Well, it is probably a reasonable bet that a lot of people already know the general answer to that question . . . However, with Atlantic Salmon reclassified from ‘least concern‘ to ‘endangered’ at the end of 2023, it is a multi-dimensional issue that deserves more attention than a simple ‘climate change and pollution’ statement.
Wild Atlantic Salmon – A powerhouse species
Note: In the following, ‘Atlantic Salmon’ refers only to the wild members of the species, not those raised in fish farms.
Atlantic Salmon are widely regarded as a species of great importance, a keystone in river and marine ecosystems. Keystone species are so named as they have an importance to the wider ecosystem which is probably disproportionate to the abundance of the species. The split lifecycle between freshwater and marine environments is part of what causes Salmon to have such a crucial role.
Atlantic Salmon play many roles within both the marine and freshwater ecosystems that contribute to both the structure and functioning of each ecosystem. Atlantic salmon begin life in freshwater before migrating to the sea for much of their adulthood, always returning to freshwater rivers for spawning. Due to this marine-freshwater traversing, Atlantic Salmon are a key transfer of nutrients from the ocean, deep into the upper catchments of rivers, a vital input which supports the wider freshwater community in these rivers.
As prey, they act as key food source for a variety of larger and smaller mammals, from Bears to Otters. Removing the Atlantic Salmon from the food web puts strain on the ability of the web to support predator and prey numbers. Increased predation on alternative prey by the many species that usually consume Salmon can then cause severe instability in prey and predators alike as the balance between them is thrown off. This can affect down to the lowest levels of producers as competitive relationships are changed, a knock-on effect to a change in grazing patterns precipitated by the instability in numbers of species at higher levels. In addition to their roles as both prey and predator, they also facilitate other ecosystem services such as providing a spawning ground for freshwater mussel larvae, who can only survive attached to the gills of juvenile Salmon.
Atlantic Salmon are also an ‘indicator species’; a species that can be used to signal the health of an ecosystem. Atlantic Salmon require clean and cold water to thrive, in both freshwater and marine environments. A decline in quality will lead to a noticeable decline in Salmon populations, which will also infer further changes across the affected ecosystem.
Atlantic Salmon and the UK – a snapshot
- Salmon are declining in number all over the world however, the UK is currently experiencing the most severe decline.
- A 2023 report found Atlantic Salmon reached the lowest numbers in England since records began in 1977.
- Of the principal salmon rivers in England, 90% fall into classification categories of ‘at risk’ or ‘probably at risk’, meaning that the number of Salmon is lower than the minimum abundance required to support sustainable populations in that river.
- The ICUN reclassified Atlantic Salmon from ‘least concern’ to ‘endangered’ following steep reductions in numbers 2010-2015 and even larger reductions projected (50-80%) in 2025.
- English chalk stream Salmon are classified separately owing to their unique chalk stream environment are assessed on a regional basis received a classification of ‘vulnerable’.
- Salmon in Scotland faired more positively, achieving ‘Least concern’.
Threats facing Wild Atlantic Salmon
Underpinning everything is the general decline in English river water quality which is linked to a host of different pollution sources:
Agricultural pollution: Affects nutrient concentration and can cause fish kills when the resulting drop in oxygen creates hypoxic and anoxic conditions.
Road run-off: Causes sedimentation of the riverbed, silting gravel beds which are key Salmon spawning grounds. It can also contain chemicals which cause harm to the aquatic environment, affecting salmon both directly and indirectly.
Destruction of riparian environment: Riparian vegetation has a key role in filtering ground and surface water before it reaches the same. It can both capture nutrients that cause eutrophication and stop sediment from entering the channel. Additionally, it provides several ecosystem services, including providing areas of shade, key in maintaining lower river temperatures and for the raising of ‘fry’ before they begin their journey to the sea. Removal of riparian buffers causes a loss of those benefits and can worsen their effect, if the replaced materials are impermeable leading to a faster and larger quantity of run-off, directly into the river. Best practice for riparian buffers is a 30m width, anything less and the effects are observable in river health.
Barriers to migration; Possibly one of the more well-publicised threats with some notable work going in to lessening this threat. Where weirs, dams and similar constructions exist, Salmon experience a block to their ability to swim upstream, preventing them from reaching their spawning ground. There are mitigation techniques including baffles, fish passes/ladders and even fish elevators that come in a variety of different constructions, meaning they can be adapted to most rivers.
Sewage other pollution sources: Salmon require cold, clean water to be able to successfully complete their life cycle and anything that interferes with that becomes a threat to their existence. In England, with both treated and untreated wastewater being released into our rivers and coastal marine waters, there will be a changeable level of pollution. During periods of prolonged pollution or where conditions are changing rapidly, leaving no time to adapt, Salmon lifecycle (and indeed the lifecycle of other aquatic life) can be affected. Siltation of spawning grounds, increase in water temperatures, decreases in available oxygen, bioavailability of heavy metals are just some of the issues caused by pollution entering the aquatic environment.
Fishing: The North Atlantic has seen a significant reduction in the species typically predated upon by juvenile Salmon, such as Blue Whiting and Sand Eels, which is causing juvenile Salmon to have reduced growth and reduces their chances of surviving to adulthood. Juvenile Salmon are not included in the international bycatch regulations and therefore are offered no mandated consideration in the altering of fishing tools to avoid bycatch.
Sea temperatures: Rising sea temperatures are a threat to many aspects of marine life and Salmon are no exception. Given time, species like Salmon can adapt to a changing in average temperature however it is accepted that currently, anthropogenic-induced temperature increases are too swift to allow marine life reasonable time to adapt their physiology.
Disease: This piece has so far only considered the survival and habitat of Wild Atlantic Salmon however fish farmed Salmon are also potentially contributing to the difficulties facing Wild Salmon. Open net-stye farms exist in coastal areas so the farm can utilise the natural currents to oxygenate the water, without any manmade infrastructure. These open-net farms have come under intense scrutiny as they allow uncontrolled exchange of fish-farm waste directly into marine water. The waste is often contaminated with viruses such as Tenacibaculum and Sea Lice. Fish that pass within close quarters of these nets have a far higher mortality rate than those with greater separation distance. As both marine and river dwelling fish, Salmon are particularly vulnerable to the effects of fish farms as disease is easily developed and spread throughout the farmed fish and transferred to the wild species that move past them.
Potential for Hope
Whilst not always a trending news topic, the decline in Wild Atlantic Salmon has not gone unnoticed. There are many organisations that are working towards stabilising the plight of the Wild Salmon and allowing their species to rebuild. Groups like The Missing Salmon Alliance and The Atlantic Salmon Trust actively work to provide evidence, strategy and actions to help protect the species. With the extensive list of threats to UK rivers, Salmon are a keystone of a far larger and more complex ecosystem, and the threat of Salmon extinction is a significant symptom of the general degradation of aquatic ecosystems both through direct anthropogenic actions and climate change.
However, the situation is not all bleak as individuals and organisations strive to improve conditions for the Salmon, both riverine and marine. In 2024, for the first time in 100 years Salmon returned to the spawning grounds in the upper course of the Derwent in Derbyshire, a landmark achievement. Schemes adopted by Natural Resources Wales (NRW) have provided easements and passes for fish on rivers where barriers to migration had been identified.
This is only a start and even with improvements to existing habitats to encourage spawning, migration and further the Salmon lifecycle, the battle is an uphill one. Warming temperatures, as well as a current and historic decline in numbers are just some of the significant challenges for the long-term recovery of Atlantic Salmon. Catchment-based approaches grounded in understanding and evidence can be extremely effective, but the marine portion of the lifecycle is beyond those controls. As with so many things, the wider picture is just as important as the close-up, and a trade-off between the two can only ever yield limited results. We need greater collaboration between organisations, governments, and a global conscience for working towards a sustainable planet before time runs out.