Scotland's wild Atlantic salmon, once abundant, are now endangered. Populations are at their lowest levels ever, and urgent action is needed to safeguard them from fish diseases, parasites, and invasive species. Once introduced, disease, parasites, and invasive species become increasingly difficult to eradicate.
Here you’ll find essential information on biosecurity measures, fish disease reporting, and invasive species identification to help you take action.
Diseases
Impact on wild salmon: Saprolegnia is a water mould that causes skin infections in both salmon and trout. Light infections may look unsightly but are usually tolerated by healthy fish and can heal if conditions allow. Saprolegnia in general is a secondary opportunistic pathogen that will infect fish that have physical damage on the skin or are immunocompromised. However, some species, like Saprolegnia parasitica, are considered primary pathogens because they can cause systemic diseases leading to debilitation, lethargy and even mortality of badly infected fish. Rising water temperatures, a result of climate change, have been linked to increased frequency and severity of outbreaks.
Current Status in Scotland: Saprolegnia is ubiquitous in the environment and Outbreaks have been reported in several rivers across Scotland.
How to Identify:
- White, cream or pale brown cotton wool-like growths on the skin and fins of fish
- These infections often start as small round patches, but can increase in size to form larger plaques covering the body
- Fish with severe infection may appear lethargic or moribund.
- Skin damage or lesions, often near infected areas, particularly around the head and fins.
Impact on Wild Salmon: Red Skin Disease (RSD) is an emerging disease of wild Atlantic salmon in Scotland. Clinical signs of the disease include hemorrhaging on the underside of the body and in some cases the flanks. The infection can vary in severity but can lead to secondary infections such as saprolegnia. RSD primarily affects salmon during their migration back to freshwater to spawn, further diminishing the already fragile salmon population.
Current Status in Scotland: The incidence of RSD has increased in recent years. While the exact cause remains unclear, environmental stressors like rising water temperatures and pollution are being investigated as contributing factors.
How to Identify:
- Petechial (pinpoint spots on the skin caused by small broken blood vessels) or rash-like (general redness and irritation) haemorrhaging on the ventral surface and in some cases the flanks
- RSD may result in secondary infections, such as saprolegnia, causing lethargy or morbidity.
Impact on Wild Salmon:
Red Vent Syndrome (RVS) affects wild Atlantic salmon, causing inflamed, swollen, and bleeding vents. While the syndrome does not appear to significantly affect the overall health or reproductive abilities of the fish, it is a visual indication of an underlying issue, possibly linked to the nematode parasite Anisakis simplex. The full implications of RVS on salmon populations are still under investigation, but the presence of this condition adds additional stress to already struggling populations.
Current Status in Scotland:
The first reports of Red Vent Syndrome in Scotland came in early June 2007, with over 50 rivers in Scotland reporting cases by the end of the year. Similar observations were noted in rivers in England and Wales. The impact of the condition on fish is not fully known, however there is currently no evidence it has either prevented salmon from spawning successfully or led to mortality during their freshwater life/stage. Learn more from Marine Directorate’s topic sheet.
How to Identify:
- Inflamed, swollen, or bleeding vent area.
- Red or discolored tissue around the vent.
- Fish may otherwise appear healthy and capable of swimming, though the condition is noticeable upon close inspection.
Parasites
Impact on Wild Salmon: Gyrodactylus salaris (Gs) is a microscopic parasite that infects the skin and gills of juvenile salmon. Once introduced, it spreads quickly, causing mass mortality in affected populations, and is extremely difficult and costly to eradicate. Gyrodactylus salaris can be spread between and from rainbow trout farms through stocking practices.
Current Status in Scotland: GS has never been identified in Scotland health zone and we are currently disease free. However, Scotland remains vigilant in preventing the introduction of Gyrodactylus salaris. Biosecurity measures are in place, but the parasite poses a significant risk if it ever enters the country’s rivers.
How to Identify:
- Small, barely visible parasites on the skin and gills of juvenile salmon.
- Fish may appear stressed and weakened.
- Visible damage to the gills.
Impact on Wild Salmon:
Sea lice (Lepeophtheirus salmonis and Caligus elongatus) are parasitic copepods that primarily affect farmed and wild salmon, feeding on their skin, mucus, and blood. Infestations can lead to significant health problems, including open wounds, increased stress, and reduced growth rates. In severe cases, sea lice can cause mortality, especially in juvenile wild salmon (smolts), and contribute to the decline of wild salmon populations in affected areas. Fish that have recently entered freshwater from the sea ("fresh run fish") may carry parasites they picked up in the marine environment. However, these parasites are typically adapted to live in saltwater and cannot survive for long in freshwater conditions.
Current Status in Scotland:
Sea lice are a well-recognised challenge in both aquaculture and wild fisheries. In farmed salmon, regular monitoring and treatment protocols are in place to manage sea lice levels, but challenges remain in reducing the transfer of lice to wild populations. The Scottish Government continues to explore effective management strategies to limit the impact of sea lice on wild salmon through the introduction of a sea lice regulatory framework.
How to Identify:
- Visible small, pale parasites attached to the skin, especially around the head and dorsal fin.
- Wounds and ulcerations may be visible where sea lice are feeding.
- Wild salmon infested with sea lice may exhibit slower swimming and overall weakened health.
Invasive Non-Native Species
Impact on wild salmon: Pink salmon is an invasive non-native species which could compete with Atlantic salmon for habitat and food. There is also a risk that it could spread disease to native species.
Current Status in Scotland: Reports of pink salmon are increasing in Scottish rivers, and efforts are underway to monitor their spread and potential impact.
How to Identify:
- Large black oval spots on the tail.
- Dark mouth.
- In freshwater, male pink salmon develop a distinctive hump.
Impact on wild salmon: Signal crayfish, native to North America, threaten native fish species by preying on their eggs and young fish. Their burrowing activities also destabilise riverbanks.
Current Status in Scotland: Although their distribution is limited, signal crayfish continue to spread through illegal human transfer and natural migration.
How to Identify:
- Larger than native crayfish, often with bright red undersides on their claws.
- Burrowing activity along riverbanks.
- Presence of crayfish in previously uninhabited areas.
Impact on wild salmon: The American mink is a voracious predator of native fish, birds, and small mammals. Escaping from fur farms in the 20th century, they have spread throughout Scotland, causing significant ecological damage.
Current Status in Scotland: American mink populations continue to be monitored and controlled in an effort to protect native species, including wild salmon.
How to Identify:
- Medium-sized dark brown mammal with a sleek body and tail.
- Often seen near watercourses.
- Tracks or signs of predation near riverbanks.
Impact on wild salmon: Invasive plants such as Japanese knotweed and giant hogweed grow along watercourses, displacing native vegetation and contributing to habitat degradation.
Current Status in Scotland: These plants spread quickly, often downstream via seeds or root fragments. Coordinated removal efforts are required to control their spread across catchments.
How to Identify:
- Dense clusters of invasive plants along riverbanks.
- Large leaves or towering stalks typical of species like giant hogweed.
- Rapid growth in areas previously free of these plants.
What can you do?
Adopt Biosecurity Measures
Prevent the spread of diseases and invasive species by following these simple biosecurity steps:
Check: Inspect your equipment, clothing, and boats for debris, plants, and animals before moving between water bodies.
Clean: Wash off any material thoroughly, using hot water or disinfectant if possible.
Dry: Ensure everything is completely dry before reuse. Invasive species can survive in damp conditions for weeks.