Seasonal differences in smolt traits and post‐smolt survival of wild Atlantic salmon, Salmo salar, migrating from a northern boreal river

Abstract

Seasonal differences in smolt traits and the post-smolt survival of wild Atlantic salmon, Salmo salar L., were investigated by smolt trapping and Carlin-tagging in the River Simojoki, northern Finland between 1991 and 2004. The annual trapping season was divided into two halves based on the median catch date. Smolt length was significantly higher during the first half of the season, while differences in smolt weight were typically small. Smolt age was always significantly higher during the first half of the season because older smolts tended to migrate earlier in the season. Many smolts migrating during the early season and almost all smolts migrating later had started their new growth, indicating that smolts grow in the spring before sea entry. The differences in recaptures between smolts tagged during the first and second halves were insignificant. Although variations in smolt traits and environmental conditions can produce inter-annual variation in post-smolt survival, their seasonal differences seem to be too small to have an effect. K E Y W O R D S :Atlantic salmon, growth, post-smolt, smolt, survival.

Evidence of changing migratory patterns of wild Atlantic salmonSalmo salarsmolts in the River Bush, Northern Ireland, and possible associations with climate change

“…Age and L F steadily reduced throughout the smolt run with the youngest, smallest fish moving towards the end of the migratory period. This pattern has been observed in other rivers including the North Esk, Scotland (Dunkley, 1986), the River Ricklean in Sweden (Österdahl, 1969) and the River Simojoki in Finland (Jutila & Jokikokko, 2008). Early run smolts tend to be older, slower growing individuals, while later running fish are often dominated by individuals with the highest growth rates, which migrate at a younger age but smaller body size (Bohlin et al , 1996; Jutila & Jokikokko, 2008).…”

“…This pattern has been observed in other rivers including the North Esk, Scotland (Dunkley, 1986), the River Ricklean in Sweden (Österdahl, 1969) and the River Simojoki in Finland (Jutila & Jokikokko, 2008). Early run smolts tend to be older, slower growing individuals, while later running fish are often dominated by individuals with the highest growth rates, which migrate at a younger age but smaller body size (Bohlin et al , 1996; Jutila & Jokikokko, 2008). A positive relationship has been demonstrated between smolt size and subsequent survival (Tipping, 1986; Eriksson, 1989; Jonsson et al , 1990).…”

Evidence of changing migratory patterns of wild Atlantic salmonSalmo salarsmolts in the River Bush, Northern Ireland, and possible associations with climate change

“…Age and L F steadily reduced throughout the smolt run with the youngest, smallest fish moving towards the end of the migratory period. This pattern has been observed in other rivers including the North Esk, Scotland (Dunkley, 1986), the River Ricklean in Sweden (Österdahl, 1969) and the River Simojoki in Finland (Jutila & Jokikokko, 2008). Early run smolts tend to be older, slower growing individuals, while later running fish are often dominated by individuals with the highest growth rates, which migrate at a younger age but smaller body size (Bohlin et al , 1996; Jutila & Jokikokko, 2008).…”

“…This pattern has been observed in other rivers including the North Esk, Scotland (Dunkley, 1986), the River Ricklean in Sweden (Österdahl, 1969) and the River Simojoki in Finland (Jutila & Jokikokko, 2008). Early run smolts tend to be older, slower growing individuals, while later running fish are often dominated by individuals with the highest growth rates, which migrate at a younger age but smaller body size (Bohlin et al , 1996; Jutila & Jokikokko, 2008). A positive relationship has been demonstrated between smolt size and subsequent survival (Tipping, 1986; Eriksson, 1989; Jonsson et al , 1990).…”

Repeatable individual variation in migration timing in two anadromous salmonids and ecological consequences

“…Thus, because of the high mortality risk at this stage (Jensen et al., 2019), a trade‐off between energy gain and survival probably exists that maintains variation in migration timing. The largest smolts are the first ones to migrate to the sea, and the size of the smolts migrating to the sea decreases throughout the season, both in this river and in other rivers (Bohlin et al., 1993; Ewing et al., 1984; Jensen et al., 2012; Jutila & Jokikokko, 2008). One explanation may be that small individuals migrate to the sea in late summer because these individuals were too small to smoltify in the spring but reached a length suitable for smoltification later in the summer and hence migrated to the sea at a later date.…”

Changes in smolt traits of Atlantic salmon (Salmo salar Linnaeus, 1758) and linkages to parr density and water temperature

“…Unfortunately, no stomach analyses of parr or smolts and their food consumption were conducted. However, the larger mean size of both 2-and 3-year-old smolts after the median migration date confirms that food resources are available and that smolts start to feed in the river before sea migration (Jutila and Jokikokko, 2008). An interesting observation was that smolts in the Simojoki River did not compensate for their decreased growth by migrating at an older age, as suggested by Hutchings and Jones (1998) and Rich et al (2009).…”

“…Unfortunately, no stomach analyses of parr or smolts and their food consumption were conducted. However, the larger mean size of both 2‐ and 3‐year‐old smolts after the median migration date confirms that food resources are available and that smolts start to feed in the river before sea migration (Jutila and Jokikokko, ).…”

Evidence of changing migratory patterns of wild Atlantic salmonSalmo salarsmolts in the River Bush, Northern Ireland, and possible associations with climate change

“…Age and L F steadily reduced throughout the smolt run with the youngest, smallest fish moving towards the end of the migratory period. This pattern has been observed in other rivers including the North Esk, Scotland (Dunkley, 1986), the River Ricklean in Sweden (Österdahl, 1969) and the River Simojoki in Finland (Jutila & Jokikokko, 2008). Early run smolts tend to be older, slower growing individuals, while later running fish are often dominated by individuals with the highest growth rates, which migrate at a younger age but smaller body size (Bohlin et al , 1996; Jutila & Jokikokko, 2008).…”

“…This pattern has been observed in other rivers including the North Esk, Scotland (Dunkley, 1986), the River Ricklean in Sweden (Österdahl, 1969) and the River Simojoki in Finland (Jutila & Jokikokko, 2008). Early run smolts tend to be older, slower growing individuals, while later running fish are often dominated by individuals with the highest growth rates, which migrate at a younger age but smaller body size (Bohlin et al , 1996; Jutila & Jokikokko, 2008). A positive relationship has been demonstrated between smolt size and subsequent survival (Tipping, 1986; Eriksson, 1989; Jonsson et al , 1990).…”

Repeatable individual variation in migration timing in two anadromous salmonids and ecological consequences

“…Thus, because of the high mortality risk at this stage (Jensen et al., 2019), a trade‐off between energy gain and survival probably exists that maintains variation in migration timing. The largest smolts are the first ones to migrate to the sea, and the size of the smolts migrating to the sea decreases throughout the season, both in this river and in other rivers (Bohlin et al., 1993; Ewing et al., 1984; Jensen et al., 2012; Jutila & Jokikokko, 2008). One explanation may be that small individuals migrate to the sea in late summer because these individuals were too small to smoltify in the spring but reached a length suitable for smoltification later in the summer and hence migrated to the sea at a later date.…”

Changes in smolt traits of Atlantic salmon (Salmo salar Linnaeus, 1758) and linkages to parr density and water temperature

“…Unfortunately, no stomach analyses of parr or smolts and their food consumption were conducted. However, the larger mean size of both 2-and 3-year-old smolts after the median migration date confirms that food resources are available and that smolts start to feed in the river before sea migration (Jutila and Jokikokko, 2008). An interesting observation was that smolts in the Simojoki River did not compensate for their decreased growth by migrating at an older age, as suggested by Hutchings and Jones (1998) and Rich et al (2009).…”

“…Unfortunately, no stomach analyses of parr or smolts and their food consumption were conducted. However, the larger mean size of both 2‐ and 3‐year‐old smolts after the median migration date confirms that food resources are available and that smolts start to feed in the river before sea migration (Jutila and Jokikokko, ).…”

Evidence of changing migratory patterns of wild Atlantic salmonSalmo salarsmolts in the River Bush, Northern Ireland, and possible associations with climate change

“…Age and L F steadily reduced throughout the smolt run with the youngest, smallest fish moving towards the end of the migratory period. This pattern has been observed in other rivers including the North Esk, Scotland (Dunkley, 1986), the River Ricklean in Sweden (Österdahl, 1969) and the River Simojoki in Finland (Jutila & Jokikokko, 2008). Early run smolts tend to be older, slower growing individuals, while later running fish are often dominated by individuals with the highest growth rates, which migrate at a younger age but smaller body size (Bohlin et al , 1996; Jutila & Jokikokko, 2008).…”

“…This pattern has been observed in other rivers including the North Esk, Scotland (Dunkley, 1986), the River Ricklean in Sweden (Österdahl, 1969) and the River Simojoki in Finland (Jutila & Jokikokko, 2008). Early run smolts tend to be older, slower growing individuals, while later running fish are often dominated by individuals with the highest growth rates, which migrate at a younger age but smaller body size (Bohlin et al , 1996; Jutila & Jokikokko, 2008). A positive relationship has been demonstrated between smolt size and subsequent survival (Tipping, 1986; Eriksson, 1989; Jonsson et al , 1990).…”

Repeatable individual variation in migration timing in two anadromous salmonids and ecological consequences

“…Thus, because of the high mortality risk at this stage (Jensen et al., 2019), a trade‐off between energy gain and survival probably exists that maintains variation in migration timing. The largest smolts are the first ones to migrate to the sea, and the size of the smolts migrating to the sea decreases throughout the season, both in this river and in other rivers (Bohlin et al., 1993; Ewing et al., 1984; Jensen et al., 2012; Jutila & Jokikokko, 2008). One explanation may be that small individuals migrate to the sea in late summer because these individuals were too small to smoltify in the spring but reached a length suitable for smoltification later in the summer and hence migrated to the sea at a later date.…”

Changes in smolt traits of Atlantic salmon (Salmo salar Linnaeus, 1758) and linkages to parr density and water temperature

“…Unfortunately, no stomach analyses of parr or smolts and their food consumption were conducted. However, the larger mean size of both 2-and 3-year-old smolts after the median migration date confirms that food resources are available and that smolts start to feed in the river before sea migration (Jutila and Jokikokko, 2008). An interesting observation was that smolts in the Simojoki River did not compensate for their decreased growth by migrating at an older age, as suggested by Hutchings and Jones (1998) and Rich et al (2009).…”

“…Unfortunately, no stomach analyses of parr or smolts and their food consumption were conducted. However, the larger mean size of both 2‐ and 3‐year‐old smolts after the median migration date confirms that food resources are available and that smolts start to feed in the river before sea migration (Jutila and Jokikokko, ).…”