The major threats to Atlantic salmon in Norway

Abstract

Atlantic salmon (Salmo salar) is an economically and culturally important species. Norway has more than 400 watercourses with Atlantic salmon and supports a large proportion of the world's wild Atlantic salmon. Atlantic salmon are structured into numerous genetically differentiated populations, and are therefore managed at the population level. Long-distance migrations between freshwater and ocean habitats expose Atlantic salmon to multiple threats, and a number of anthropogenic factors have contributed to the decline of Atlantic salmon during the last decades. Knowledge on the relative importance of the different anthropogenic factors is vital for prioritizing management measures. We developed a semi-quantitative 2D classification system to rank the different anthropogenic factors and used this to assess the major threats to Norwegian Atlantic salmon. Escaped farmed salmon and salmon lice from fish farms were identified as expanding population threats, with escaped farmed salmon being the largest current threat. These two factors affect populations to the extent that they may be critically endangered or lost, with a large likelihood of causing further reductions and losses in the future. The introduced parasite Gyrodactylus salaris, freshwater acidification, hydropower regulation and other habitat alterations were identified as stabilized population threats, which have contributed to populations becoming critically endangered or lost, but with a low likelihood of causing further loss. Other impacts were identified as less influential, either as stabilized or expanding factors that cause loss in terms of number of returning adults, but not to the extent that populations become threatened. Management based on population specific reference points (conservation limits) has reduced exploitation in Norway, and overexploitation was therefore no longer regarded an important impact factor. The classification system may be used as a template for ranking of anthropogenic impact factors in other countries and as a support for national and international conservation efforts.

Modelled salmon lice dispersion and infestation patterns in a sub-arctic fjord

“…The low water temperatures lead to slower generation cycles of the lice (Samsing et al, 2016b), and the wild salmon populations in Finnmark have been relatively unaffected by the salmon farming industry (Bjørn et al, 2007;Svåsand et al, 2017). This in contrast to the situation on the west coast of Norway, where the long-standing intensive aquaculture industry combined with higher water temperatures have caused concentrations of lice that may have contributed to a serious decimation of wild Atlantic salmon stocks (Skaala et al, 2014;Taranger et al, 2015;Svåsand et al, 2016;Forseth et al, 2017).…”

“…Salmon lice epizootics can represent a welfare issue for the farmed fish, and the repeated delouse treatments of farmed fish to keep the lice level under the permitted level cause significant economic losses to the industry. Further, the salmon lice are regarded as a serious threat for wild anadromous salmonids (Ashley, 2007;Costello, 2009a, b;Forseth et al, 2017). The Norwegian government has chosen mortality on wild salmonids due to salmon lice infestation as the index of carrying capacity for environmentally sustainable growth of the aquaculture industry (Anon., 2014(Anon., -2015Taranger et al, 2015;Vollset et al, 2018).…”

Environmental problems and regulation in the aquaculture industry. Insights from Norway

“…Ecologically, they may interact through competition, predation, hybridization, colonization, and spreading diseases and parasites (Forseth et al 2017). Escaped farmed salmon may hence have a number of effects on the natural growth and economic value of wild salmon.…”

“…The lice are mainly a problem for the salmon postsmolts on their seaward migration journey, as they have to pass the farm areas before they reach their offshore winter habitat. A recent study ranks the high sea lice densities, together with escaped farmed salmon from aquaculture, as the two most significant and expanding threats to the wild salmon populations in Norway (Forseth et al 2017). Salmon aquaculture increases the sea lice density in the fjords and along the coast because they amplify the number of hosts for the lice by a magnitude of 100 (Heuch et al 2005).…”

“…Consequently, Norwegian authorities have taken a particular responsibility to protect the species and its populations. According to the scientific board of salmon management, the two most severe challenges for this species is the escapement from fish farms and the high sea lice densities (Anon 2017a, Forseth et al 2017). …”