Long-Term Trends in Freshwater and Marine Growth Patterns in Three Sub-Arctic Atlantic Salmon Populations

The rapid warming of the Northern hemisphere has especially challenged the evolvability of anadromous fish species, such as Atlantic salmon (Salmo salar), which must cope with drastically different environments depending on their life-history stage. We studied the long-term trends in, and the effects of environmental factors and life-history traits on, Atlantic salmon growth rates in both freshwater and in the ocean using c. 35,000 scale samples collected across 48 years from spawners returning to three tributaries of the subarctic River Teno in the northernmost parts of Finland and Norway (70 & DEG; N). The freshwater growth has decreased in all three populations and spending more than three juvenile years in freshwater before the sea migration comes at the expense of growth. On the other hand, returning mature salmon (one-sea-winter, 1SW) showed increased growth at the sea with increasing marine temperatures, which results in larger sizes at return in 1SW spawners. We did not observe such trends in growth rates in larger, two-sea-winter salmon. Here, we report the contrasting responses in Atlantic salmon growth rates to a warming climate depending on the life-history stage.

Automated summary

This study analyzed the data collected from samples of Atlantic salmon in Finland and Norway and found that the rapid warming of the Northern hemisphere has different effects on the growth rates of Atlantic salmon in freshwater and in the ocean. The growth rate in freshwater has decreased, and spending more time in freshwater before migrating to the sea affects the growth. On the other hand, mature returning salmon in the ocean showed increased growth with increasing marine temperatures, resulting in larger sizes at return.