Table 3.

Anthropogenic factors potentially resulting in fitness changes and thus alterations to the cost–benefits of migration v. residency or migration destination in Salmo trutta and other salmonines

Factor	Impact on migrants	References
Partial barriers to downstream and upstream migration resulting from water offtake, hydroelectric generation, etc.	Increased energy expenditure. Increased risk of predation. Migration speed of smolts significantly slower. High downstream passage mortality of S. trutta kelts at hydropower stations. Upstream may be size selective and thus change size–age at maturity. Multiple partial barriers have an effect equivalent to an impassable barrier. Partial barriers resulting in a reduction of MAR alleles in Oncorhynchus mykiss. Removal of six partially impassable weirs in a Danish river resulted in nine‐fold increase in spawning S. trutta over 12 year period.	Apgar et al., 2017; Birnie‐Gauvin et al., 2018; Buddendorf et al., 2019; Haugen, 2008; Huusko et al., 2017; Jepsen et al., 1998; Ostergren & Rivinoja, 2008; Van Puijenbroek et al., 2018;
Complete barrier to upstream migration resulting from; e.g., construction of water storage reservoirs and hydropower stations without fish passes.	Anadromous populations extinct. Most of 72 anadromous S. trutta populations in Finland now lost. Change in destination; e.g., anadromous become lacustrine‐adfluvial migrants.	Holecek & Scarnecchia, 2013; Leitwein et al., 2017; Soininen et al., 2018
Regulation of river flows. Also, redirection of water to hydropower stations.	Un‐naturally high and low flows resulting in decrease in or elimination of migrants. Delays and increased energy expenditure. Changes in speed of migration. Fluvial–adfluvial became river‐resident due to reduced habitat quality.	Garcia‐Vega et al., 2017; Sandlund & Jonsson, 2016
Increased infestation by sea lice Lepeophtheirus salmonis associated with Salmo salar farming.	Reduced marine survival with 50%–100% mortality within 15 km of farms in Norway. Problems with osmoregulation. Earlier return to rivers with lower growth and fewer offspring thus reducing advantage of migration.	Gargan et al., 2016; Halttunen et al. 2017; Moore et al., 2018; Poole et al., 2006; Skaala et al., 2014; Taranger et al., 2015; Thorstad et al., 2015;
Increased predation by piscivorous birds and mammals in downstream sections of rivers, in lakes, and at sea.	Reduced survival. Increased energy expenditure in predator avoidance. Greater increase in predation at sea tips balance in favour of potamodromy. Predation through lakes and on sea entry main factor determining number of returning anadromous S. trutta in Denmark. High predation by great cormorants Phalacrocorax carbo key mortality factor in some rivers & lakes. Heavy pike Esox lucius predation at river‐to‐lake confluences.	Berejikian et al., 2016; Healy et al., 2017; Jepsen et al., 2018, 2019; ; Kennedy et al., 2018; Schwinn et al., 2018
Increased exploitation. Differential life history, size, and sex exploitation.	Reduced marine survival due to exploitation either directly or as a by‐product. Greater exploitation of (larger) migrants than (smaller) river‐residents resulting in selection for latter. Selection for earlier age of maturity, run timing and time of spawning.	Czorlich et al., 2018; Hollins et al., 2018; Kallio‐Nyberg et al., 2018; Koeck et al., 2018; Syrjanen et al., 2018; Thériault et al., 2008; Tillotson & Quinn, 2018
Climate change.	Changes in river flows and water temperature influencing feeding, migration timing, spawning and juvenile survival. Increased metabolic cost of upstream migration. Decreased marine productivity and increased freshwater productivity and growth rates tipping balance in favour of potamodromy–river‐residency. Possibly direct effect of temperature on life history.	Finstad & Hein, 2012; García‐Vega et al., 2018; Hermoso & Clavero, 2011; Jonsson & Jonsson, 2011, 2019; ; Lennox et al., 2018; Peiman et al., 2017; Piou & Prévost, 2012.
Interbreeding with stocked fertile hatchery reared / farm S. trutta.	Decreased genetic tendency for migration. Reduced marine cf. freshwater survival.	Ferguson, 2007; Ferguson et al., 2017; Thrower & Hard, 2009;

MAR: migration‐associated region.