Variation in the post‐smolt growth pattern of wild one sea‐winter salmon (Salmo salar L.), and its linkage to surface warming in the eastern North Atlantic Ocean

Abstract

Variation in circulus spacing on the scales of wild Atlantic salmon is indicative of changes in body length growth rate. We analyzed scale circulus spacing during the post‐smolt growth period for adult one sea‐winter salmon (n = 1947) returning to Scotland over the period 1993–2011. The growth pattern of the scales was subjectively and visually categorized according to the occurrence and zonal sequence of three intercirculus spacing criteria (“Slow”, “Fast” and “Check” zones). We applied hierarchical time‐series cluster analysis to the empirical circulus spacing data, followed by post hoc analysis of significant changes in growth patterns within the 20 identified clusters. Temporal changes in growth pattern frequencies showed significant correlation with sea surface temperature anomalies during the early months of the post‐smolt growth season and throughout the Norwegian Sea. Since the turn of the millennium, we observed (a) a marked decrease in the occurrence of continuous Fast growth; (b) increased frequencies of fish showing an extended period of initially Slow growth; and (c) the occurrence of obvious growth Checks or hiatuses. These changes in post‐smolt growth pattern were manifest also in decreases in the mean body length attained by the ocean midwinter, as sea surface temperatures have risen.

Time series covering up to four decades reveals major changes and drivers of marine growth and proportion of repeat spawners in an Atlantic salmon population

“…Long‐term studies by Todd et al ( 2020 ) and Jonsson et al ( 2016 ) also observed a negative effect of SST on marine growth in Atlantic salmon populations. It has been postulated that increasing SST causes an indirect negative effect on growth through climate changes influencing prey availability (Jonsson et al, 2016 ; Todd et al, 2008 , 2020 ). In the present study, marine growth fell to an all‐time low for the smolt year classes around 2007, just as zooplankton abundance dropped sharply from a high level at about 10–15 g/m 2 down to about half the biomass (Figure 3A ).…”

“…Earlier studies have also found conflicting influences of SST on marine growth (Bacon et al, 2009 ; Jensen et al, 2011 ; Todd et al, 2020 ), highlighting the fact that conclusions concerning drivers of marine growth rates of Atlantic salmon may differ among studies covering different regions and time periods. Long‐term studies by Todd et al ( 2020 ) and Jonsson et al ( 2016 ) also observed a negative effect of SST on marine growth in Atlantic salmon populations. It has been postulated that increasing SST causes an indirect negative effect on growth through climate changes influencing prey availability (Jonsson et al, 2016 ; Todd et al, 2008 , 2020 ).…”

Ecological regime shift in the Northeast Atlantic Ocean revealed from the unprecedented reduction in marine growth of Atlantic salmon

“…A sudden reduction in body growth was also observed among Atlantic mackerel following this reduction in plankton. A reduced growth of Atlantic salmon around 2005 has also been observed in France and Scotland ( 8 , 21 ), suggesting that the factors affecting southern Norwegian populations affected salmon populations across a large geographic area. We hypothesize that the oceanographic change in Arctic water caused a synchronous regime shift among trophic levels across a large area of the Northeastern Atlantic Ocean.…”

“…Atlantic salmon ( Salmo salar ) migrate from rivers around the North Atlantic Ocean and spend at least one year in the ocean, where they feed before returning to their natal rivers ( 7 ). Upon return, a proportion of the salmon is captured by anglers, who report size of each fish and often provide researchers with a scale sample ( 8 ). Experienced scale readers can reconstruct individual growth in length at different periods at sea by analyzing growth pattern in the scales ( 9 , 10 ).…”

“…Long-term biobanking of scales collected by anglers provides a unique opportunity to recover information about marine conditions at a large geographic scale, treating the Atlantic salmon as remote oceanographers that sample information on growth conditions in the distant marine waters across decades. Studies of growth have revealed that size at age of Atlantic salmon has been decreasing in large parts of the Northeast Atlantic along with reduced survival, particularly affecting southern populations ( 8 , 11 , 12 ). Several studies have suggested that changes in size and age at maturation ( 12 , 13 ) of Atlantic salmon may be related to ocean warming and subsequent changes to marine ecosystem functioning ( 14 – 19 ).…”