Atlantic Salmon undergo long-distance migrations between freshwater and marine habitats during which they are exposed to multiple threats which have led to a significant overall decline in the Atlantic Salmon population since the late 1990s (Forseth 2017). The causes of this decline are, however, complex and yet to be fully determined and understood and many declines cannot be fully explained (Crozier
et al. 2017). In its freshwater phase the species is strongly affected by hydropower development, and by water pollution and sedimentation, primarily caused by logging and agricultural activities (especially in spawning habitats) and in-stream river infrastructure. Reduction in water flow and increase in water temperature as a result of climate change is also of main concern in the southern parts of the species range. This has led to the degradation of juvenile and spawning habitat (MAFF and NAWAD 2000) and blocking of upstream and downstream migrations. This may also alter migration phenology, creating a temporal mismatch between fish ecological requirements and the environmental conditions. In the marine phase there is concern over declines in post-smolt marine survival rates. Several potential reasons have been proposed for this decline, including: i) fish farming resulting in transfer of disease to wild fish, and localised increases in sea lice infestations causing increased post-smolt mortality; ii) changes in sea temperature, ocean currents, and marine ecosystems, potentially affecting migration routes and feeding opportunities, and iii) industrial fishing, which can affect marine-phase salmon both indirectly due to over-exploitation of their food source (e.g. sand eel fisheries), or directly by post-smolts being netted intentionally or inadvertently as a by-catch, and; iv) changes in sea temperature and ocean currents (e.g. Forseth
et al. 2017; Vollset
et al. 2016, 2022; Czorlich
et al. 2022).
Pollution from industrial and domestic sources and agriculture remains a major threat to Atlantic Salmon during the freshwater phase of its lifecycle throughout much of its range. Acid rain caused by the combustion of fossil fuels in power plants, other industry and means of transport has resulted in acidification of watersheds and loss of many Atlantic Salmon populations. Anadromous salmonids may be affected through low pH in itself, or from aluminium released from the soil at low pH that attaches to fish gills and impacts osmoregulation when migrating to sea. Overstocking of livestock, and the extensive use of fertilisers has led to increased input of phosphate and nitrate leading to eutrophication in many rivers. Siltation of spawning gravels is also often an impact of activities such as forestry, agriculture, and mining. Arable cultivation in particular can lead to siltation of rivers and increased water turbidity. In intensive cattle farming regions, recurrent accidental overflow of silage pits generates often lethal levels of organic pollution. Agricultural practices can also alter flow regimes leading to changes in the size and composition of gravels beds essential for spawning and as habitat for juvenile salmon.
River infrastructure and engineering has had a major impact on salmon. The construction of dams, weirs and barriers for utilities, power generation, navigation weirs and flood control in the early part of the 20
th century throughout Europe has been a major cause for the decline of salmon through habitat fragmentation blocking access to spawning and feeding grounds in many cases. More than one million barriers fragment Europe’s rivers (Belletti
et al. 2020), of which an estimated 100,000 are obsolete such that their removal has the potential to benefit many salmon subpopulations (Gough
et al. 2018).
Salmon farming has increased dramatically over the last 20 years, posing a number of potentially serious impacts to wild Atlantic Salmon and to the marine environment (Cowan
et al. 2016). Potential impacts include: i) spread of disease and parasites from farmed fish to wild fish; ii) large-scale escapes of farmed fish, iii) release of chemicals employed to prevent or treat disease outbreaks in farmed fish, iv) accumulation of organic waste below fish farms, and; v) overharvesting of salmon food sources, such as sand eels, for production of salmon feed, noting that feed is becoming increasingly plant-based (Cottrell
et al. 2021). Given the intense ongoing debate on the potential impacts of salmon farming on wild fish, each aspect is dealt with below in more detail. Repeated invasions of escaped farmed fish into wild fish populations are reported to potentially reduce the fitness of the native populations leading toward local extinctions in extreme cases (Cowan
et al. 2016). Farmed salmon are usually the result of hybridisation of different stocks with selection favouring farm conditions. Large numbers of farmed salmon escape and - as they have no homing site - move to any river and hybridise with wild stocks. Usually life time survival rates are much less than in wild salmon and thus farm escapees drain the reproductive success and pollute the genetic and adaptive identity of wild populations. Genetic introgression of escaped farmed salmon represents an existential threat to the viability of many wild salmon populations. For instance, in Norway there were indications of genetic introgression from escaped farmed salmon in the wild population in two-thirds of the screened rivers (159 of 239 rivers), of which 68 populations were severely affected (29% of the screened populations) (Diserud
et al. 2020). Large scale releases have, also been cited as a potential threat to Baltic salmon genetic diversity (Palmé
et al. 2012). Wild salmon populations suffer a loss of local adaptation when they interbreed with farmed salmon owing to the introgression of maladapted genotypes and life history traits (McGinnity
et al. 2003, Bolstad
et al. 2017). Genetic introgression of farmed salmon imposes an extra impediment to the natural process of adaptation and may reduce the ability of Atlantic salmon to adapt to rapid environmental changes such as climate change (Thorstad
et al. Finally, there is also an impact on marine food webs given that several studies have showed overlap in wild and farmed fish diet (e.g. Jacobsen and Hansen 2001, but see Olsen and Skilbrei 2010).
The potential impact of sea lice (Lepeophtheirus salmonis) is also of great concern (Thorstad and Finstad 2018) as it indicates that, for wild salmon stocks experiencing poor marine survival, there could be a reduction in salmon returning to the river of up to 39% or more as a consequence of sea ice infestations (Cowan et al. 2016, Johnsen et al. 2020). Sea lice, an ectoparasitic copepod, can occur in large numbers on captive salmon and can infect wild smolts as they migrate past salmon farms in estuaries (Whelan 2010). The primary concern relating to sea lice originating from aquaculture is the impact on post-smolt salmon, which are susceptible to lice infections. For instance in Norway, salmon lice is regarded as the biggest threat to Atlantic Salmon populations, together with escaped farmed salmon (Forseth et al. 2017).
Overfeeding in salmon farms, fish excretion, and the use of chemicals and/or antibiotics have been reported to result in localised pollution impacts in the marine environment, however evidence of impacts on wild salmon is still required.
Another parasite of concern is the monogenetic trematode Gyrodactylus salaris, which is native in Baltic salmon, but introduced to Norway where it has caused mass mortalities in wild fish (Johnsen and Jensen 1991). Gyrondactylus salaris is a freshwater ectoparasite on parr; it is spread by salmon parr (or trout) used for stocking. This threat has been greatly reduced, because successful eradication programs have strongly reduced the number of rivers infected with the parasite, and the Atlantic Salmon stocks have been re-established from live gene banks. The number of rivers with known occurrence of the parasite has been reduced from 51 to eight, due to the eradication measures, and eradications measures are planned for the eight remaining rivers.
Overfishing at sea, and in particular with drift nets, has been a major threat to the species, and fisheries controls have been implemented. The species is also impacted as bycatch in the mackerel fishery in the North Norwegian Sea but the level of the bycatch is to be determined (ICES 2005). During the period 1983-2016 there has been a marked reduction in exploitation which peaked in 1973 with a harvest of some 3.5 million salmon (NASCO 2019). Regulatory measures agreed by NASCO have greatly reduced the interception of salmon in the distant-water fisheries at West Greenland and around the Faroe Islands. These fisheries now only account for a small proportion of the total catch. Despite the reduction in fishing pressures one hypothesis for the decline in marine survival is the potential for “fishery induced evolution” to influence life history characteristics, such as maturation timing, which in turn may impact mortality at sea (Crozier et al. 2017).
Invasive species may also threaten Atlantic Salmon. A new invasive species raising concerns is the Pacific Pink Salmon (Oncorhynchus gorbuscha). These fish, originally introduced to some Russian rivers from the 1950s, have spread westwards to rivers particularly in Northern Norway (Sandlund et al. 2019, Pauli et al. 2022). Since 2017, they have occurred in large numbers in rivers in the Northeast Atlantic Ocean and Barents Sea, and are found in rivers in a number of European countries. They spawn at a different time from Atlantic Salmon, and have a two-year life-cycle. The potential impacts are not clear at this time, so the precautionary approach is being recommended with removal of the species where encountered. An increasing abundance of reproducing pink salmon will most likely present hazards to biodiversity and river ecosystems (Hindar et al.2020).
Climate change is expected to impact biotic and abiotic factors that affect salmon survival at both a regional and oceanic scale, but the relative impact of, and the interaction between these factors remain poorly understood (ICES 2017). Even if there is a total cessation of emissions the environment will be exposed to continued climate change for at least 50 years, with continued effects in the world’s oceans lasting many centuries. Atlantic Salmon populations will have to cope with these changes if they are to persist. These potential impacts have been reviewed (ICES 2017, see also Graham and Harrod 2009) and summarised as follows: “It appears that Global Climate Change, resulting in rising sea, stream and air temperatures, is a major driver acting on a wide range of factors influencing the Atlantic Salmon’s life cycle. In freshwater, this can cause lethal increases in temperature for ova, juveniles, or adults in streams with few thermal refugia, perhaps ultimately resulting in losses of individual populations. However, in areas where lethal temperatures are not exceeded, growth and ultimately smolt production and adult population size could increase. But faster growth and higher stream temperatures could also result in earlier migrating smolts, which could reduce marine survival as there can be a mismatch in migration timing and optimum food availability. Globally rising stream temperatures could also facilitate population expansion into habitats that until recently were below the minimal temperature requirements for salmon (e.g. Bilous and Dunmall 2020). In the freshwater habitat rising stream temperatures can also be a factor in the expansion of the range or population size of invasive species that negatively impact on Atlantic Salmon stocks. This can include the deliberate or unintentional introduction of non-native strains of salmon. In the marine environment, the effects of rising water temperatures are, just like in freshwater, not uniform across subpopulations. This is probably a result of the extremely complex interactions between multitudes of factors at sea and the interaction between the events in the freshwater phase and their subsequent success in the marine environment. As mortality is strongly linked with growth, it appears that a lack of feeding opportunities (either qualitatively or quantitatively) is the main factor. Changes in food webs have already altered prey availability and quality in some areas, and CC induced changes to teleconnections like the NAO can change migration routes for post-smolts moving them into areas of reduced feeding potential. Add to this changing predator fields due to rising SST, as well as increases in mortality due to parasites, and continued low marine survival appears very likely in many stocks, something that has been observed since the mid-1990s. Towards the northern end of the distribution range expansion might occur, as will a general increase in productivity of many stocks. It is doubtful if this will compensate for losses elsewhere in the range. And the loss of southern stocks can also mean the loss of very unique genetic types, altering the overall genetic structure and future evolutionary potential.”
Finally there is an emerging hypothesis that changes in large-scale climate conditions in the Northwest Atlantic have apparently caused a “phase shift” in ecosystem productivity, altering trophic pathways that influence growth, survival and abundance of many species (Crozier et al. 2017). This hypothesis is supported by another study which has reported an abrupt decrease in the growth of Atlantic Salmon in the Northeast Atlantic Ocean in 2004 following a marked decrease in the extent of Arctic water in the Norwegian Sea, a subsequent warming of spring water temperature before Atlantic Salmon enter the sea, and an approximately 50% reduction of zooplankton across large geographic areas of the Northeast Atlantic Ocean (Vollset et al. 2022).
