine Scotland Science, SEPA and SNH in an attempt to ensure that the industry operates in a manner compatible with the needs of wild salmonids.
ASFB sit on the Ministerial Group for Sustainable Aquaculture and are involved in various working groups as part of that process, including the Interactions Working Group, which was established specifically to look at issues relating to interactions between wild fish and aquaculture. Although there remain a number of challenging issues to overcome, ASFB continues to engage in constructive dialogue with the industry and government in an attempt to make progress on these issues.
- ASFB-RAFTS Aquaculture Policy Paper (Feb 2012) (pdf)
- Ministerial Group for Sustainable Aquaculture
- RAFTS Managing Interactions Aquaculture Project
Following the completion of the licensed trial reintroduction of European beavers in Knapdale, Argyll, the Minister for Environment, Climate Change and Land Reform will in 2015 consider whether to allow a formal introduction programme to start. Fishery interests in Scotland hold concerns about the potential effect of beavers on migratory fish, principally due to construction of dams which could impede fish movement. In light of these concerns, the Beaver/Salmon Working Group was set up to consider the issues surrounding beavers and migratory fish.
ASFB is a member of the Group which is presently examining the potential impacts of beavers on salmon habitat and how these might be modelled to gain a better understanding. The Group, which is chaired by Professor Roger Wheater, comprises a range of experts on fish, beavers and general ecology and will report in January 2015.
In Tayside, a number of beavers have either escaped or been deliberately released into the wild, an offence under the Wildlife and Countryside Act. SNH estimate there are about 100 beavers living in the wild in the Tay catchment. Despite concern at the potential for an important and unwelcome precedent to be set, in March 2012 the then Environment Minister, Stewart Stevenson announced that these animals would not be removed and instead would be monitored until the end of the Knapdale beaver trial in 2015, when a decision will be made about the future re-introduction of beavers to Scotland as a whole.
- ASFB Beaver Introduction Policy Paper
- Tayside Beaver Study Group
- Scottish Beaver Trial
- Beaver FAQs: The Tweed Foundation (pdf)
Catch and Release
District Salmon Fishery Boards and Fishery Trusts are involved in a number of means of increasing salmon and sea trout populations including habitat maintenance and enhancement, mitigation of diffuse pollution, removal of in-stream barriers to migration, removal of invasive non-native species and stocking programmes. However, the simplest means for anglers and ghillies to make a positive and significant contribution to increasing fish populations is catch and release.
The proportion of the rod catch accounted for by catch and release has increased dramatically since 1994, when such information was first recorded in Scotland. Provisional data for the 2012 season demonstrates that 74% of the annual rod catch of salmon was released compared to less than 8% in 1994. This proportion of the rod catch accounted for by catch and release is the highest recorded to date. Crucially, less than 1% of rod caught spring salmon were released in 1994 while 91% were released in 2012. For sea trout the proportion of the rod catch accounted for by catch and release was 70% compared to 8.5% in 1994.
Numerous angling and radio-tracking studies have demonstrated high survival rates and successful spawning for salmon released after capture – up to 100% under certain conditions. However, the longer a fish is out of water, or poorly handled, the less chance it has of survival. Scotland’s wild fisheries organisations have combined with their counterparts south of the border to produce a new angler’s guide on Catch and Release for Salmon. The guide’s primary purpose is to provide practical advice and guidance to anglers to maximise the survival of salmon which anglers choose to release for conservation purposes.
More recently the Atlantic Salmon Federation have produced an excellent video demonstrating techniques of catch and release. This video can be viewed below (Copyright Atlantic Salmon Federation):
Climate change has been identified as a threat to Atlantic salmon. The species’ developmental rate is directly related to water temperature, and increasing temperature in freshwater may result in smolts developing more rapidly and entering the ocean at a suboptimal time in relation to their planktonic food sources.
In addition, as air temperatures warm, much of the snow that feeds the river systems is expected to melt earlier. This will lead to a reduction in the flow of many rivers in the spring and summer, which will increase water temperatures further and may reduce the overall optimal habitat available to the Atlantic salmon.
The effects of climate change on river ecosystems are now becoming apparent. River temperatures have increased over the last 20–30 years in upland Scotland by up to 1°C per decade in ways that reflect global trends in air temperature.
The first priority in mitigating these effects is to control atmospheric concentrations of greenhouse gases. However, with further climate change inevitable in the short to medium term, attention is now focusing on the development of accommodation and adaptation strategies, through which adverse effects on species or ecosystems can be minimized. Some of the key needs with respect to developing adaptation strategies for rivers and their biodiversity were summarised by Ormerod (2009 – Aquatic Conserv: Mar. Freshw. Ecosyst. 19: 609–613):
1. To quantify and predict the direct effects of climate change on species distribution, population processes, life cycles, community composition, species interactions and ecological processes.
2. To assess which river ecosystems are most vulnerable to climate change: for example alpine, montane, lowland or urban.
3. To minimize the adverse effects on river biodiversity of actions taken to mitigate climate change, for example large-scale hydropower development or land-use change for biofuels and carbon sequestration.
4. To control stressors that will exacerbate climate-change effects.
5. To buffer rivers against temperature gain. At least in climatic zones that have riparian trees as natural ecosystem features, the restoration of riparian zones appears to be an effective mechanism for offsetting the highest temperatures and creating thermal refuges. The benefits from effective riparian management are multiple; for example from energy flow, sediment control, nutrient retention and habitat enhancement (Larsen et al., 2009).
6. To increase resilience or resistance to climate change in rivers and their associated ecosystems. There are likely to be conservation benefits in restoring water retention in catchment soils; restoring functional linkages among catchments, riparian zones, floodplains and rivers; and making integrated land–water conservation a reality.
It is clear that survival of salmon and sea trout during their marine migration phase has fallen over the last 40 years. Some of this reduced survival can be explained by changes in sea surface temperature and subsequent contraction of feeding grounds. These issues can only be addressed by controlling atmospheric concentrations of greenhouse gases and as highlighted in point 4 above, a key strategy for managing adaptation of species sensitive to climate change is to minimise additional pressures such as those which are man- induced.
In the specific case of Atlantic salmon and sea trout, this involves concentrating on those factors which fishery managers and the Scottish Government can address. These include the improvement of degraded freshwater habitat, removal of barriers to migration, reduction in exploitation where necessary – including mixed stock fisheries, ensuring marine and terrestrial renewables are deployed in an environmentally sensitive and well-informed manner and addressing the negative effects of salmon farming.
In Scotland, individual DSFBs promote conservation policies appropriate to individual rivers. The provisional overall catch and release rate for salmon in 2013 was 80%, rising to 92% for spring salmon. In addition, ASFB is working with Marine Scotland in the development of a Climate Change Adaptation Plan for Scotland, working to minimise and mitigate the potential negative consequences of fish farming and marine renewables and seeking to reduce exploitation in mixed stock fisheries (where such action is necessary to conserve threatened stocks).
- Keeping-Rivers Cool: Getting ready for climate change by creating riparian shade. EA Guidance Manual (Aug 2012) (pdf)
- US National Fish, Wildlife and Plants Climate Adaptation Strategy
Diffuse pollution is the release of potential pollutants from a range of activities that individually may have no effect on the water environment, but at the scale of a catchment can have a significant impact (i.e. reduction in water quality, decrease in wildlife, etc.). Diffuse sources of pollution include run-off from roads, houses and commercial areas, run-off from farmland, and seepage into groundwater from developed landscapes of all kinds.
Diffuse pollution from land use activities has a significant impact on water quality. To achieve the objectives of the Water Framework Directive (WFD), the quality of Scotland’s water environment must be maintained and, where necessary, improved.
Rural diffuse pollution arises from land use activities such as livestock grazing, cultivation of land to grow crops and from forestry operations. Such activities can give rise to a release of potential pollutants the pressures and impacts of which include: eutrophication; loss of biodiversity; silting of fish spawning grounds; and impacts on human health through drinking water or bathing water pollution. The pollutants of concern include the nutrients nitrogen and phosphorus, sediment, pesticides, biodegradable substances, ammonia and micro-organisms.
ASFB is a member of the Diffuse Pollution Management Advisory Group, a partnership that focuses on improving Scotland’s water environment by reducing rural diffuse pollution. The main purpose of this advisory group is to: help create a robust governance, decision-making and coordination framework for the effective delivery of rural diffuse pollution RBMP actions in Scotland; and ensure input from a cross section of rural, environmental and biodiversity interests.
DPMAG have developed a two tiered strategy approach to reduce diffuse pollution in Scotland:
- a national campaign to prevent water bodies from deterioration in status and make improvement where they are not far from a status boundary. DPMAG has developed a national awareness campaign to support the national campaign;
- a targeted approach in catchments where the extent of diffuse pollution problem on the water environment requires a more focused approach. Fourteen priority catchments have been selected for the first RBMP cycle.
A major output from DPMAG is the rural diffuse pollution plan for Scotland which is designed to ensure that the key stakeholders in Scotland work in a coordinated way to reduce diffuse pollution from rural sources.
- Rural diffuse pollution plan for Scotland (SEPA) (pdf)
- Diffuse Pollution Management Advisory Group Website
- Guidance on Diffuse Pollution
DSFBs are statutory regulators and written consent must be obtained from the relevant Board prior to the artificial stocking of salmon and sea trout in the Board’s district. ASFB, in conjunction with RAFTS, have produced guidelines to assist Boards in this regulatory function; hatcheries and restocking projects can be effective management actions if carried out in the appropriate circumstances. Inappropriate stocking practice can be ineffective and costly at best, and at worst positively damaging to native stocks and sub-stocks of salmon. This is becoming more evident as our understanding deepens in relation to the genetic structures of salmon. The guidelines are designed to assist Boards in the decision making process.
- Stocking, Genetics, Broodstock Management (Marine Scotland Science Topic Sheet – 2010) (pdf)
- Genetics & run timing (Marine Scotland Science Topic Sheet – 2010) (pdf)
- Natural Breeding, healthier stocks (Marine Scotland Science Topic Sheet – 2009) (pdf)
- ASFB-RAFTS Salmon stocking policy paper (Aug 2008) (pdf)
- DSFB Stocking flowchart (Aug 2008) (pdf)
- Hatchery Work in Support of Salmon Fisheries (FRS Report – 2007) (pdf)
- Marine Scotland guidance on applications to fish in the close season or to use otherwise illegal methods of fishing
More information on GS can be found here.
The first large hydro-electric scheme in Scotland was built in the 1890s. The establishment of the North of Scotland Hydro-electric Board in 1943 led to a succession of new schemes in the following 30 years. Many of these major hydro schemes were built before the full ecological impact was understood and at a time when methods to resolve the problems were poorly developed. There is currently a renewed interest in hydro-electric power, arising from Government incentives to achieve a higher proportion of energy from renewable sources.
The effect of a hydro-electric scheme on fish depends on the site, the type of scheme, and the design, construction and operation of its various elements. There are four main types: run of river; storage;
pump-storage; and catchment transfer. The potential effects on fish are:
Barriers to migration and access to spawning grounds
Weirs and impounding dams may obstruct the passage of salmon or other fish unless an effective fish pass is provided. Temporary obstructions may also occur during the construction of a scheme (e.g. building of coffer dams or culverts on temporary roads) and the needs of fish must be considered.
Physical damage from turbines
Depending on the design of a turbine, fish passing through it may be killed or injured from changes in pressure or by being struck by turbine blades. It may therefore be necessary to provide screens at the intake to keep fish out. Screens may also be necessary at the downstream end of a tailrace to prevent fish entering it.
Fish may be adversely affected by pollution arising during the construction and operation of a scheme.
Transfer of water between catchments
As well as the hydrological effects of catchment transfer, fish can gain access to previously uninhabited watercourses via the tunnel used to transfer water. This could potentially lead to the transfer of water-borne infection. Salmon are thought to recognize chemical attractants in their home waters, so catchment transfer also has the potential to mislead fish up a ‘wrong’ tributary.
Other changes in hydrology
Hydro-electric schemes change the hydrology within the area of the development, and, in cases of catchment transfer, beyond it. These changes include:
- In run of river schemes there is usually a large reduction of flow in the river channel between the water intake and the discharge from the power station. This may also occur downstream from the impounding dam of a storage scheme. If a sufficient residual flow is not provided, the stocks of fish in these areas will be lost or significantly reduced and the passage of migratory salmon will be prevented. A required residual flow is usually called a ‘compensation flow’.
- If a stretch of river is dammed, the resulting reservoir may destroy fish spawning grounds or nursery areas. In storage schemes, depending on the pattern of generation, the flow downstream of the power station may fluctuate markedly and this could adversely affect fish or fisheries. The water level in the reservoir may also fluctuate, affecting fish stocks.
- Catchment transfers can result in substantial changes in flow in the affected catchments with consequential effects on fish or fisheries.
- There are other indirect effects, such as changes in sediment transport which can alter the character of a river bed, which in turn may affect fish stocks.
Generators of electricity are required to, “avoid, so far as possible, causing injury to fisheries or to the stocks of fish in any waters” (Schedule 9 of the Electricity Act 1989).
ASFB work at a strategic level, in order to influence the prioritisation of the CAR-license review process towards ensuring free passage of wild salmonids and ensuring that adequate compensation flows are provided to allow fish to migrate through the river. A key driver for this is the River Basin Management Planning process which prioritises such work between the 6-year planning cycles.
More information on invasive non-native species can be found here.
More information on the ‘check, clean, dry‘ campaign can be found here.
- A UK-wide Marine policy statement
- A national marine plan
- Regional marine plans
The marine planning system will set out the Scottish Ministers policies for the sustainable development of the Scottish Marine Area. The planning system will also contribute to marine conservation via the Scottish Government’s 3-pillar approach to marine nature conservation. It is therefore important to ensure that the plans take appropriate account of the protection of wild salmonids, especially with regard to aquaculture, marine renewables and mixed stock fisheries.
ASFB works with officials across Marine Scotland, and as a member of the Marine Strategy Forum to ensure that the final National Marine Plan is compatible with the needs of wild salmonids.
The Scottish Government has committed to meeting a stated target of the equivalent of 100% of Scotland’s electricity demand from renewable sources by 2020. Last year the Sustainable Development Commission undertook a comprehensive study of Tidal Power in the UK. The study found that tidal stream technology could provide 5% of the UK’s electricity – 58% of this potential is around the Pentland Firth in Scotland. However, there are significant uncertainties as to the environmental effects of marine renewable devices due to the large number of different, novel devices under development and currently undergoing consenting.
ASFB are members of the steering group overseeing the national strategy into strategic research for offshore marine renewables and anadromous fish – a process designed to develop and help deliver a credible and effective research strategy. It is clear however that the strategy will not be developed in time to inform several of the developments currently in the consenting process. It is vital that adequate resources are made available to this work, in order that these key questions can be answered, in a robust and timeous manner. This would allow migratory fish interests to approach the consenting process in the knowledge that a strategy is in place to address the legitimate concerns relating to possible negative interactions resulting from the construction and on-going operation of these developments.
The main concerns with regard to the effects of marine renewables on migratory salmonids are the effects of noise during construction and operation, the effects of electro-magnetic fields (EMFs) during operation and from associated cabling and potential of some forms of sub-sea devices to inflict physical damage on fish. The potential effects of these issues on migratory salmonids must be considered in the light of the considerable uncertainties regarding the behaviour and migratory routes of these fish. Post-consent monitoring is absolutely key, particularly in the light of the conclusion of the report into monitoring of the Robin Rigg wind farm that there is a 1 in 5 chance that the wind farm could be depressing Atlantic salmon abundance in potentially affected rivers by as much as 40% without being detected.
ASFB is working with Marine Scotland, Marine Scotland Science, SNH, The Crown Estate and the renewables industry to find the best way to fill these knowledge gaps in order that proper assessments of the potential impacts of proposed schemes on wild salmonids can be made.
- National Research and Monitoring Strategy for Diadromous Fish (NRMSD)
- Potential Influence of Robin Rigg Wind Farm on Atlantic Salmon (Marine Scotland Publication) (Feb 2013) (pdf)
- ASFB-RAFTS advice to DSFBs and Fishery Trusts on Marine Renewables Planning Process (Mar 2012) (pdf)
- Salmonid Migration Routes & Renewables (Marine Scotland Science) (pdf)
- SNH report on potential effects of electromagnetic fields and subsea noise from marine renewable energy developments on Atlantic salmon, sea trout and European eel (pdf)
Mixed Stock Fisheries (MSFs) are defined by the North Atlantic Salmon Conservation Organisation (NASCO) as fisheries that exploit a significant number of salmon from two or more river stocks. Whilst mixed stock fisheries are not inherently detrimental to stock status (any fishery, single stock or mixed stock, only poses a problem if it exploits stocks where there is no exploitable surplus) without knowing how many fish come from which rivers it is not possible to know the impact of the catch on individual catchments or to apply targeted conservation measures in the fisheries to protect individual catchments. Mixed stock fisheries are therefore considered by all interested northern hemisphere countries to present particular difficulties for management and species conservation.
Salmon fishing rights in Scotland (freshwater and marine) are private heritable titles that are registered separately from land. As such these titles can be bought and sold like any other property. Net fishings are not licensed in Scotland and the only limits to catch are through a statutory framework of national regulations with regard to: the meshes, materials and dimensions of nets used; the method of fishing to be used; and by weekly and annual close times for salmon fishing. Other than these restrictions on effort, the only other limiting factors are those of an economic nature to the commercial fishermen (i.e. the availability of fish and supply to, and demand of, the available market). Since the mid-19th century many netting rights have been purchased by rod fishery interests in order to increase the spawning escapement to the river system. Originally, these buyouts would have been viewed primarily as a means to expose more fish to the rod fishery. However, in recent years, a greater understanding of the salmon resource and its associated pressures (local, national and international) mean that such actions have been taken primarily in the interests of salmon conservation and river stocks as a whole. The rod fishery has also responded to declines in stocks through a range of measures – catch and release policies are now widespread throughout Scotland and in 2013 80% of salmon, 77% of sea trout and 92% of spring salmon caught in the rod fishery, were released back into the river.
The key issues relating to mixed stock fisheries relate to:
- the ability to assess the harvestable surplus available to salmon fisheries and allocating any surplus between rod fisheries and net fisheries in an equitable manner ;
- ensuring that all fisheries operate within the legal framework designed to ensure that exploitation is controlled and sustainable;
- the burden of paying for the management of fisheries is allocated fairly between rod and net fisheries.
The Wild Fisheries Review recognised these key issues. The Scottish Government is currently considering the recommendations and have recently committed to consulting on a conservation measure to ban the killing of wild salmon except under licence, along with an accompanying carcass tagging scheme to help enforcement. We continue to work with Marine Scotland in an attempt to develop a positive solution to these key issues and, in particular, exploitation of the fragile spring stock component.
- Report of the Scottish Mixed Stock Salmon Fisheries Working Group (2010) (pdf)
- Fisheries Management Focus Area Review Group 2009 (pdf)
- NASCO Fisheries Focus Area Report (2008) (pdf)
A variety of birds and animals can cause serious damage to fisheries. The law recognises this and permits licences to be granted to control piscivorous birds and seals. Licences to shoot seals are granted under the Marine (Scotland) Act 2010. Control of piscivorous birds is determined under the Wildlife and Countryside Act 1981.
Crucial to granting of a licence to control piscivorous birds is the gathering of evidence to demonstrate ‘serious damage’ with which to support licence applications. ASFB supports its member Boards in the collection of the relevant evidence. Experience shows that predation can be particularly high at certain times, for example during the seaward migration of smolts in Spring. By this stage, young salmon preparing for the marine environment have left their natal streams and are effectively ‘irreplaceable’. It is important therefore that predation ‘bottlenecks’ such as these can be dealt with through appropriate, evidence based licensing.
Scotland is now facing an unprecedented number of terrestrial windfarm development applications as the Scottish Government strives to meet renewables targets. By their very nature these developments will occur in freshwater catchments and some of them will have the potential to impact sensitive parts of river catchments for migratory and non-migratory fish species. Most wind farm developments are now large scale industrial projects often taking place in fragile upland landscapes and ecologies that require a full assessment of the risks associated with such projects. Not only do the turbine bases require a considerable amount of excavation but there also tends to be extensive road building and river crossings required to build and service wind farm developments. Risks associated with such projects include:
- Impacts on hydrology and habitat during construction due to large scale excavation in fragile habitats
- Extensive road building with accompany culverting, drainage, run off and sedimentation issues
- Possible long term impacts on the hydrology in the catchment
- Impacts on fish behaviour relating to turbine movement
- Impacts relating to transit of high voltage cabling
Large scale terrestrial wind farms have been built in important river catchments with little or no observable impact on either water quality, quantity or fish populations. Equally, there are have been examples of catastrophic failure of wind farm developments (Derry Bran – Republic of Ireland) and also significant water quality impacts during construction – particularly during periods of high run off. There is therefore potential for considerable long and short term damage to the freshwater environment. By deploying a proper risk assessment, appropriate construction methods and good contractor supervision, these risks can be reduced to manageable levels.
- ASFB-RAFTS advice to DSFBs and Fishery Trusts on Terrestrial Windfarm Planning Process (Mar 2012) (pdf)
- ASFB Wind Farm Policy (Apr 2009) (pdf)
Ulcerative Dermal Necrosis
In 2012 Ulcerative Dermal Necrosis (UDN) was diagnosed by Marine Scotland Science on one salmon in the River Spey. Monitoring was extended to other rivers with the intention of sampling fish with symptoms that are consistent with UDN, prior to the onset of colonisation by the freshwater fungus Saprolegnia (whose spores are always present), for further investigations. To date there is no evidence that it is widespread and instances may well prove to be isolated. It is possible that UDN is endemic in wild salmon populations but generally at such low levels that it attracts little attention.
There was no evidence in the late 1960s and early 1970s, when it was common in most rivers, that it impacted on salmon populations, although salmon are now much less abundant than they were then. The evidence from that period is that affected fish can recover and spawn successfully. Eggs from affected fish hatched normally, and there is no evidence that salmon fry or parr can be impacted.
UDN is not an officially notifiable disease and there is inconsistency in scientific literature regarding its status; although often called a “disease” this is disputed by many as no bacteria or virus or other causative agent for it has ever been conclusively identified. The alternative theory is that it starts as a skin “condition” initiated by some unknown environmental factor or factors in the sea which subsequently allows infection by other freshwater organisms. In the late 1960s, attempts to transmit UDN from sick to healthy fish gave no clear or consistent results.
“UDN” is therefore the description of the symptoms, not the name of a causative agent, and stands for Ulcerative Dermal Necrosis (= death of skin, with ulcers). Fish come in from the sea with ulcers (what triggers the condition in fish out at sea is unknown), typically on the head but also on other areas of their bodies that lack scales, and the bare flesh can then be attacked by the freshwater fungus Saprolegnia. In the past it was most prevalent in cold water temperatures and the ulcers can heal as the water warms up in summer, allowing complete recovery. However, if the fungus infection takes hold and extends the area of skin damage, the fish’s ability to control the movement of water and salts in and out of its body is lost, leading to the fish’s death. UDN is not the cause of death; the effects of secondary infestation by fungus and other organisms is responsible.
ASFB takes the recent diagnosis of UDN in salmonid fish very seriously and therefore we have helped initiate a project to investigate this issue further. ASFB have coordinated the funding of a PhD studentship at the University of Stirling investigating skin lesions in salmon, associated with recent entry into freshwater. This studentship is funded by a number of District Salmon Fishery Boards and the River Tweed Commission and a significant contribution from Marine Scotland. Sandra Schlittenhardt has been appointed to this role, co-supervised by Prof Randolph Richards (University of Stirling) and Dr David Bruno (Marine Scotland Science). It is hoped that the project will give us a far better understanding of the relationship between skin lesions and secondary infections and, ultimately, will allow informed management decisions to be taken in the event of an outbreak of disease.