The iconic wild Atlantic salmon is in decline across Scotland. Their unique lifestyle – spawning in rivers and migrating thousands of miles downstream, across the North Atlantic and back again – means they face a huge range of pressures during their life span (as illustrated in the figure below). This blog summarises some of the main reasons for their decline, discussed in more detail in SPICe briefing SB 19-48 on Wild Salmon.
Rivers
Young salmon spend 2-3 years in rivers before migrating downstream. Adult salmon also migrate upstream through rivers to their spawning grounds, swimming against the flow to get there.
Water temperature
Salmon have specific temperature requirements at different life stages. Temperatures which are too high can kill fish. Climate change, riparian (riverside) vegetation, river orientation and flow rate all affect water temperature. A 2-3°C increase has already been observed on the River Spey over the last century.
Water quality
Salmon are particularly sensitive to water quality and will be affected by pH, dissolved oxygen levels, nutrient pollutants, pesticides and suspended solids. Water quality is strongly determined by land use, with agricultural run-off a key cause of deterioration. Flow rate and acid rain also affect water quality. Overall, the water quality status of Scotland’s rivers has improved in the last couple of decades.
Water flow
Salmon have different flow requirements at different stages of their lives. Too high a flow rate can damage spawning habitats, transport fish downstream and prevent adult migration. Too low a flow rate can reduce oxygen and increase sediment deposition in spawning grounds. It can also reduce available habitat and food and prevent adult migration. Flow rate is affected by abstraction (removal), barriers, river engineering, land use and climate change, leading to more extremes in flow rate.
Predators
Salmon may be eaten by various birds, mammals or larger fish e.g. cormorants, goosanders and otters. Although fish-eating birds can take a large number of fish, there is not much evidence showing an impact on salmon at the population level.
Barriers
Adult salmon migrating upstream to their spawning grounds may be delayed or stopped at man-made or natural barriers, such as dams, weirs and waterfalls. This can lead to fish dying or abandoning their migration. Man-made obstacles are often associated with hydroelectric power generation and can be harder to pass than natural obstacles, due to their different design.
Food supply
Young salmon in rivers predominantly eat aquatic insect larvae. Abundance of this food source is affected by riparian vegetation, water quality, flow rate and water temperature.
Illegal poaching
Fish poaching is the most common wildlife crime in Scotland, with most poaching relating to salmon. New regulations in 2016 specify that all Scottish caught salmon must be tagged, to help prevent illegally caught fish reaching the market.
Invasive, non-native species
The introduction of non-native species may affect salmon through competition, predation or habitat alteration. These include the pink salmon, signal crayfish, American mink and Himalayan balsam.
Estuaries and the coast
At a certain age, young salmon migrate downstream to the sea, transferring from freshwater to marine habitats in estuaries and at the coast. Here they face new challenges as they make their way into the open ocean.
Salmon farming
Scotland produces a large amount of farmed salmon on the west coast and islands. Farmed salmon are kept in pens in freshwater lochs (as young fish) and then marine water (as adults). However, many fish escape each year with the risk that they may spawn with wild fish or transfer parasites – known as sea-lice – to wild salmon.
Predators
Salmon may be eaten by seals, birds and larger fish, as they pass through estuaries and along the coast. The impact of seal predation on salmon is not well understood, but anecdotal evidence suggests that this may affect the number of salmon caught by anglers. Research suggests the impact of seals will be greatest on smaller rivers in spring, when salmon numbers are lower.
Overfishing
Net fishing for salmon takes place in estuaries and at the coast. However, this has been in decline since the 1960s and it is now illegal to kill salmon in coastal waters; very few net fisheries – within estuaries – remain.
The open ocean
Salmon migrate thousands of miles across the North Atlantic as adults, reaching the coastal waters of Norway, Greenland and even North America. The full extent of these migrations is still unknown, making it difficult to fully understand the risks and pressures they face.
Ocean temperatures
North Atlantic sea surface temperatures have been increasing since the 1970s. Human caused climate change and natural climate cycles have been shown to correlate with salmon abundances.
Food supply
Salmon eat a range of food in the ocean, including plankton and small fish. Climate change has affected the distribution of these food sources and studies have found salmon abundances to be correlated with a decline in the range of various species.
Predation
Various fish, sea birds and marine mammals eat salmon in the ocean, including cod, tuna, sharks and whales. Understanding the effect of predation in the marine realm is difficult due to the rarity of salmon in the ocean. Climate change may increase their vulnerability to predation in certain areas.
Overfishing and bycatch
During migration, salmon may reach a range of mixed-stock fisheries in the coastal waters of other countries, such as Greenland. Since the 1970s, conservation measures, based on safe biological limits for exploitation, have led to substantial declines in salmon caught at these fisheries. Salmon may also be caught in fisheries targeting other species.
Marine energy
Development of marine renewables such as tidal and wind turbines may affect migrating salmon such as through influencing migratory patterns, noise disturbance, interactions with turbines and habitat loss.
The declining trend in wild salmon has been observed across the North Atlantic. This has prompted the North Atlantic Salmon Conservation Organisation, together with the North Pacific Anadromous Fish Commission, to launch the International Year of the Salmon 2019. This aims to raise awareness and stimulate research to help understand and protect the salmon. In response, Salmon and Trout Conservation and Sir David Attenborough have made a video about the ‘King of Fish’.
Kate Ashley, Research Intern, SPICe