Complex relationships between marine growth and age at maturity in Atlantic salmon

Abstract Drastic declines in abundance of Atlantic salmon have been attributed to ecosystem change at sea reducing growth in body size, which in turn could impact survival because mortality at sea may be size-selective. A less scrutinized hypothesis proposes that poor growth conditions at sea may also increase mortality if slow growing individuals need more time at sea to become sexually mature with a longer marine residence period also increasing mortality risk. However, published analyses of the relationship or reaction norm between marine growth and age at maturity in salmon suggest that poor growth is linked with both earlier and later maturation. Hence, understanding why these opposing norms of reaction for age at maturity arise may be critical for elucidating the effects of maturation timing on the dynamics of Atlantic salmon populations. As a step toward this goal, we explain how developmental threshold theory can account for the contrasting norms of reaction that have been observed between marine growth and age at maturity in salmon. To aid our explanation, we construct and interpret a growth in body length at sea vs. age at maturity reaction norm from nearly 4000 individuals returning to the River Namsen in Norway, showing that rapid marine growth can be linked with later maturation and that maturation age can be influenced by shifts in growth occurring during the marine migration. We discuss possibilities for future studies that might yield a better understanding of the relationship between growth at sea and age at maturity in Atlantic salmon.