A critical life stage of the Atlantic salmon Salmo salar: behaviour and survival during the smolt and initial post‐smolt migration

Abstract

The anadromous life cycle of Atlantic salmon Salmo salar involves long migrations to novel environments and challenging physiological transformations when moving between salt-free and salt-rich waters. In this article, (1) environmental factors affecting the migration behaviour and survival of smolts and post-smolts during the river, estuarine and early marine phases, (2) how behavioural patterns are linked to survival and (3) how anthropogenic factors affect migration and survival are synthesized and reviewed based on published literature. The timing of the smolt migration is important in determining marine survival. The timing varies among rivers, most likely as a consequence of local adaptations, to ensure sea entry during optimal periods. Smolts and post-smolts swim actively and fast during migration, but in areas with strong currents, their own movements may be overridden by current-induced transport. Progression rates during the early marine migration vary between 0.4 and 3.0 body lengths s(-1) relative to the ground. Reported mortality is 0.3-7.0% (median 2.3) km(-1) during downriver migration, 0.6-36% (median 6.0) km(-1) in estuaries and 0.3-3.4% (median 1.4) km(-1) in coastal areas. Estuaries and river mouths are the sites of the highest mortalities, with predation being a common cause. The mortality rates varied more among studies in estuaries than in rivers and marine areas, which probably reflects the huge variation among estuaries in their characteristics. Behaviour and survival during migration may also be affected by pollution, fish farming, sea lice Lepeophtheirus salmonis, hydropower development and other anthropogenic activities that may be directly lethal, delay migration or have indirect effects by inhibiting migration. Total mortality reported during early marine migration (up to 5-230 km from the river mouths) in the studies available to date varies between 8 and 71%. Hence, the early marine migration is a life stage with high mortalities, due to both natural and human influences. Factors affecting mortality during the smolt and post-smolt stages contribute to determine the abundance of spawner returns. With many S. salar populations in decline, increased mortality at these stages may considerably contribute to limit S. salar production, and the consequences of human-induced mortality at this stage may be severe. Development of management actions to increase survival and fitness at the smolt and post-smolt stages is crucial to re-establish or conserve wild populations.

High host densities dilute sea lice Lepeophtheirus salmonis loads on individual Atlantic salmon, but do not reduce lice infection success

“…A laminar current speed of 16.2 ± 0.5 cm s −1 (mean ± SE), equivalent to a swimming speed of 0.7 body lengths (BL) s −1 , was generated in the raceways by a propeller (Minn Kota RT80/EM, Johnson Outdoors Marine Electronics) with adjustable speed followed by a honeycomb (5 mm opening, 102 mm thick, PC 5.0 G4, Plascore). Current speed was measured with a Vectrino velocimeter (Nortek) and was set at this level to reflect typical daytime swimming speeds of salmon (Oppedal et al 2011a, Thorstad et al 2012. Underwater video cameras (SeaVision SV27) were mounted in the middle of each raceway, with the field of view covering the back of the raceway.…”

“…The static water conditions during artificial infections or immobile fish contrast with natural infection settings for both farmed and wild salmon, where water currents may change infection dynamics (Revie et al 2003). Here, we used swimming chambers with a fixed current speed that reflected the normal swimming speeds of salmon in farmed (Oppedal et al 2011a) and wild environments (Thorstad et al 2012). This enabled more realistic swimming behaviours during infection to test how host density influences infection intensity, infection success, prevalence and degree of aggregation of sea lice L. salmonis on Atlantic salmon (Fig.…”

Linking Behavior, Physiology, and Survival of Atlantic Salmon Smolts During Estuary Migration

“…Elevated marine mortality in recent years, due in large part to changes in ocean climate (Friedland et al 2003;Mills et al 2013;Friedland et al 2014), is thought to be a driver behind the recovery failure of many endangered populations of Atlantic Salmon Salmo salar (Chaput et al 2005;Chaput 2012;Miller et al 2012;Mills et al 2013;Lacroix 2014). However, high mortality during migration through freshwater and estuarine corridors (Thorstad et al 2012b;Hayes and Kocik 2014) also likely contributes to reduced population sizes (Parrish et al 1998). Estimates of marine survival also often include estuarine mortality due to the difficulty in separating these processes (Friedland 1998).…”

“…The transition of Atlantic Salmon smolts to saltwater is recognized as a period of high mortality in estuaries (Lacroix 2008;Kocik et al 2009;Dempson et al 2011) and fjords (Gudjonsson et al 2005;Svenning et al 2005;Thorstad et al 2012b). This period is marked by high predation risk (Hvidsten and Lund 1988;Kocik et al 2009;Hawkes et al 2013), physiological stresses (Handeland et al 1997), and novel environmental conditions .…”

“…In response to these challenges, the process of smolting involves a synchronous suite of changes in physiology, morphology, and behavior that, in concert, enhance the probability of survival following successful saltwater entry . Therefore, the seasonal timing of smolt runs is important Thorstad et al 2012b). …”