Scottish wild salmon strategy

4. Pressures on wild Atlantic salmon

Salmon have complex habitat requirements that vary over their life cycle. Changes to the quantity and quality of habitat, feeding opportunities and predation pressures can have substantial impacts on salmon survival, growth and population status in rivers, estuaries, and the open ocean. A brief description of each of these pressures is provided below. In most cases pressures do not operate independently of one another but act in conjunction to negatively impact salmon survival and can be amplified by climate change effects.

Exploitation

Salmon suffer direct and indirect mortality through legal and illegal forms of fishing, including rod- and-line, coastal and in-river net fisheries. Voluntary catch and release measures, changes to annual close times to protect vulnerable spring stocks and, since 2016, statutory prohibitions on the killing of salmon in coastal waters and certain inland waters have reduced fisheries-related mortality in recent years. Mortality can also occur through catch and release fisheries, exacerbated by high temperatures.

Predation

Salmon are consumed by many species of predator. Those considered to present the greatest risk include other fish (e.g., trout, pike, eels), birds (e.g., cormorants, goosanders) and mammals, including seals. The effects of predation can be exacerbated in the presence of anthropogenic pressures including barriers and impoundments that alter habitats and disrupt salmon migration.

Disease and parasites

Salmon can be host to a wide range of pathogens and parasites that can affect growth and survival. Diseases can be bacterial (e.g., Furunculosis) and viral (e.g., infectious salmon anaemia). Red Vent Syndrome (RVS) caused by the parasite, Anisakis, has been highlighted as a cause for concern in recent years.

Sea lice

Sea lice are a naturally occurring parasite of wild fish that impair performance and can kill salmon smolts^[12] above threshold levels. Salmon farms can substantially elevate levels of sea lice in coastal habitats and potentially increase risks to wild salmon growth and mortality under some local conditions.

Genetic introgression

Escaped farmed Atlantic salmon can negatively impact wild Atlantic salmon through direct competition in fresh water. Breeding of escaped fish with wild Atlantic salmon can disrupt adaptive genetic selection with negative consequences for fitness and thus the viability of wild populations.

Invasive non-native species

Species introduced outside of their native range (e.g., North American signal crayfish, American mink, pink salmon) can have direct (e.g., predation, competitive exclusion) and indirect (e.g., habitat alteration) negative effects on Atlantic salmon populations. Non-native plants (e.g., giant hogweed, Japanese knotweed) may have impacts on salmon by their effect on river bank erosion.

Water quality

Salmon require clean, well oxygenated water to thrive. Point source (e.g., septic tanks or licensed discharges) and diffuse (e.g., acidification, eutrophication, sedimentation) pollution can cause direct mortality, or stress that affects subsequent growth and survival. Fine sediment can alter the suitability of habitats and suffocate eggs.

Water quantity

Salmon prefer specific water flow characteristics, including depth and velocity, that vary across life stages. Too little water can reduce the availability and suitability of river habitat, causing increased mortality. Too much water can affect feeding success or in extreme circumstances displace fish from habitats.

Thermal habitat

Salmon are a cold water adapted species that are highly sensitive to river temperature. Temperatures may be elevated broadly due to climate change and locally due to point source thermal effluents from industry and discharges from dams, which in some instances may alternatively have a cooling effect. During the warm summer of 2018, about 70% of Scotland’s rivers experienced temperatures that could cause stress in salmon.

Instream and riparian habitat

Riparian (river-side) habitat affects water quality, temperature, food availability and channel shape and structure. The loss of natural riparian woodland can increase temperatures and have other detrimental impacts, while excessive over-shading by commercial forestry can reduce instream salmon growth and numbers and exacerbate acidification.

The physical characteristics of rivers and their banks (riparian zone), including the shape of the river channel and the bed material (substratum), affect hydraulic conditions and the availability of shelter and refuges for salmon. Engineering activities, such as straightening, dredging and bank reinforcement, can negatively affect the quality and quantity of salmon habitat.

Obstacles to fish passage

Man-made barriers to migration, including dams, weirs, bridge foundations and culverts can completely prohibit the migrations necessary to complete the lifecycle of salmon. Where barriers are partial, they can impede salmon migration, deplete energy reserves of the fish, and increase the likelihood of predation and illegal exploitation.

Marine development

Activities in the marine and estuarine environments, including dredging and maintenance of harbours, have the potential to affect salmon through impacts on water quality and noise. Marine renewable developments also may affect salmon through noise, impacts on water quality, strike (in the case of turbines) and effects on local direct electromagnetic fields used by fish for migration.

High seas

Growth and survival of Atlantic salmon on the high seas may be influenced by predators, food availability, fisheries, and costs of metabolism. Climate change has elevated sea surface temperatures, influencing metabolic costs directly and potentially affecting growth and survival of Atlantic salmon indirectly through changes in the ecosystem and hence food availability and/or predation risk.

Other pressures

Potential pressures as diverse as numbers of terrestrial insects falling into streams and activities of inshore fisheries might have significant impacts on salmon growth and mortality, have probably changed over time but have not been assessed.

4.1 Assessing impacts of pressures

In some cases, the impact of a pressure at a given location is unequivocal, for example where a dam fully obstructs the passage of salmon. However, in many if not most cases, there is uncertainty regarding impact level. In some instances, it may be possible to quantify an impact within confidence bands through experimentation, but this is seldom the case, and results may not be transferable and generalisable. Mathematical modelling can be an important tool for predicting impacts and generalising results transported from specific local experimental assessments. Expert opinion has played an important role in combining evidence and judgement into frameworks for pragmatic management of pressures. Fisheries Management Scotland together with Scottish Government have produced a Pressures Tool that uses local expert opinion to collate impressions of the relative extents of and regional variations in the pressures affecting Atlantic salmon (for publication in 2022).

A key principle in managing pressures is to use the best available scientific evidence, whilst taking into account scientific uncertainty (through the precautionary principle). Adaptive management is an important approach for refinement whereby outcomes of management interventions are monitored and fed back to refine subsequent actions.